The object that can be used to set a field to read-only in all applications of the RESTful Application Programming model (RAP) is the behaviour definition. The behaviour definition is a CDS artefact that defines the business logic and the UI behaviour of a business object. A business object is a CDS entity that represents a business entity or concept, such as a customer, an order, or a product. The behaviour definition can specify the properties of the fields of a business object, such as whether they are mandatory, read-only, or transient. These properties are valid for all applications that use the business object, such as transactional, analytical, or draft-enabled apps12. For example:
The following code snippet defines a behaviour definition for a business object ZI_PB_APPLICATION. It sets the field APPLICATION to read-only for all applications that use this business object:
define behavior for ZI_PB_APPLICATION { field ( read only ) APPLICATION; … } You cannot do any of the following:
A) Service definition: A service definition is a CDS artefact that defines the interface and the binding of a service. A service is a CDS entity that exposes the data and the functionality of one or more business objects as OData, InA, or SQL services. A service definition can specify the properties of the fields of a service, such as whether they are filterable, sortable, or aggregatable. However, these properties are only valid for the specific service that uses the business object, not for all applications that use the business object12.
C) Projection view: A projection view is a CDS artefact that defines a view on one or more data sources, such as tables, views, or associations. A projection view can select, rename, or aggregate the fields of the data sources, but it cannot change the properties of the fields, such as whether they are read-only or not. The properties of the fields are inherited from the data sources or the behaviour definitions of the business objects12.
D) Metadata extension: A metadata extension is a CDS artefact that defines additional annotations for a CDS entity, such as a business object, a service, or a projection view. A metadata extension can specify the properties of the fields of a CDS entity for UI or analytical purposes, such as whether they are visible, editable, or hidden. However, these properties are only valid for the specific UI or analytical application that uses the metadata extension, not for all applications that use the CDS entity12.

The purpose of a foreign key relationship between two tables in the ABAP Dictionary is to ensure the integrity of data in the corresponding database tables. A foreign key relationship defines a logical link between a foreign key table and a check table, where the foreign key fields of the former are assigned to the primary key fields of the latter. This means that the values entered in the foreign key fields must exist in the check table, otherwise the system will reject the entry. This way, the foreign key relationship prevents the insertion of invalid or inconsistent data in the database tables.
A foreign key relationship also serves to document the relationship between the two tables in the ABAP Dictionary, but this is not its primary purpose. A foreign key relationship does not necessarily create a corresponding foreign key relationship in the database, as this depends on the database system and the settings of the ABAP Dictionary. Some database systems do not support foreign keys at all, while others require additional steps to activate them. Therefore, the foreign key relationship in the ABAP Dictionary is mainly a logical concept that is enforced by the ABAP runtime environment.
References: Foreign Keys (SAP Library – ABAP Dictionary), Foreign Keys (SAP Library – BC – ABAP Dictionary)
https://help.sap.com/doc/saphelp_snc70/7.0/en-US/cf/21ea77446011d189700000e8322d00/content.htm

Explanation
A CDS view is a data definition that defines a data structure and a data selection from one or more data sources. A CDS view consists of several parts, but two of them are:
Extension: An extension is an optional clause that allows a CDS view to extend another CDS view by adding new elements, annotations, or associations. The extension clause has the syntax EXTEND VIEW view_name WITH view_name. The first view_name is the name of the CDS view that is being extended, and the second view_name is the name of the CDS view that is doing the extension1.
Field list: A field list is a mandatory clause that specifies the elements of the CDS view. The field list has the syntax SELECT FROM data_source { element_list }. The data_source is the name of the data source that the CDS view selects data from, and the element_list is a comma-separated list of elements that the CDS view exposes. The elements can be fields of the data source, expressions, associations, or annotations2.
The following example shows a CDS view that extends another CDS view and defines a field list:
@AbapCatalog.sqlViewName: ‘ZCDS_EXT’ define view Z_CDS_Extension extend view Z_CDS_Base with Z_CDS_Extension as select from ztable { // field list key ztable.id as ID, ztable.name as Name, ztable.age as Age, // extension @Semantics.currencyCode: true ztable.currency as Currency } The other options are not parts of a CDS view, but rather related concepts:
Partitioning attributes: Partitioning attributes are attributes that are used to partition a table into smaller subsets of data. Partitioning attributes are defined in the ABAP Dictionary for transparent tables and can improve the performance and scalability of data access. Partitioning attributes are not part of the CDS view definition, but rather the underlying table definition3.
Semantic table attributes: Semantic table attributes are attributes that provide additional information about the meaning and usage of a table. Semantic table attributes are defined in the ABAP Dictionary for transparent tables and can be used to enhance the data modeling and consumption of the table. Semantic table attributes are not part of the CDS view definition, but rather the underlying table definition4.
References: 1: Extending CDS Views | SAP Help Portal 2: SELECT List – ABAP Keyword Documentation 3:
Partitioning Attributes – ABAP Keyword Documentation 4: Semantic Table Attributes – ABAP Keyword Documentation

