SQL Programming
  • Introduction to SQL
  • Data Definition Language
    • DATA TYPES
    • CREATE DATABASE
    • DROP DATABASE
    • CREATE TABLE Statement
    • CONSTRAINTS
      • SQL PRIMARY KEY Constraint
      • SQL NOT NULL Constraint
      • SQL UNIQUE Constraint
      • SQL FOREIGN KEY Constraint
      • SQL DEFAULT Constraint
      • SQL CHECK Constraint
    • DROP & TRUNCATE TABLE Statement
    • ALTER TABLE Statement
    • AUTO INCREMENT Field
    • SQL SELECT INTO Statement
    • CREATE INDEX
    • DDL Practice Problems
  • Data Manipulation Language
    • INSERT INTO Statement
    • INSERT INTO SELECT Statement
    • DELETE Statement
    • UPDATE Statement
    • DML Practice Problems
  • SQL NULL Values
  • SQL Comments
  • Data Query Language
    • SQL SELECT DISTINCT Statement
    • SQL WHERE Clause
      • SQL And, Or, Not (Logical Operators)
      • SQL Wildcards
      • SQL LIKE Operator
      • SQL IN Operator
      • SQL BETWEEN Operator
      • SQL WHERE Exercises
    • SQL TOP CLAUSE
    • SQL Order By
    • SQL Server Scalar Functions (Non-aggregate)
      • SQL MIN() and MAX()
      • SQL COUNT(), AVG() and SUM() Functions
      • Functions Problems
    • SQL ISNULL Functions
    • SQL Aliases
    • SQL CASE Statement
    • SQL SET Operator
    • SQL GROUP BY Statement
    • SQL HAVING Clause
    • Analytical Function
    • SQL Joins
      • SQL INNER JOIN Keyword
      • SQL LEFT JOIN Keyword
      • SQL RIGHT JOIN Keyword
      • SQL FULL OUTER JOIN Keyword
      • SQL Self JOIN
      • SQL CROSS JOIN
    • SQL Subqueries
    • SQL EXISTS Operator
    • SQL ANY and ALL Operators
  • CREATE VIEWS
  • Common Table Expressions (CTE)
  • Data Control Language
  • SQL Stored Procedures for SQL Server
  • Practice Papers
  • SQL PROJECT
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  1. Data Definition Language

CONSTRAINTS

SQL constraints are used to specify rules for data in a table.

SQL Create Constraints

Constraints can be specified when the table is created with the CREATE TABLE statement, or after the table is created with the ALTER TABLE statement.

Syntax

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

SQL Constraints

SQL constraints are used to specify rules for the data in a table.

Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.

Constraints can be column level or table level. Column level constraints apply to a column, and 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 a NOT NULL and UNIQUE. Uniquely identifies each row in a table

  • FOREIGN KEY - Uniquely identifies a row/record in another table

  • CHECK - Ensures that all values in a column satisfies a specific condition

  • DEFAULT - Sets a default value for a column when no value is specified

  • INDEX - Used to create and retrieve data from the database very quickly

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Last updated 6 years ago

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