Expressions are used within constraints to generate a value that is true.
There are three types of expressions usable for constraints:
- Comparisons with operators
A comparison expression consists of two attributes or values, separated by a comparison operator, like ‘=’, ‘<=’ and ‘>’.
//Sales.Customer[Name = 'Jansen']
This query retrieves all customers whose name is ‘Jansen’.
//Sales.Order[TotalPrice < 50.00]
This query retrieves all orders for which the total price is less than 50.00 euros.
//Sales.Customer[Sales.Customer_Order/Sales.Order/HasPayed = false()]
This query retrieves all customers who have at least one unpaid order.
//Sales.Customer[Name = City]
This query retrieves all customers who have the same name as the city they live in.
//Sales.Customer[Sales.Customer_Order = 'ID_124123512341']
This query retrieves the customer who placed the order with the given unique identification number.
The same result can be retrieved by doing the following query:
//Sales.Customer[Sales.Customer_Order/Sales.Order/ID = 'ID_124123512341']
However, it is strongly recommended not to use this notation. This is because its execution is inefficient and results in a lower performance due to manner in which it is processed by the database.
See this page for information on the available functions.
The last type of expression, the exist expression, can be used to check whether a specific association is filled or not.
This query retrieves all customers who have placed at least one order.
This query retrieves all customers who have not placed any orders.