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.

Pros

• Easy to use with a learn-by-doing approach
• Offers quality content
• Gamified in-browser coding experience
• The price matches the quality
• Suitable for learners ranging from beginner to advanced

Main Features

• Free certificates of completion
• Focused on data science skills
• Flexible learning timetable

GET 25% OFF

Pros

• Simplistic design (no unnecessary information)
• High-quality courses (even the free ones)
• Variety of features

Main Features

• Nanodegree programs
• Suitable for enterprises
• Paid Certificates of completion

UP TO 70% OFF

Pros

• A wide range of learning programs
• University-level courses
• Easy to navigate
• Verified certificates
• Free learning track available

Main Features

• University-level courses
• Suitable for enterprises
• Verified certificates of completion

FREE COURSES

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.