A CHECK condition can be applied as a constraint on the table, instead of on a single column. Such CHECK conditions typically specify that two values in a row being entered or modified have a proper relation to each other. Column CHECK conditions are held individually in the system tables, and can be replaced or deleted individually. This is more flexible behavior, and CHECK conditions on individual columns are recommended where possible.
For example, in a library database, the date_returned column for a particular entry must be later than (or the same as) the date_borrowed entry:
ALTER TABLE loan ADD CHECK(date_returned >= date_borrowed)
