Objective and Introduction

Objective: This segment introduces how to link two tables using FOREIGN KEY.

Introduction: In relational databases, a foreign key is a column (field) or group of columns. A parent table stores the primary key, and the child table references the parent with a foreign key. Refer to the following syntax for creating such references, or to link the tables using a foreign key.

Selecting all the records from a parent table
mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| DB                 |
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
5 rows in set (0.00 sec)

mysql> USE DB;
Database changed

mysql> SHOW TABLES;
+--------------+
| Tables_in_DB |
+--------------+
| holders      |
+--------------+
1 row in set (0.01 sec)

mysql> SELECT * FROM holders;
+-------------+---------------+---------+------------+----------+--------+
| account_no  | name          | city    | dob        | bank     | amount |
+-------------+---------------+---------+------------+----------+--------+
| 25622348989 | James Moore   | Phoenix | 1985-05-26 | Barclays |   5000 |
| 25622348990 | Donald Taylor | Irvine  | 1990-08-20 | Citi     |   7000 |
| 25622348991 | Edward Parkar | Irvine  | 1994-01-29 | ICICI    |  95000 |
| 25622348992 | Ryan Bakshi   | Mumbai  | 1982-01-14 | Citi     |  50000 |
| 25622348993 | Marie Peters  | Ribe    | 1967-01-05 | DZBank   |  12250 |
| 25622348994 | Aanya         | Delhi   | 1975-08-18 | SBI      | 105000 |
| 25622348995 | James Moore   | NULL    | 1978-06-26 | Citi     |  97800 |
+-------------+---------------+---------+------------+----------+--------+
7 rows in set (0.00 sec)

mysql> DESC holders;                                                          
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| account_no | bigint      | NO   | PRI | NULL    |       |
| name       | varchar(30) | NO   |     | NULL    |       |
| city       | varchar(20) | YES  |     | NULL    |       |
| dob        | date        | YES  |     | NULL    |       |
| bank       | varchar(10) | YES  |     | NULL    |       |
| amount     | bigint      | NO   |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
Creating a table with foreign key
mysql> CREATE TABLE loans(
    -> loan_id INT AUTO_INCREMENT PRIMARY KEY,
    -> loan_type VARCHAR(20) NOT NULL,
    -> loan_amount BIGINT,
    -> account_no BIGINT,
    -> FOREIGN KEY(account_no) REFERENCES holders(account_no));
Query OK, 0 rows affected (1.79 sec)

mysql> DESC loans;
+-------------+-------------+------+-----+---------+----------------+
| Field       | Type        | Null | Key | Default | Extra          |
+-------------+-------------+------+-----+---------+----------------+
| loan_id     | int         | NO   | PRI | NULL    | auto_increment |
| loan_type   | varchar(20) | NO   |     | NULL    |                |
| loan_amount | bigint      | YES  |     | NULL    |                |
| account_no  | bigint      | YES  | MUL | NULL    |                |
+-------------+-------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)

mysql> SHOW TABLES;
+--------------+
| Tables_in_DB |
+--------------+
| holders      |
| loans        |
+--------------+
2 rows in set (0.00 sec)
What Next?
MySQL NOT NULL Constraint MySQL ALTER TABLE (ADD, DROP, MODIFY, CHANGE, RENAME TO) MySQL DROP a TABLE and DATABASE
Advertisement