Quick Reference
The CONSTRAINT keyword is used to create or delete a constraint from an existing table.
ADD CONSTRAINT
The following adds a constraint named “key_customer” that is a PRIMARY KEY constraint on multiple columns (id and last_name).
ALTER TABLE Customers
ADD CONSTRAINT key_customer PRIMARY KEY (id, last_name);DROP a DEFAULT CONSTRAINT (on a column)
MySQL:
ALTER TABLE Customers
ALTER city DROP DEFAULT;SQL Server:
ALTER TABLE Persons
ALTER COLUMN city DROP DEFAULT;DROP a UNIQUE CONSTRAINT
MySQL:
ALTER TABLE Customers
DROP INDEX uc_customer;SQL Server:
ALTER TABLE Customers
DROP CONSTRAINT uc_customer;DROP a PRIMARY KEY CONSTRAINT
MySQL:
ALTER TABLE Customers
DROP PRIMARY KEY;SQL Server:
ALTER TABLE Customers
DROP CONSTRAINT pk_customer;DROP a FOREIGN KEY CONSTRAINT
MySQL:
ALTER TABLE Customers
DROP FOREIGN KEY fk_customer;SQL Server:
ALTER TABLE Customers
DROP CONSTRAINT fk_customer;DROP a CHECK CONSTRAINT
MySQL:
ALTER TABLE Customers
DROP CHECK chk_customer;SQL Server:
ALTER TABLE Customers
DROP CONSTRAINT chk_customer;SQL Notes:
- Any work being done to modify the structure of a database or delete tables or the the database itself should only be done after making a recent backup
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