What happens when a unique constraint is added?
If a UNIQUE constraint is added to a column that has duplicated values, the Database Engine returns an error and does not add the constraint. The Database Engine automatically creates a UNIQUE index to enforce the uniqueness requirement of the UNIQUE constraint.
When to create null function in unique constraint?
Create a function that returns NULL if the RecordStatus is not one of the values you want to consider in your constraint (and the RecordStatus otherwise) and create an index over that. That’ll have the advantage that you don’t have to explicitly examine other rows in the table in your constraint, which could cause you performance issues.
How to use conditional unique constraint in SQL Server?
For earlier versions of SQL Server, please see this answer. You could move the deleted records to a table that lacks the constraint, and perhaps use a view with UNION of the two tables to preserve the appearance of a single table. You can do this in a really hacky way… Create an schemabound view on your table.
Can you create unique constraint for recordstatus column?
Plenty of gotchas because of that. Because, you are going to allow duplicates, a unique constraint will not work. You can create a check constraint for RecordStatus column and a stored procedure for INSERT that checks the existing active records before inserting duplicate IDs.
How are unique constraints referenced in SQL Server?
A UNIQUE constraint can be referenced by a FOREIGN KEY constraint. When a UNIQUE constraint is added to an existing column or columns in the table, by default, the Database Engine examines the existing data in the columns to make sure all values are unique.
Can a check constraint be applied to multiple columns?
You can also apply a single CHECK constraint to multiple columns by creating it at the table level. For example, a multiple-column CHECK constraint could be used to confirm that any row with a country_region column value of USA also has a two-character value in the state column. This allows for multiple conditions to be checked in one location.
Can a constraint trigger be created in PostgreSQL?
Finally, PostgreSQL has the option to create “constraint triggers” with CREATE CONSTRAINT TRIGGER. It sounds like such triggers could be used to avoid the race condition. Constraint triggers respect the MVCC rules, so they cannot “peek” at uncommitted rows of concurrent transactions.
How to drop a unique constraint in MySQL?
To drop a UNIQUE constraint, you use can use DROP INDEX or ALTER TABLE statement: For example, the following statement drops the uc_name_address constraint on the suppliers table:
When to use check constraint in ALTER TABLE?
The CHECK constraint being added specifies that there must be at least one row in table CheckTbl. However, because there are no rows in the table against which to check the condition of this constraint, the ALTER TABLE statement succeeds.