☞ They are some set of rules created to apply data validations.
☞ They ensure accuracy, correctness and reliability of data.
☞ It is not mandatory to define constraints for each attribute of a table.
☞ SQL provides the following constraints :
1. PRIMARY KEY 2. NOT NULL 3. UNIQUE 4. DEFAULT 5. FOREIGN KEY
☞ The field which uniquely identifies each record in a table.
☞ It ensures that the column cannot have NULL values.
☞ NULL means missing/unknown/not applicable values.
☞ It ensures that all the values in a column are distinct/unique.
☞ It allows to assign a default value if the value is not provided or given.
☞ The column which refers to value of an attribute defined as the primary key in another table.