Explanation
SAP recommends that you eliminate the following types of legacy code when you review modifications as part of an SAP S/4HANA system conversion:
Code that now is identical to a standard SAP object. This type of code is redundant and unnecessary, as it does not provide any additional functionality or customization. It can also cause conflicts or errors during the system conversion, as the standard SAP object may have changed or been replaced in SAP S/4HANA. Therefore, you should delete this type of code and use the standard SAP object instead.
Code that can be redesigned as a key user extension. This type of code is usually related to UI or business logic adaptations that can be achieved using the in-app tools provided by SAP S/4HANA. By redesigning this type of code as a key user extension, you can simplify and standardize your code base, reduce maintenance efforts, and avoid compatibility issues during the system conversion. Therefore, you should migrate this type of code to the key user extensibility framework and delete the original code.
The other types of legacy code are not recommended to be eliminated, as they may still be relevant or necessary for your business processes. However, you should still review and adjust them according to the SAP S/4HANA simplification items and best practices. These types of code are:
Code that supports a critical business process. This type of code is essential for your business operations and cannot be easily replaced or removed. However, you should check if this type of code is compatible with SAP S/4HANA, and if not, you should adapt it accordingly. You should also consider if this type of code can be optimized or enhanced using the new features and capabilities of SAP S/4HANA.
Code that has less than 10% usage according to usage statistics. This type of code is rarely used and may not be worth maintaining or converting. However, you should not delete this type of code without verifying its relevance and impact on your business processes. You should also consider if this type of code can be replaced or consolidated with other code that has higher usage or better performance.
References: Custom Code Management (CCM) During an SAP S/4HANA Conversion, Custom Code Migration Guide for SAP S/4HANA 2020

Explanation
The statement that can be used to expose the field “connid” of the data source “demo_ods_assoc_spfli” in the element list is A. demo_ods_assoc_spfli.connid,. This statement uses the dot notation to access the field
“connid” of the data source “demo_ods_assoc_spfli”, which is an association defined on line #4. The association “demo_ods_assoc_spfli” links the data source “demo_ods” with the table “spfli” using the field
“carrid”. The statement also ends with a comma to separate it from the next element in the list12.
You cannot do any of the following:
B: demo_ods_assoc_spfli-connid/: This statement uses the wrong syntax to access the field “connid” of the data source “demo_ods_assoc_spfli”. The dash notation is used to access the components of a structure or a table, not the fields of a data source. The statement also ends with a slash, which is not a valid separator for the element list12.
C: spfli-connid,: This statement uses the wrong data source name to access the field “connid”. The data source name should be “demo_ods_assoc_spfli”,not “spfli”. The statement also uses the wrong syntax to access the field “connid”, as explained above12.
D:_spfli.connid/: This statement uses the wrong data source name and the wrong separator to access the field “connid”. The data source name should be “demo_ods_assoc_spfli”, not “_spfli”. The statement also ends with a slash, which is not a valid separator for the element list12.
References: 1: ABAP CDS – SELECT, select_list – ABAP Keyword Documentation – SAP Online Help 2: ABAP CDS – SELECT, from – ABAP Keyword Documentation – SAP Online Help

