Violating Constraints

Query :

• UPDATE emp SET department_id = 29 WHERE department_id = 30;

Error :

• insert or update on table "emp" violates foreign key constraint "emp_ref_dept_fk"

DETAIL :

• Key (deptno)=(29) is not present in table "dept"
• Department 29 does not exist

You cannot delete a row that contains a primary key that is used as a foreign key in another table.

Query :

• DELETE FROM departments WHERE department_id = 60;

Error :

• update or delete on table "dept" violates foreign key constraint "emp_re f_dept_fk" on table "emp";

Detail :

• Key (deptno)=(10) is still referenced from table "emp".
  

CHECK Constraint

CHECK Constraint :

Defines a condition that each row must satisfy

• The following expressions are not allowed :
• References to CURR_VAL, NEXT_VAL and CTID pseudocolumns
• Queries that refer to other values in other rows
• ..., salary numeric(2)CONSTRAINT emp_salary_minCHECK (salary > 0),...

CREATE TABLE : Example