What is Database Constraint?

This article describes about the database constraint in SQL. Summary of the article:

• What is Database Constraint?
• Types of Database constraints
• How to make a Great Primary Key?
• Primary Key Constraint
• Foreign Key Constraint
• Unique Key Constraint
• Not null Constraint
• Check Constraint

What is Database Constraint?
Database constraint is user-defined structures that express the behaviors of column. It is used to protect wrong entry.

Types of Database constraints
There are five types of database constraints. Such:

1. Primary Key Constraint
2. Foreign Key Constraint
3. Unique Key Constraint
4. Not null Constraint
5. Check Constraint

Primary Key Constraint
A column value will be unique among all the rows in a table. It does not allow any null value. Therefore, a primary key constraint has the behaviors of both Not Null and Unique Key Constraints.

Foreign Key Constraint
We can’t insert a foreign key value which is not present in the primary key column of that foreign key.

Unique Key Constraint
A column value will be unique among all the rows in a table. It allows only a single Null value in that column. Depending on the design, a table may have more than one unique key constraint.

Not null Constraint
It makes a column mandatory. This means you can’t insert a row in the table without providing a valid data.

Check Constraint
Check Constraints are table-level constraint. It doesn’t make the column mandatory.

That’s all about the constraint of a database.