Secondary keys are additional keys that can be defined for internal tables to optimize the access to the table using fields that are not part of the primary key. Secondary keys can be either sorted or hashed, depending on the table type and the uniqueness of the key. Secondary keys have the following characteristics1:
* A. Secondary keys must be chosen explicitly when you actually read from an internal table. This means that when you use a READ TABLE or a LOOP AT statement to access an internal table, you have to specify the secondary key that you want to use with the USING KEY addition. For example, the following statement reads an internal table itab using a secondary key sec_key:
READ TABLE itab USING KEY sec_key INTO DATA(wa).
If you do not specify the secondary key, the system will use the primary key by default2.
* B. Multiple secondary keys are allowed for any kind of internal table. This means that you can define more than one secondary key for an internal table, regardless of the table type. For example, the following statement defines an internal table itab with two secondary keys sec_key_1 and sec_key_2:
DATA itab TYPE SORTED TABLE OF ty_itab WITH NON-UNIQUE KEY sec_key_1 COMPONENTS field1 field2 sec_key_2 COMPONENTS field3 field4.
You can then choose which secondary key to use when you access the internal table1.
* D. Sorted secondary keys do NOT have to be unique. This means that you can define a sorted secondary key for an internal table that allows duplicate values for the key fields. A sorted secondary key maintains a predefined sorting order for the internal table, which is defined by the key fields in the order in which they are specified. For example, the following statement defines a sorted secondary key sec_key for an internal table itab that sorts the table by field1 in ascending order and field2 in descending order:
DATA itab TYPE STANDARD TABLE OF ty_itab WITH NON-UNIQUE SORTED KEY sec_key COMPONENTS field1 ASCENDING field2 DESCENDING.
You can then access the internal table using the sorted secondary key with a binary search algorithm, which is faster than a linear search3.
The following are not characteristics of secondary keys for internal tables, because:
* C. Hashed secondary keys do NOT have to be unique. This is false because hashed secondary keys must be unique. This means that you can only define a hashed secondary key for an internal table that does not allow duplicate values for the key fields. A hashed secondary key does not have a predefined sorting order for the internal table, but uses a hash algorithm to store and access the table rows. For example, the following statement defines a hashed secondary key sec_key for an internal table itab that hashes the table by field1 and field2:
DATA itab TYPE STANDARD TABLE OF ty_itab WITH UNIQUE HASHED KEY sec_key COMPONENTS field1 field2.
You can then access the internal table using the hashed secondary key with a direct access algorithm, which is very fast.
* E. Secondary keys can only be created for standard tables. This is false because secondary keys can be created for any kind of internal table, such as standard tables, sorted tables, and hashed tables. However, the type of the secondary key depends on the type of the internal table. For example, a standard table can have sorted or hashed secondary keys, a sorted table can have sorted secondary keys, and a hashed table can have hashed secondary keys1.
References: 1: Secondary Table Key – ABAP Keyword Documentation 2: READ TABLE – ABAP Keyword Documentation 3: Sorted Tables – ABAP Keyword Documentation : Hashed Tables – ABAP Keyword Documentation

