SQL CONSTRAINTS

In this tutorial, we will learn what is CONSTRAINT and how to use SQL.

SQL CONSTRAINTS

SQL CONSTRAINT is assigned to a column or a set of columns of the table that prevents the column from certain types of inconsistent values insertions in those columns.
SQL CONSTRAINT ensures the column to follow the specified rules. i.e. NOT NULL constraint ensures that column cannot have NULL values.
SQL CONSTRAINT can be assigned when the table is created OR after the table is created using an ALTER TABLE statement.
SQL CONSTRAINT is used to enforce data integrity. This ensures the accuracy and reliability of the data in the table.

Adding constraints with the CREATE TABLE statement

Syntax - Adding constraints with the CREATE TABLE statement

CREATE TABLE table_name (
    column1 datatype constraint,
    column2 datatype constraint,
    column3 datatype constraint,
    ....
    columnN datatype constraint,
);

The most commonly used SQL Constraint are as follow:

NOT NULL: It ensures that the column can not have a NULL value.
UNIQUE: It ensures that all the values in a column are different or unique. A table can have multiple unique constraints.
PRIMARY KEY: It is a combination of a NOT NULL and UNIQUE constraint. PRIMARY KEY constraint is used to uniquely identify each row and only one primary key constraint can be created for each table.
FOREIGN KEY: It prevents any actions that would destroy a link between tables with the corresponding data values. A FOREIGN KEY in one table points to a primary key in another table.
CHECK: It ensures that all values in a column satisfy a specific condition.
DEFAULT: It is used to set a default value for a column when NULL or no value is specified to a column.
INDEX: It is used to create and retrieve data from the database very quickly.