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SQL Constraint List and Syntax

Reading time 1 min
Published Aug 9, 2017
Updated Oct 11, 2019

SQL Constraint: Main Tips

  • An SQL constraint defines a rule to the data records in tables.
  • When an infraction against the data record action and the constraint occurs that action is stopped by it.
  • They can be defined after or when creating the table.
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Constraint Syntax and Quick Reference

CREATE TABLE table_name (
    column1 datatype constraint_one,
    column2 datatype constraint_two,
    column3 datatype constraint_three,

The main constraints in SQL are:

  • NOT NULL - there is no NULL value contained in a column.
  • UNIQUE - every row in a specific column needs a unique value.
  • PRIMARY KEY - a combination of UNIQUE and NOT NULL: columns must have an identity which is unique. This helps when you need to locate a specific data record faster.
  • FOREIGN KEY - one table's SQL referential integrity matches another table's.
  • CHECK - a defined condition is met within the value.
  • DEFAULT - stock column value.

SQL Constraint: Summary

  • You can define a constraint when you create a table, or afterrwards.
  • Constraints stop data record actions with data that breaks the constraint rule.