After you created a database table in the RESTful Application Programming model (RAP), the next step is to create a projection view on the database table. A projection view is a CDS artefact that defines a view on one or more data sources, such as tables, views, or associations. A projection view can select, rename, or aggregate the fields of the data sources, but it cannot change the properties of the fields, such as whether they are read-only or not. The properties of the fields are inherited from the data sources or the behaviour definitions of the business objects12. For example:
The following code snippet defines a projection view ZI_AGENCY on the database table /DMO/AGENCY:
define view ZI_AGENCY as select from /dmo/agency { key agency_id, agency_name, street, city, region, postal_code, country, phone_number, url } The projection view is used to expose the data of the database table to the service definition, which is the next step in the RAP. The service definition is a CDS artefact that defines the interface and the binding of a service. A service is a CDS entity that exposes the data and the functionality of one or more business objects as OData, InA, or SQL services. A service definition can specify the properties of the fields of a service, such as whether they are filterable, sortable, or aggregatable12. For example:
The following code snippet defines a service definition ZI_AGENCY_SRV that exposes the projection view ZI_AGENCY as an OData service:
define service ZI_AGENCY_SRV { expose ZI_AGENCY as Agency; }
You cannot do any of the following:
A) A metadata extension: A metadata extension is a CDS artefact that defines additional annotations for a CDS entity, such as a business object, a service, or a projection view. A metadata extension can specify the properties of the fields of a CDS entity for UI or analytical purposes, such as whether they are visible, editable, or hidden. However, a metadata extension is not the next step after creating a database table in the RAP, as it is not required to expose the data of the database table to the service definition. A metadata extension can be created later to customize the UI or analytical application that uses the service12.
C) A data model view: A data model view is a CDS artefact that defines a view on one or more data sources, such as tables, views, or associations. A data model view can select, rename, or aggregate the fields of the data sources, and it can also change the properties of the fields, such as whether they are read-only or not. The properties of the fields are defined by the annotations or the behaviour definitions of the data model view. A data model view is used to define the data model of a business object, which is a CDS entity that represents a business entity or concept, such as a customer, an order, or a product. However, a data model view is not the next step after creating a database table in the RAP, as it is not required to expose the data of the database table to the service definition. A data model view can be created later to define a business object that uses the database table as a data source12.
D) A service definition: A service definition is a CDS artefact that defines the interface and the binding of a service. A service is a CDS entity that exposes the data and the functionality of one or more business objects as OData, InA, or SQL services. A service definition can specify the properties of the fields of a service, such as whether they are filterable, sortable, or aggregatable. However, a service definition is not the next step after creating a database table in the RAP, as it requires a projection view or a data model view to expose the data of the database table. A service definition can be created after creating a projection view or a data model view on the database table12.

SAP recommends that you eliminate the following types of legacy code when you review modifications as part of an SAP S/4HANA system conversion:
* Code that now is identical to a standard SAP object. This type of code is redundant and unnecessary, as it does not provide any additional functionality or customization. It can also cause conflicts or errors during the system conversion, as the standard SAP object may have changed or been replaced in SAP S/4HANA. Therefore, you should delete this type of code and use the standard SAP object instead.
* Code that can be redesigned as a key user extension. This type of code is usually related to UI or business logic adaptations that can be achieved using the in-app tools provided by SAP S/4HANA. By redesigning this type of code as a key user extension, you can simplify and standardize your code base, reduce maintenance efforts, and avoid compatibility issues during the system conversion. Therefore, you should migrate this type of code to the key user extensibility framework and delete the original code.
The other types of legacy code are not recommended to be eliminated, as they may still be relevant or necessary for your business processes. However, you should still review and adjust them according to the SAP S/4HANA simplification items and best practices. These types of code are:
* Code that supports a critical business process. This type of code is essential for your business operations and cannot be easily replaced or removed. However, you should check if this type of code is compatible with SAP S/4HANA, and if not, you should adapt it accordingly. You should also consider if this type of code can be optimized or enhanced using the new features and capabilities of SAP S/4HANA.
* Code that has less than 10% usage according to usage statistics. This type of code is rarely used and may not be worth maintaining or converting. However, you should not delete this type of code without verifying its relevance and impact on your business processes. You should also consider if this type of code can be replaced or consolidated with other code that has higher usage or better performance.
References: Custom Code Management (CCM) During an SAP S/4HANA Conversion, Custom Code Migration Guide for SAP S/4HANA 2020

