Dasar penggunaan Foreign Key (FK) di database relasional (RDBMS)

fk-basic.sql

Nurs-MacBook-Pro:~ nurhidayat$ /Applications/XAMPP/bin/mysql -u root -p
Enter password: 
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Your MariaDB connection id is 307
Server version: 10.1.38-MariaDB Source distribution

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> use test
Database changed
MariaDB [test]> 
MariaDB [test]> -- header table
MariaDB [test]> -- primary key id
MariaDB [test]> create table orders (
    ->     id int auto_increment primary key,
    ->     code varchar(50) not null,
    ->     order_date int not null default 0,
    ->     required_date int null,
    ->     remarks text
    -> );
Query OK, 0 rows affected (0.04 sec)

MariaDB [test]> 
MariaDB [test]> -- detail table
MariaDB [test]> -- primary key id
MariaDB [test]> -- foreign key refer ke header table
MariaDB [test]> create table order_details (
    ->     id int auto_increment primary key,
    ->     order_id int not null,
    ->     item_id int not null,
    ->     quantity int not null default 0,
    ->     unit_price int not null default 0,
    ->     remarks text,
    ->     foreign key (order_id) references orders (id)
    ->         on delete restrict on update cascade
    -> );
Query OK, 0 rows affected (0.04 sec)

MariaDB [test]> -- coba insert ke table orders
MariaDB [test]> insert into orders (code, order_date)
    ->     values ('PO.0001', unix_timestamp(now()))
    ->     , ('PO.0002', unix_timestamp(now()));
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> 
MariaDB [test]> -- lihat isi table orders
MariaDB [test]> select * from orders;
+----+---------+------------+---------------+---------+
| id | code    | order_date | required_date | remarks |
+----+---------+------------+---------------+---------+
|  4 | PO.0001 | 1561262020 |          NULL | NULL    |
|  5 | PO.0002 | 1561262020 |          NULL | NULL    |
+----+---------+------------+---------------+---------+
2 rows in set (0.01 sec)

MariaDB [test]> 
MariaDB [test]> -- coba insert ke table order_details
MariaDB [test]> -- menggunakan nilai order_id dari id di table orders
MariaDB [test]> -- dalam contoh ini, kita pakai order_id = 4
MariaDB [test]> insert into order_details (order_id, item_id, quantity, unit_price)
    ->     values (4, 1, 10, 10000), (4, 2, 10, 20000), (4, 3, 10, 15000);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [test]> 
MariaDB [test]> -- lihat isi table order_details
MariaDB [test]> select * from order_details;
+----+----------+---------+----------+------------+---------+
| id | order_id | item_id | quantity | unit_price | remarks |
+----+----------+---------+----------+------------+---------+
|  1 |        4 |       1 |       10 |      10000 | NULL    |
|  2 |        4 |       2 |       10 |      20000 | NULL    |
|  3 |        4 |       3 |       10 |      15000 | NULL    |
+----+----------+---------+----------+------------+---------+
3 rows in set (0.00 sec)

MariaDB [test]> 
MariaDB [test]> -- coba insert ke table order_details
MariaDB [test]> -- menggunakan nilai order_id fiktif
MariaDB [test]> -- kita coba pakai order_id = 9 yg tidak ada di table orders
MariaDB [test]> insert into order_details (order_id, item_id, quantity, unit_price)
    ->     values (9, 1, 10, 10000);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails 
(`test`.`order_details`, CONSTRAINT `order_details_ibfk_1` FOREIGN KEY (`order_id`) REFERENCES `orders` (`id`) ON UPDATE CASCADE)
MariaDB [test]> 
MariaDB [test]> -- perhatikan error message yang muncul karena kita mencoba
MariaDB [test]> -- untuk insert data menggunakan nilai FK order_id, yaitu 2
MariaDB [test]> -- yang tidak ada pasangannya di table orders
MariaDB [test]>