Add Constraint SQL - sql - sql tutorial - learn sql

SQL Constraints are rules, which is used to limit the type of data that can go into a table.
To maintain the accuracy & integrity of the data within the table.
Constraints are used to make sure that the integrity of data is maintained in the database.
Constraints can be divided into two types as follows,

Column level constraints :

limits only column data

Table level constraints :

limits whole table data
Following are the most used constraints that can be applied to a table.

Not null
Unique
Primary key
Foreign key
Check
Default

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SQL > ALTER TABLE > Add Constraint Syntax

Sometimes we may decide to add a new constraint to an existing table (to see what are the different types of constraints that can be located on a database table, ref. to the CONSTRAINT section).

The syntax for adding a constraint in SQL is,

ALTER TABLE "table_name"
ADD [CONSTRAINT_NAME]
[CONSTRAINT_TYPE][CONSTRAINT_CONDITION];

For example, assuming our initial 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 add a UNIQUE constraint to the "Address" column. To do this, we type in the following:

MySQL:

ALTER TABLE Employee ADD CONSTRAINT Wikitechy UNIQUE (Address);

Oracle:

ALTER TABLE Employee ADD CONSTRAINT Wikitechy UNIQUE (Address);

SQL Server:

ALTER TABLE Employee ADD CONSTRAINT Wikitechy UNIQUE (Address);

where Wikitechy is the name of the constraint.

Alter Table Add Constraint

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