The following are the explanations for each statement:
* C: This statement is valid. Class CL1 uses the interface. This is because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The INTERFACES statement makes the class compatible with the interface and inherits all the components of the interface. The class can then use the interface components, such as the method ml, by using the interface component selector ~, such as ifl~ml12
* E: This statement is valid. Class CL1 implements the interface. This is because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The INTERFACES statement makes the class compatible with the interface and inherits all the components of the interface. The class must then provide an implementation for the interface method ml in the implementation part of the class, unless the method is declared as optional or abstract12
* D: This statement is valid. In class CL2, the interface method is named ifl~ml. This is because class CL2 has a data member named m0_ifl of type REF TO ifl, which is a reference to the interface ifl. The interface ifl defines a method ml, which can be called using the reference variable m0_ifl. The interface method ml has the name ifl~ml in the class, where ifl is the name of the interface and the character ~ is the interface component selector12 The other statements are not valid, as they have syntax errors or logical errors. These statements are:
* A: This statement is not valid. In class CL1, the interface method is named ifl~ml, not if-ml. This is because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The interface ifl defines a method ml, which can be called using the class name or a reference to the class. The interface method ml has the name ifl~ml in the class, where ifl is the name of the interface and the character ~ is the interface component selector. Using the character – instead of the character ~ will cause a syntax error12
* B: This statement is not valid. Class CL2 does not use the interface, but only has a reference to the interface. This is because class CL2 has a data member named m0_ifl of type REF TO ifl, which is a reference to the interface ifl. The interface ifl defines a method ml, which can be called using the reference variable m0_ifl. However, class CL2 does not implement the interface ifl, nor does it inherit the interface components. Therefore, class CL2 does not use the interface, but only references the interface12 References: INTERFACES – ABAP Keyword Documentation, CLASS – ABAP Keyword Documentation

Explanation
The <source> clause in the CDS View Entity Data Definition can be used to specify the data source for the view entity. The <source> clause can accept different types of data sources, depending on the type of the view entity1.
A database table from the ABAP Dictionary: This is a valid type of data source for a CDS View Entity Data Definition. A database table from the ABAP Dictionary is a table that is defined in the ABAP Dictionary using the keyword TABLE or TABLE OF. The name of the database table must be unique within its namespace and must not contain any special characters2.
A CDS DDIC-based view: This is also a valid type of data source for a CDS View Entity Data Definition. A CDS DDIC-based view is a view that is defined in the Core Data Services using the keyword DEFINE VIEW ENTITY. The name of the CDS DDIC-based view must be unique within its namespace and must not contain any special characters3.
You cannot do any of the following:
An external view from the ABAP Dictionary: This is not a valid type of data source for a CDS View Entity Data Definition. An external view from the ABAP Dictionary is a view that is defined in an external application using any language supported by SAP, such as SQL, PL/SQL, or Java. The name of the external view must be unique within its namespace and must not contain any special characters4.
A database view from the ABAP Dictionary: This is not a valid type of data source for a CDS View Entity Data Definition. A database view from the ABAP Dictionary is a view that is defined in an external application using any language supported by SAP, such as SQL, PL/SQL, or Java. The name of the database view must be unique within its namespace and must not contain any special characters4.
References: 1: CDS DDL – DEFINE VIEW ENTITY – ABAP Keyword Documentation – SAP Online Help 2:
ABAP Dictionary Tables – SAP Online Help 3: CDS DDL – DEFINE VIEW ENTITY – ABAP Keyword Documentation – SAP Online Help 4: ABAP Dictionary Views – SAP Online Help

Explanation
The following are the explanations for each statement:
C: This statement is valid. Class CL1 uses the interface. This is because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The INTERFACES statement makes the class compatible with the interface and inherits all the components of the interface. The class can then use the interface components, such as the method ml, by using the interface component selector ~, such as ifl~ml12 E: This statement is valid. Class CL1 implements the interface. This is because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The INTERFACES statement makes the class compatible with the interface and inherits all the components of the interface. The class must then provide an implementation for the interface method ml in the implementation part of the class, unless the method is declared as optional or abstract12 D: This statement is valid. In class CL2, the interface method is named ifl~ml. This is because class CL2 has a data member named m0_ifl of type REF TO ifl, which is a reference to the interface ifl. The interface ifl defines a method ml, which can be called using the reference variable m0_ifl. The interface method ml has the name ifl~ml in the class, where ifl is the name of the interface and the character ~ is the interface component selector12 The other statements are not valid, as they have syntax errors or logical errors. These statements are:
A: This statement is not valid. In class CL1, the interface method is named ifl~ml, not if-ml. This is because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The interface ifl defines a method ml, which can be called using the class name or a reference to the class. The interface method ml has the name ifl~ml in the class, where ifl is the name of the interface and the character ~ is the interface component selector. Using the character – instead of the character ~ will cause a syntax error12 B: This statement is not valid. Class CL2 does not use the interface, but only has a reference to the interface. This is because class CL2 has a data member named m0_ifl of type REF TO ifl, which is a reference to the interface ifl. The interface ifl defines a method ml, which can be called using the reference variable m0_ifl. However, class CL2 does not implement the interface ifl, nor does it inherit the interface components. Therefore, class CL2 does not use the interface, but only references the interface12 References: INTERFACES – ABAP Keyword Documentation, CLASS – ABAP Keyword Documentation

