Databases & Data Storage
Constraint
Definition
In SQL, a constraint is a rule that is applied to a column or a table to limit the type of data that can go into it. This ensures the accuracy and reliability of the data in the database.
Why It Matters
Constraints are the primary mechanism for enforcing data integrity at the database level. They prevent invalid or corrupt data from ever being saved.
Contextual Example
A `CHECK` constraint could be used on a `ProductPrice` column to ensure that the price is always greater than zero. The database would reject any attempt to insert or update a product with a negative price.
Common Misunderstandings
- Common SQL constraints include `NOT NULL`, `UNIQUE`, `PRIMARY KEY`, `FOREIGN KEY`, and `CHECK`.
- If you try to perform an action that violates a constraint, the database will return an error.