The OLD() function
The OLD() function is a USoft-specific extension of the SQL language that it is not part of the ANSI-SQL standard. You can use the OLD() function to refer to the previous value of a column. This is the value the column had the previous time the record was stored. In constraint SQL, OLD( column-name ) refers to the old value of the column, and column-name to the new value.
•When a record is inserted, all old column values are taken to be NULL.
•When a record is deleted, all new column values are taken to be NULL.
As an example, consider the following sequence of statements:
This table shows old and new values when you execute these 3 statements:
Old and new column values In INSERT, UPDATE, and DELETE
In a constraint, you can refer to the old values of the columns of ONE table only. This is called the "transition table".
To check that a column value does not increase by more than 10 at a time, you can write a domain constraint. In the Do Not Allow That field of the constraint, enter:
To check that a cancel date is initially (= when a reservation is inserted) left empty, it is recommended you set transition properties. But you can also express the rule simply by writing the following constraint SQL:
where res_id is the primary key column and, therefore, the last WHERE clause implies that the record is being inserted.
Because the OLD() function appears in your statement, the constraint checker automatically sets transition properties when you have the constraint checked. You are free to set these properties to different values.