Explanation:
The sequence in which the constructors will be executed is as follows:
* Class constructor of superl. This is because the class constructor is a static method that is executed automatically before the class is accessed for the first time. The class constructor is used to initialize the static attributes and components of the class. The class constructor of the superclass is executed before the class constructor of the subclass, as the subclass inherits the static components of the superclass12
* Class constructor of subl. This is because the class constructor is a static method that is executed automatically before the class is accessed for the first time. The class constructor is used to initialize the static attributes and components of the class. The class constructor of the subclass is executed after the class constructor of the superclass, as the subclass inherits the static components of the superclass12
* Instance constructor of superl. This is because the instance constructor is an instance method that is executed automatically when an instance of the class is created using the statement CREATE OBJECT.
The instance constructor is used to initialize the instance attributes and components of the class. The instance constructor of the superclass is executed before the instance constructor of the subclass, as the subclass inherits the instance components of the superclass. The instance constructor of the subclass must call the instance constructor of the superclass explicitly using super->constructor, unless the superclass is the root node object12
* Instance constructor of subl. This is because the instance constructor is an instance method that is executed automatically when an instance of the class is created using the statement CREATE OBJECT.
The instance constructor is used to initialize the instance attributes and components of the class. The instance constructor of the subclass is executed after the instance constructor of the superclass, as the subclass inherits the instance components of the superclass. The instance constructor of the subclass must call the instance constructor of the superclass explicitly using super->constructor, unless the superclass is the root node object12 References: Constructors of Classes – ABAP Keyword Documentation, METHODS – constructor – ABAP Keyword Documentation

The purpose of a foreign key relationship between two tables in the ABAP Dictionary is to ensure the integrity of data in the corresponding database tables. A foreign key relationship defines a logical link between a foreign key table and a check table, where the foreign key fields of the former are assigned to the primary key fields of the latter. This means that the values entered in the foreign key fields must exist in the check table, otherwise the system will reject the entry. This way, the foreign key relationship prevents the insertion of invalid or inconsistent data in the database tables.
A foreign key relationship also serves to document the relationship between the two tables in the ABAP Dictionary, but this is not its primary purpose. A foreign key relationship does not necessarily create a corresponding foreign key relationship in the database, as this depends on the database system and the settings of the ABAP Dictionary. Some database systems do not support foreign keys at all, while others require additional steps to activate them. Therefore, the foreign key relationship in the ABAP Dictionary is mainly a logical concept that is enforced by the ABAP runtime environment.
Reference:
https://help.sap.com/doc/saphelp_snc70/7.0/en-US/cf/21ea77446011d189700000e8322d00/content.htm

When processing a loop with the statement DO… ENDDO, the system variable that contains the implicit loop counter is sy-index. The loop counter is a numeric value that indicates how many times the loop has been executed. The loop counter is initialized to 1 before the first execution of the loop and is incremented by 1 after each execution. The loop counter can be used to control the number of loop iterations or to access the loop elements by index. The loop counter can also be accessed or modified within the loop body, but this is not recommended as it may cause unexpected results or errors1.
For example, the following code snippet uses the loop counter sy-index to display the numbers from 1 to 10:
DO 10 TIMES. WRITE: / sy-index. ENDDO.
The output of this code is:
1 2 3 4 5 6 7 8 9 10

