Alter Table Drop Constraint Syntax

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 TABLEsection:
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. 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. con_first is the name of the constraint.
Oracle:
ALTER TABLE customer DROP CONSTRAINT con_first;
SQL Server:
ALTER TABLE customer DROP CONSTRAINT con_first;