CDS provides extended support for treating relationships between entities. Such relationships are often reflected as foreign key relationships in the database, but they are not treated as first-class citizens in SQL , especially in DML. Therefore, using them to query data from several tables often requires writing non-trivial join conditions.

CDS treat relationships, called associations, as first-class citizens to simply reading relationships between entities, using them and defining CDS views for reuse, as follows: -

1. Associations can be defined within CDS view using the ASSOCIATION TO syntax, followed by the target entity and an ON condition specifying how entities are related.
2. Specifying an alias for the target entity and a cardinality is optional.
3. Path expressions can be used to navigate along with associations.

Exposed associations are helpful for the following reasons:

• The Data preview in the ADT supports navigation along with the association
• Higher-Level CDS views defined based on the exposing view can use any column of the associated entity. This can simplify defining views on top of the other views. When querying from such a higher-level CDS View, the corresponding JOIN is only executed on the database if the higher-level view uses the association in a path expression. This can be considered to JOIN on Demand.

Open SQL supports simple path expressions to allow consuming associations in CDS views. Only simple path expressions without filter expressions are supported so far, but they are supported in all clauses of the SELECT statement, such as the field list, WHERE clause, ORDER BY clause and so on.

Association's names must be prefixed with a backslash. Deeper chains of associations and paths are also supported.

The filter expression is a conditional expression with the following constraints:

• The left-hand side of each condition must be a field of the target of the association.
• The right-hand side of each condition must be a field of the target of the association, a literal, or a parameter.
• No path or other expressions or functions can be used.