MySQL Constraints Explained: Types & Usage

In MySQL, a constraint is a mechanism used to enforce rules in a table. It ensures that the data in the table meets specific conditions, thereby providing data integrity and validity.

Constraints can be defined when creating a table or added, modified, or removed in an existing table.

The common types of constraints include the following:

1. Primary key is used to uniquely identify each row of data in a table. A table can have only one primary key, and the values of the primary key cannot be duplicated.
2. FOREIGN KEY: Used to establish relationships between tables. It links corresponding columns in two tables, ensuring data consistency between the related tables.
3. UNIQUE Constraint: Ensures that the values in a column of a table are unique and does not allow duplicates.
4. NOT NULL constraint: Ensures that the value of a column in a table cannot be empty.
5. Check constraint is used to restrict values in a column to meet specific conditions, which could be a range, data type, etc.
6. DEFAULT constraint is used to set a default value for a column. If a value is not provided for that column during data insertion, the default value will be used.

The use of constraints can effectively ensure the integrity and consistency of data, preventing data that does not comply with business rules from being inserted into a table.