What are database constraints? [closed]

Answer 1

1. UNIQUE constraint (of which a PRIMARY KEY constraint is a variant). Checks that all values of a given field are unique across the table. This is X-axis constraint (records)

2. CHECK constraint (of which a NOT NULL constraint is a variant). Checks that a certain condition holds for the expression over the fields of the same record. This is Y-axis constraint (fields)

3. FOREIGN KEY constraint. Checks that a field's value is found among the values of a field in another table. This is Z-axis constraint (tables).