define view ZI_SalesOrder as select from ZSalesOrderHeader
{
    key SalesOrderID,
    CustomerID,

    association [0..1] to ZI_Customer as _Customer 
        on _Customer.CustomerID = ZSalesOrderHeader.CustomerID

    // Exposing the association
    _Customer
} 

Key Points:

• You can reuse exposed associations in other views.
• Useful in modular CDS design.
• Can be traversed via path expressions (e.g., _Customer.Name).

Ad-hoc Associations

Ad-hoc associations are associations that are used inline (on-the-fly) via path expressions, without needing to expose the association explicitly in the SELECT list. If an association (like _Customer) is already defined in a view (exposed or implicit), you can use it ad-hoc in the SELECT or WHERE clause without re-exposing it.

define view ZI_SalesOrder as select from ZSalesOrderHeader
{
    key SalesOrderID,
    _Customer.Name    // Ad-hoc usage of the association
}
 

define view ZSalesOrder as select from sales_order
association [0..1] to customer as _Customer on $projection.customer_id = _Customer.id
{
key sales_order_id,
customer_id,
_Customer.name
}

Here, _Customer is an association. It won’t trigger a join unless _Customer.name is accessed.

define view ZSalesOrderJoin as select from sales_order
inner join customer on sales_order.customer_id = customer.id
{
sales_order.id,
customer.name
}

When to Use What?

Use Association:

• When modeling data relationships.
• When you want flexibility and better performance for optional fields.
• When the associated data isn’t always needed.

Use Join:

• When you always need data from both sources.
• When performing aggregations or filters based on joined fields.
• When doing complex multi-joins with specific types (e.g. inner join).