Get Started: SQL Constraints

In This Article, You Will Know About SQL Constraints. 

Before moving ahead, let’s know a bit about the SQL ALTER TABLE Statement.

Table of Contents

SQL Constraints

The SQL Constraints are used to assign a specified rule on the data in a table.

SQL Create Constraints

There are mainly two ways to implement SQL Constraints –

  • When the table is created with the CREATE TABLE statement
  • After creating the table with the ALTER TABLE statement

Syntax

				
					CREATE TABLE table_name (
    Column_name1 column_datatype constraint,
    Column_name2 column_datatype constraint,
    Column_name3 column_datatype constraint,
    ....
);
				
			

Different SQL Constraints

The SQL Constraints are used to specify the rules on the data.

The Constraints represent the maximum number of data types a table can have. It helps to make data more readable as well as accurate to understand. If any rule doesn’t match with Constraints and data action, then action will be canceled.

Constraints can have two types of levels –

  1. Column level constraints apply to a column.
  2. Table level constraints apply to the whole table.

The following constraints are commonly used in SQL:

NOT NULL – Ensures that a column cannot have a NULL value

UNIQUE – Ensures that all values in a column are different

PRIMARY KEY – A combination of NOT NULL and UNIQUE. Uniquely identifies each row in a table

FOREIGN KEY – Prevents actions that would destroy links between tables

CHECK – Ensures that the values in a column satisfy a specific condition

DEFAULT – Sets a default value for a column if no value is specified

CREATE INDEX – Used to create and retrieve data from the database very quickly.

If you find anything incorrect in the above-discussed topic and have further questions, please comment below.

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