Alter Table Add Constraint Syntax

The SQL syntax for ALTER TABLE Add Constraint is
ALTER TABLE "table_name"
ADD [CONSTRAINT_NAME] [CONSTRAINT_TYPE] [CONSTRAINT_CONDITION]
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 add a UNIQUE constraint to the "Address" column. To do this, we type in the following:
MySQL:
ALTER TABLE customer ADD CONSTRAINT con_first UNIQUE (first_name);
Oracle:
ALTER TABLE customer ADD CONSTRAINT con_first UNIQUE (first_name);
SQL Server:
ALTER TABLE customer ADD CONSTRAINT con_first UNIQUE (first_name);
where con_first is the name of the constraint.