SQL constraints are used to specify rules for the data in a table. These are used to limit the type of data in a table. Constraints ensure the accuracy and reliability of the data in the database.
SQL constraints are
NOT NULL : It make sure that a column cannot store NULL value
e.g CREATE table CUSTOMER (customer_id int NOT NULL , Name varchar(60));
UNIQUE : It make sure that each row for a column must have unique value
e.g CREATE table CUSTOMER (customer_id int NOT NULL UNIQUE, Name varchar(60));
PRIMARY KEY : Primary key is one of the important constraints.It is make sure that each row of a column is unique and not null. Primary key uniquely identifies each record in a database.
e.g CREATE table CUSTOMER (customer_id int NOT NULL PRIMARY KEY, Name varchar(60));
FOREIGN KEY: Foreign key is used to relate two tables, A foreign key is a field in a table that matches another field(primary key) of another table.
e.g CREATE table Order_info(order_id int PRIMARY KEY,product_name varchar(60) NOT NULL,
c_id int FOREIGN KEY REFERENCES CUSTOMER (customer_id));
CHECK : CHECK is used to restrict the value of a column between a given range.
e.g CREATE table CUSTOMER (customer_id int NOT NULL CHECK(s_id > 100)
, Name varchar(60));