What are different Integrity Constraints?

Integrity constraints are rules that help to ensure the quality of information in a database. They enforce the business rules associated with a database and prevent the entry of invalid information into tables. Here are the different types of integrity constraints:

1. Domain Constraints: These are the most basic form of constraints. They define the valid set of values that a column or attribute can have in a database table. For example, a column storing age values might have a domain constraint specifying that the age must be a positive integer.

2. Entity Integrity Constraints: These ensure that the primary key of a table cannot have NULL values. This is because the primary key is used to identify individual records in a table, and if it were NULL, it would not be possible to identify records uniquely.

3. Referential Integrity Constraints: These ensure that the relationship between two tables remains consistent. More specifically, this means that any foreign key value can only be in a column if that value exists in the corresponding primary key of another table.

4. Key Constraints: These ensure that a column (or set of columns) has a unique value for each row of a table. The most common type of key constraint is the primary key, but there can also be unique constraints that require a column's value to be unique but do not identify the row.

5. Check Constraints: These allow you to specify a condition on a column that the data entered into it must meet. For example, you might have a check constraint on an 'age' column specifying that the age must be less than 150.

6. Not NULL Constraints: These ensure that a column cannot have a NULL value. This is different from an entity integrity constraint, which applies only to primary key columns. A not NULL constraint can be applied to any column, whether it is part of the primary key or not.