SQL constraints are rules used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database. If there is any violation between the constraint and the data action, the action is aborted.
Constraints can be specified when the table is created with the CREATE TABLE statement, or after the table is created with the ALTER TABLE statement.
The following are some of the most commonly used constraints in SQL:
NOT NULL - Ensures that a column cannot have a NULL value.UNIQUE - Ensures that all values in a column are different.PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Uniquely identifies each record in a table.FOREIGN KEY - Uniquely identifies a record in another table, creating a link between the two tables.CHECK - Ensures that all values in a column satisfy a specific condition.DEFAULT - Sets a default value for a column when no value is specified.INDEX - Used to create and retrieve data from the database very quickly.We will cover each of these in detail in the following chapters.