SQL Drop Constraint Syntax

Constraint can be placed on a table to limit the type of data that can go into a table. Since we can specify constraints on a table, there needs to be a way to remove this constraint as well. In SQL, this is done via the ALTER TABLE statement.

The SQL syntax for ALTER TABLE Drop Constraint is

ALTER TABLE "table_name"
DROP [CONSTRAINT|INDEX] "CONSTRAINT_NAME"

Let's look at the example. Assuming our starting point is the "customer" table created in the CREATE TABLE section:

Table customer

Column Name	Data Type
First_Name	char(50)
Last_Name	char(50)
Address	char(50)
City	char(50)
Country	char(25)
Birth_Date	date

Assume we want to drop the UNIQUE constraint on the "Address" column, and the name of the constraint is "con_first." To do this, we type in the following:

MySQL:
ALTER TABLE customer DROP INDEX con_first;

Note that MySQL uses DROP INDEX for index-type constraints such as UNIQUE.

Oracle:
ALTER TABLE customer DROP CONSTRAINT con_first;

SQL Server:
ALTER TABLE customer DROP CONSTRAINT con_first;