In database terms, a CHECK constraint is a type of constraint that checks data before it enters the database.
CHECK constraints help maintain data integrity, because they prevent invalid data entering the database.
Continue readingTag: constraints
Create a CHECK Constraint in SQLite
In SQLite, you can create a CHECK constraint by adding the applicable code within the CREATE TABLE statement when creating the table.
If a table has a CHECK constraint on it, and you try to insert or update data that violates the CHECK constraint, the operation will fail with an error.
Return All Disabled Constraints in SQL Server (T-SQL Example)
Here’s some T-SQL code you can use to get a list of all CHECK and foreign key constraints in a SQL Server database.
See if a Table has a DEFAULT Constraint in SQL Server using OBJECTPROPERTY()
You can use the OBJECTPROPERTY() function in SQL Server to see whether or not a table has a DEFAULT constraint.
To do this, pass the table’s object ID as the first argument, and TableHasDefaultCnst as the second argument. The function returns a 1 or a 0 depending on whether or not it has a DEFAULT constraint.
A return value of 1 means that the table has a DEFAULT constraint, and a value of 0 means that it’s not.
Use OBJECTPROPERTY() to Find Out if an Object is a CHECK Constraint in SQL Server
In SQL Server you can use the OBJECTPROPERTY() function to find out whether or not an object is a CHECK constraint.
This function accepts two parameters: the object ID, and the property for which you’re checking it for.
Therefore, you can pass the object ID as the first argument, and IsCheckCnst as the second, and the function will return either a 1 or a 0 depending on whether or not it’s a CHECK constraint.
A return value of 1 means that it is a CHECK constraint, and a value of 0 means that it’s not.
Return All Foreign Keys & CHECK Constraints in a SQL Server Database (T-SQL Examples)
In SQL Server, you can use Transact-SQL to return a list of all foreign keys and CHECK constraints for the current database.
The examples on this page query two system views in order to retrieve this information: sys.foreign_keys and sys.check_constraints. You can query each one separately, or use UNION to display them all in a single result set.
Modify a CHECK Constraint in SQL Server using T-SQL
If you already have an existing CHECK constraint in SQL Server, but you need to modify it, you’ll need to drop it and recreate it. There’s no ALTER CONSTRAINT statement or anything similar.
So to “modify” an existing constraint:
- Drop the constraint using
ALTER TABLEwithDROP CONSTRAINT. - Create the new constraint using
ALTER TABLEwithADD CONSTRAINT.
Add a CHECK Constraint to an Existing Table in SQL Server (T-SQL)
This article demonstrates how to add a CHECK constraint to an existing table.
You can add a constraint to an existing table by using the ALTER TABLE statement along with the ADD CONSTRAINT argument. Examples below.
Rename a CHECK Constraint in SQL Server using T-SQL
You can use the sp_rename system stored procedure to rename a CHECK constraint in SQL Server.
The purpose of this stored procedure is to allow you to rename user-created objects in the current database. So you can also use it to rename other objects such as tables, columns, alias data types, etc.
How to Restore Trust in a Foreign Key Constraint in SQL Server (T-SQL Examples)
In SQL Server, a foreign key constraint (and a CHECK constraint) can be either trusted or not trusted.
When a constraint is trusted, this means that the constraint has been verified by the system. When it’s not trusted, the constraint has not been verified by the system.
Basically, when you have an untrusted constraint, you could also have invalid data in your database. By this I mean you could have data that violates the constraint.
This means that you’re no longer maintaining referential integrity within your relationships, which is not normally good practice when looking after a relational database in production.
In this article I’ll check my existing constraints for their “trustworthiness”, and then I’ll update them to become trustworthy once again.