Data validation is often a topic of great importance when it comes to databases.
Since information is constantly being updated, deleted, queried, or moved around, having valid data is a must.
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However, in SQL, the not null constraint can only be placed on a single column.
Finally, the primary key constraint is a mixture of the unique constraint and the not null constraint meaning the no two rows can have the same values in their columns and that a column must have data.
Referential integrity is a key aspect in data integrity that is usually associated with two tables; the lookup table and the data table.
Typically, referential integrity is applied when data is inserted, deleted, or updated.
The main types of constraints in SQL are check, unique, not null, and primary constraints.
Check constraints are used to make certain that a statement about the data is true for all rows in a table.
The unique constraint ensures that no two rows have the same values in their columns.
The not null constraint is placed on a column and states that data is required in that column.
The inserts and updates to the data table prevented by referential integrity happen in the foreign key column.
Referential integrity will prevent inputting data in the foreign key column that is not listed in the lookup table.
However, the inserts and updates allowed by referential integrity occur when the data inserted is located in the lookup table.