SQL Constraint List and Syntax

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.

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.