The Versatile Check Constraint
If constraints can be considered “cool,” then the check constraint[85] has to be the coolest. This is a flexible type of constraint you’ll use in multiple ways.
Any condition that you can express using SQL that evaluates to a Boolean result can be written for a check constraint. Create a Boolean condition rule that’s checked any time data is inserted, updated, or deleted.
Like other types you’ve seen so far, check constraints are added to a table.
For Rails developers, you’ll want to know that support for check constraints was added to Active Record in version 6.1,[86] and support for if_exists was added later.[87] This means helper methods can be used from migrations to create check constraints.
Check constraints ...
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