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ABAP CDS - SELECT, association
Defines an association of the name _assoc in a SELECT statement of a CDS view. An association associates the current CDS view as a source data source with the target data source target specified in the definition of the association using an ON condition cond_exp . A data source target can be a database table defined in ABAP Dictionary, a classic view, an external view, or a CDS entity.
An association of a SELECT statement in a CDS view can be accessed as follows:
When a CDS view is activated with path expressions, every association specified here is transformed to a join expression. The source data source represents the left side and the target data source represents the right side. The ON condition of the association is added to the ON condition of the join. By default, the category of the join is determined by where the path expression is used:
This setting can be overwritten when specifying the association in a path expression using an attribute.
When specifying the ON condition, the following special rules apply:
Defines the cardinality of the target data source of a CDS view, which is defined with an association ASSOCIATION. The square brackets [ ] are part of the syntax. For min and max, positive integers (including 0) and asterisks (*) can be specified:
If the cardinality is not defined explicitly, the cardinality "to 1" is used implicitly ([min..1]).
When specified, the cardinality is used mainly to document the semantics of the data model. The cardinality is not validated at runtime but can produce syntax check warnings.
The specified cardinality is evaluated by the syntax check for paths specified in the CDS DDL of CDS
or in Open SQL. Currently, a warning is produced if a value greater than 1 is specified for max,
indicating that a path specified in this way influences the cardinality of the results set.
... AS _assoc
Defines the name _assoc of an association defined using ASSOCIATION of a CDS view. If no name is defined explicitly using AS, _assoc is set implicitly to the name of the target data source. The name _assoc must comply with the naming rules for names.
It is advisable to use an underscore _ as the first character of the association name.
Example of a simple association. The following CDS view provides the same result as the CDS view DEMO_CDS_SCARR_SPFLI in the joins example, as shown in the program DEMO_CDS_ASSOCIATION using an assertion. Furthermore, the association spfli_scarr is published to be used from outside in the SELECT list by specifying a path that contains only the name of an association. The program DEMO_CDS_ASSOCIATION also shows how the association can be accessed by specifying a path in Open SQL.
The following CDS view sales_order_invoice_header returns information about sales invoices and works with the following databases: snwd_so_inv_head, snwd_so, snwd_bpa, snwd_so_inv_item.
Two associations are defined:
The source data source fields used in the ON conditions - node_key and buyer_guid - are part of the SELECT list. Here the recommended prefix $projection is used instead of the prefixes snwd_so_inv_head or snwd_so_inv_head.
The association _buyer is not listed in the SELECT list and can only be used in path expressions of the current SELECT statement. This association can be specified in the WHERE condition due to the cardinality [1..1]. The association _invoice_items is not accessed in path expressions of the current SELECT statement. However, this association is listed in the SELECT list, which means it can be used in path expressions of other CDS views. This association cannot be specified in a WHERE condition due to the cardinality [1..*].
The CDS view can be accessed in an ABAP program with a simple SELECT statement (Open SQL).
The complexity of the actual query is wrapped transparently in the CDS view for the application programmer. When the view is accessed, the join (defined by the association _invoice_items) between snwd_so_inv_head and snwd_so_inv_item is not built, because there are no path expressions that need to access the join.
The CDS view sales_order_invoice_header mentioned above is used as the data source in the definition of the CDS view sales_order_invoice_items. This data source is used to access the published association _invoice_items. The elements of the association are accessed in this view. There is no visual indication that it is the result of a join. This join between snwd_so_inv_head and snwd_so_inv_item is created when the CDS view sales_order_invoice_items is activated. The other association _buyer of the CDS view sales_order_invoice_header cannot be accessed.
... WITH DEFAULT FILTER cond_exp
Defines a standard filter condition for a path expression.
When the syntax check evaluates a cardinality specified using [min..max], the default filter condition is respected alongside the ON condition.