The correct expression to change a given string value ‘mr joe doe’ into ‘JOE’ in an ABAP SQL field list is C: SELECT FROM TABLE dbtabl FIELDS Of1, substring(upper(‘mr joe doe’), 4, 3) AS f2_sub_up, f3,…
This expression uses the following SQL functions for strings12:
* upper: This function converts all lowercase characters in a string to uppercase. For example, upper(‘mr joe doe’) returns ‘MR JOE DOE’.
* substring: This function returns a substring of a given string starting from a specified position and with a specified length. For example, substring(‘MR JOE DOE’, 4, 3) returns ‘JOE’.
* AS: This keyword assigns an alias or a temporary name to a field or an expression in the field list. For example, AS f2_sub_up assigns the name f2_sub_up to the expression substring(upper(‘mr joe doe’), 4,
3).
You cannot do any of the following:
* A. SELECT FROM TABLE dbtabl FIELDS Of1, upper(left( ‘mr joe doe’, 6)) AS f2_up_left, f3,…:
This expression uses the wrong SQL function for strings to get the desired result. The left function returns the leftmost characters of a string with a specified length, ignoring the trailing blanks. For example, left( ‘mr joe doe’, 6) returns ‘mr joe’. Applying the upper function to this result returns ‘MR JOE’, which is not the same as ‘JOE’.
* B. SELECT FROM TABLE dbtabl FIELDS Of1, left(lower(substring( ‘mr joe doe’, 4, 3)), 3) AS f2_left_lo_sub, f3,…: This expression uses unnecessary and incorrect SQL functions for strings to get the desired result. The lower function converts all uppercase characters in a string to lowercase. For example, lower(substring( ‘mr joe doe’, 4, 3)) returns ‘joe’. Applying the left function to this result with the same length returns ‘joe’ again, which is not the same as ‘JOE’.
* D. SELECT FROM TABLE dbtabl FIELDS Of1, substring(lower(upper( ‘mr joe doe’ ) ), 4, 3) AS f2_sub_lo_up, f3,…: This expression uses unnecessary and incorrect SQL functions for strings to get the desired result. The lower function converts all uppercase characters in a string to lowercase, and the upper function converts all lowercase characters in a string to uppercase. Applying both functions to the same string cancels out the effect of each other and returns the original string. For example, lower(upper( ‘mr joe doe’ ) ) returns ‘mr joe doe’. Applying the substring function to this result returns
‘joe’, which is not the same as ‘JOE’.
References: 1: SQL Functions for Strings – ABAP Keyword Documentation – SAP Online Help 2: sql_func – String Functions – ABAP Keyword Documentation – SAP Online Help

The <source> clause in the CDS View Entity Data Definition can be used to specify the data source for the view entity. The <source> clause can accept different types of data sources, depending on the type of the view entity1.
* A database table from the ABAP Dictionary: This is a valid type of data source for a CDS View Entity Data Definition. A database table from the ABAP Dictionary is a table that is defined in the ABAP Dictionary using the keyword TABLE or TABLE OF. The name of the database table must be unique within its namespace and must not contain any special characters2.
* A CDS DDIC-based view: This is also a valid type of data source for a CDS View Entity Data Definition. A CDS DDIC-based view is a view that is defined in the Core Data Services using the keyword DEFINE VIEW ENTITY. The name of the CDS DDIC-based view must be unique within its namespace and must not contain any special characters3.
You cannot do any of the following:
* An external view from the ABAP Dictionary: This is not a valid type of data source for a CDS View Entity Data Definition. An external view from the ABAP Dictionary is a view that is defined in an
* external application using any language supported by SAP, such as SQL, PL/SQL, or Java. The name of the external view must be unique within its namespace and must not contain any special characters4.
* A database view from the ABAP Dictionary: This is not a valid type of data source for a CDS View Entity Data Definition. A database view from the ABAP Dictionary is a view that is defined in an external application using any language supported by SAP, such as SQL, PL/SQL, or Java. The name of the database view must be unique within its namespace and must not contain any special characters4.
References: 1: CDS DDL – DEFINE VIEW ENTITY – ABAP Keyword Documentation – SAP Online Help 2:
ABAP Dictionary Tables – SAP Online Help 3: CDS DDL – DEFINE VIEW ENTITY – ABAP Keyword Documentation – SAP Online Help 4: ABAP Dictionary Views – SAP Online Help

