PostgreSQL Insert Record: Insert rows into a table to ensure that a set of columns contains a unique value that must have existed in the referencing table

12. Write a SQL statement to insert rows into the table employees in which a set of columns department_id and manager_id contains a unique value and that combined value must have existed into the table departments.

Sample Solution:

Code:

Here is the code to create a sample table departments :

CREATE TABLE departments ( 
DEPARTMENT_ID integer NOT NULL UNIQUE, 
DEPARTMENT_NAME varchar(30) NOT NULL, 
MANAGER_ID integer DEFAULT NULL, 
LOCATION_ID integer DEFAULT NULL, 
PRIMARY KEY (DEPARTMENT_ID,MANAGER_ID) 
);

Now input three rows into the table departments:

INSERT INTO departments VALUES(60,'SALES',201,89);
INSERT INTO departments VALUES(61,'ACCOUNTS',201,89);
INSERT INTO departments VALUES(80,'FINANCE',211,90);

Here is the command to see the list of inserted rows :

postgres=# SELECT * FROM departments;
 department_id | department_name | manager_id | location_id
---------------+-----------------+------------+-------------
            60 | SALES           |        201 |          89
            61 | ACCOUNTS        |        201 |          89
            80 | FINANCE         |        211 |          90
(3 rows)

Here is another table employees :

CREATE TABLE employees ( 
EMPLOYEE_ID integer NOT NULL PRIMARY KEY, 
FIRST_NAME varchar(20) DEFAULT NULL, 
LAST_NAME varchar(25) NOT NULL, 
JOB_ID varchar(10) NOT NULL, 
SALARY decimal(8,2) DEFAULT NULL, 
MANAGER_ID integer DEFAULT NULL, 
DEPARTMENT_ID integer DEFAULT NULL, 
FOREIGN KEY(DEPARTMENT_ID,MANAGER_ID) 
REFERENCES  departments(DEPARTMENT_ID,MANAGER_ID)
);

Now insert the rows in the table employees.

INSERT INTO employees VALUES(510,'Alex','Hanes','CLERK',18000,201,60);
INSERT INTO employees VALUES(511,'Kim','Leon','CLERK',18000,211,80);

Here is the command to see the list of inserted rows :

postgres=# SELECT * FROM employees;
 employee_id | first_name | last_name | job_id |  salary  | manager_id | department_id
-------------+------------+-----------+--------+----------+------------+---------------
         510 | Alex       | Hanes     | CLERK  | 18000.00 |        201 |            60
         511 | Kim        | Leon      | CLERK  | 18000.00 |        211 |            80
(2 rows)

The value against department_id and manager_id combination (60,201) and (80,211) are unique in the departmentis(parent) table so, there is no problem arise to insert the rows in the child table employees.

Now insert another row in the employees table :

INSERT INTO employees VALUES(512,'Kim','Leon','CLERK',18000,80,211);

Now see the output :

postgres=# INSERT INTO employees VALUES(512,'Kim','Leon','CLERK',18000,80,211);
ERROR:  insert or update on table "employees" violates foreign key constraint "employees_department_id_fkey"
DETAIL:  Key (department_id, manager_id)=(211, 80) is not present in table "departments".

Here in the above, the value against department_id and manager_id combination (211,80) does not matching with the same combination in departments(parent table) table and that is why the child table employees can not contain the combination of values including department_id and manager_id as specified. Here the primary key - foreign key relationship is being violated and shows the above message.

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