SQL
Check Constraint
Check constraints allow you to specify an arbitrary Boolean expression. Any columns that *do not* satisfy this expression violate the constraint. For example, we could enforce that the `name` column does not contain spaces using the following `CHECK` constraint.  
```sql
CREATE TABLE students (name VARCHAR CHECK (NOT contains(name, ' ')));
INSERT INTO students VALUES ('this name contains spaces');
```  
```console
Constraint Error: CHECK constraint failed: students
```