In this blog, I will explain SQL Constraints. They are used to specify rules for the data in a table. The following SQL Constraints are commonly used (Primary key, foreign key, unique key, Composite key, null, Not Null, check).

Primary Key
The Primary key must contain be a unique value. It is the primary column and can’t have the null value. It uniquely identifies each row in a database table.
 
Syntax
    CREATE TABLE TB_NAME(Column1 datatype,column2 datatype PRIMARY KEY(Column1)) 

Foreign Key
1. Foreign key always refers to the primary key column.
2. Foreign key accepted to duplicate value.
 
Syntax
    CREATE TABLE TB_NAME(column1 datatype FOREIGN KEY REFERENCES(primary_key column_name),cloumn2 datatype) 

Unique Key
 
The unique key is the same as the primary key, but one row is accepted for the null value.
 
Syntax
    CREATE TABLE TB_NAME(Column_name datatatype UNIQUE,column_name2 datatype) 

Composite key
A composite key is a set of multiple keys that, together, uniquely identifies each record
 
Syntax
    CREATE TABLE TB_NAME(Column1 datatype,column2 datatype PRIMARY KEY(Column1,column2)) 

Not Null
Forces a column not to accept NULL values
 
Syntax
    CREATE TABLE TB_NAME(Column1 datatype,column2 datatype NOT NULL) 

Check
The CHECK constraint is used to limit the value range that can be placed in a column.
 
Syntax
    CREATE TABLE TB_NAME(MARKS INT CHECK(MARKS<=100))