The data declarations that will always work without truncation or rounding for the assignment gv_target = gv_source are B and C. This is because the target data type string is a variable-length character type that can hold any character string, including those of data types c (fixed-length character) and d (date). The assignment of a character or date value to a string variable will not cause any loss of information or precision, as the string variable will adjust its length to match the source value12.
You cannot do any of the following:
A) DATA gv_source TYPE string, to DATA gv_target TYPE c.: This data declaration may cause truncation for the assignment gv_target = gv_source. This is because the target data type c is a fixed-length character type that has a predefined length. If the source value of type string is longer than the target length of type c, the source value will be truncated on the right to fit the target length12.
D) DATA gv_source TYPE p LENGTH 8 DECIMALS 3. to DATA gv_target TYPE p LENGTH 16 DECIMALS 2.: This data declaration may cause rounding for the assignment gv_target = gv_source. This is because the target data type p is a packed decimal type that has a predefined length and number of decimal places. If the source value of type p has more decimal places than the target type p, the source value will be rounded to the target number of decimal places12.

Explanation
ABAP SQL arithmetic expressions have different restrictions depending on the data type of the operands. The following are some of the restrictions:
Floating point types and integer types can be used in the same expression, as long as the integer types are cast to floating point types using the cast function. For example, CAST ( num1 AS FLTP ) / CAST ( num2 AS FLTP ) is a valid expression, where num1 and num2 are integer types.
The operator / is allowed only in floating point expressions, where both operands have the type FLTP or f. For example, num1 / num2 is a valid expression, where num1 and num2 are floating point types. If the operator / is used in an integer expression or a decimal expression, a syntax error occurs.
Decimal types and integer types can be used in the same expression, as long as the expression is a decimal expression. A decimal expression has at least one operand with the type DEC, CURR, or QUAN or p with decimal places. For example, num1 + num2 is a valid expression, where num1 is a decimal type and num2 is an integer type.
The operator ** is allowed only in floating point expressions, where both operands have the type FLTP or f. For example, num1 ** num2 is a valid expression, where num1 and num2 are floating point types.
If the operator ** is used in an integer expression or a decimal expression, a syntax error occurs.
References: sql_exp – sql_arith – ABAP Keyword Documentation, SQL Expressions, Arithmetic Calculations – ABAP Keyword Documentation

The ABAP SQL clause that allows the use of inline declarations is the INTO clause. The INTO clause is used to specify the target variable or field symbol where the result of the SQL query is stored. The INTO clause can use inline declarations to declare the target variable or field symbol at the same position where it is used, without using a separate DATA or FIELD-SYMBOLS statement. The inline declaration is performed using the DATA or @DATA operators in the declaration expression12. For example:
* The following code snippet uses the INTO clause with an inline declaration to declare a local variable itab and store the result of the SELECT query into it:
SELECT * FROM scarr INTO TABLE @DATA (itab).
* The following code snippet uses the INTO clause with an inline declaration to declare a field symbol
<fs> and store the result of the SELECT query into it:
SELECT SINGLE * FROM scarr INTO @<fs>.
You cannot do any of the following:
* FROM: The FROM clause is used to specify the data source of the SQL query, such as a table, a view, or a join expression. The FROM clause does not allow the use of inline declarations12.
* INTO CORRESPONDING FIELDS OF: The INTO CORRESPONDING FIELDS OF clause is used to specify the target structure or table where the result of the SQL query is stored. The INTO CORRESPONDING FIELDS OF clause does not allow the use of inline declarations. The target structure or table must be declared beforehand using a DATA or FIELD-SYMBOLS statement12.
* FIELDS: The FIELDS clause is used to specify the columns or expressions that are selected from the data source of the SQL query. The FIELDS clause does not allow the use of inline declarations. The FIELDS clause must be followed by an INTO clause that specifies the target variable or field symbol where the result is stored12.
References: 1: SELECT – ABAP Keyword Documentation – SAP Online Help 2: Inline Declarations – ABAP Keyword Documentation – SAP Online Help