Constraints in sql | Sql constraint | Drop constraint in sql - sql - sql tutorial - learn sql

• Constraints 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.
• Drop of constrain can remove primary, foreign and other keys from the column
• SQL > ALTER TABLE > Drop Constraint Syntax
• The SQL syntax to remove a constraint from a table is,

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

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

Table Employee

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

• 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 Employee DROP INDEX Con_First;

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

Oracle:

ALTER TABLE Employee DROP CONSTRAINT Con_First; 

SQL Server:

ALTER TABLE Employee DROP CONSTRAINT Con_First;

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