Understand the different types of constraints used to maintain data integrity.
Integrity Constraints are a set of rules used to maintain the quality and consistency of information in a database. They prevent invalid or corrupt data from being entered, ensuring the accuracy and reliability of the data. There are several types of integrity constraints. Domain Constraints specify the set of valid values for a particular attribute. This can include data type (e.g., INT, VARCHAR), CHECK constraints (e.g., age > 18), and NOT NULL constraints. Entity Integrity Constraint states that the primary key of a table can never be NULL. Since the primary key is used to uniquely identify each record, it must have a value. Referential Integrity Constraint, enforced by foreign keys, ensures that a foreign key value either matches an existing primary key value in the parent table or is NULL. This prevents 'dangling records' – for example, an employee record pointing to a non-existent department. Key Constraints ensure that every table has a primary key and that its values are unique. In addition to these, there are other constraints like UNIQUE, which ensures that all values in a column are distinct (similar to a primary key, but allows one NULL value), and DEFAULT, which provides a default value for a column when none is specified. By properly defining these constraints at the time of table creation, we can delegate the responsibility of data validation to the DBMS itself, leading to a more robust and reliable system.