SQL Constraints are rules enforced on data columns on a table to maintain data integrity and accuracy. These constraints are used to specify the limit of data that can be entered into the table columns. They ensure the data entered into the table is consistent and follows a set of rules.
There are several types of constraints that can be used in SQL, including NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, DEFAULT
The NOT NULL constraint ensures that a column cannot have a NULL value. The UNIQUE constraint ensures that the data in a column is unique and no duplicate values are allowed. The PRIMARY KEY constraint is used to enforce a unique identifier for each record in a table and it cannot contain NULL values. The FOREIGN KEY constraint is used to establish a relationship between two tables. The CHECK constraint is used to limit the data that can be entered into a column based on a specific condition. The DEFAULT constraint is used to set a default value for a column.
By using constraints in SQL, you can maintain the integrity and accuracy of data in a database. Constraints are specified during table creation or can be added later using the ALTER TABLE statement.