How to Join Tables
By Will Mayall
21-October-2014
Contents
Overview ................................................................................................................................................... 1
The Table Definitions .................................................................................................................... 1
Creating the Join .............................................................................................................................. 4
Summary ...................................................................................................................................................... 5
Overview
This document is intended to help creating joins on
multiple tables.
The Table Definitions
In this White Paper, the will be 5 tables involved,
user, linked_role_user, role, permission, and
linked_role_permission.
mysql> show create table user\G
*************************** 1. row ***************************
Table: user
Create Table: CREATE TABLE `user` (
`id` mediumint(9) unsigned NOT NULL AUTO_INCREMENT,
`create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`customer_id` mediumint(9) unsigned NOT NULL,
`state` tinyint(4) unsigned NOT NULL,
`username` varchar(40) NOT NULL,
`password_hash` varchar(255) NOT NULL,
1
`password_salt` varchar(10) NOT NULL,
`sso_key` varchar(255) NOT NULL,
`display_name` varchar(70) NOT NULL,
`title` varchar(35) NOT NULL,
`email` varchar(255) NOT NULL,
`manager_id` mediumint(9) unsigned NOT NULL,
`last_state_change_time` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`user_preferences` text,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
1 row in set (0.04 sec)
mysql>
mysql> show create table user\G
*************************** 1. row ***************************
Table: user
Create Table: CREATE TABLE `user` (
`id` mediumint(9) unsigned NOT NULL AUTO_INCREMENT,
`create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`customer_id` mediumint(9) unsigned NOT NULL,
`state` tinyint(4) unsigned NOT NULL,
`username` varchar(40) NOT NULL,
`password_hash` varchar(255) NOT NULL,
`password_salt` varchar(10) NOT NULL,
`sso_key` varchar(255) NOT NULL,
`display_name` varchar(70) NOT NULL,
`title` varchar(35) NOT NULL,
`email` varchar(255) NOT NULL,
`manager_id` mediumint(9) unsigned NOT NULL,
`last_state_change_time` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`user_preferences` text,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
1 row in set (0.04 sec)
2
mysql>
mysql> show create table role\G
*************************** 1. row ***************************
Table: role
Create Table: CREATE TABLE `role` (
`id` tinyint(4) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(20) NOT NULL,
`description` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
1 row in set (0.01 sec)
mysql>
mysql> show create table permission\G
*************************** 1. row ***************************
Table: permission
Create Table: CREATE TABLE `permission` (
`id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(20) NOT NULL,
`description` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8
1 row in set (0.03 sec)
mysql>
mysql> show create table link_role_permission\G
*************************** 1. row ***************************
Table: link_role_permission
Create Table: CREATE TABLE `link_role_permission` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`role_id` tinyint(4) unsigned NOT NULL,
3
`permission_id` smallint(5) unsigned NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8
1 row in set (0.02 sec)
mysql>
Creating the Join
The query to join these tables would look like:
select u.username,r.name, p.name
from user u
join link_role_user lru on u.id = lru.user_id
join role r on lru.role_id = r.id
join link_role_permission lrp on r.id = lrp.role_id
join permission p on lrp.permission_id = p.id;
+----------+-----------------+----------------------+
| username | name
| name
|
+----------+-----------------+----------------------+
| user
| CustomerUser
| dashboard
|
| user
| CustomerUser
| search
|
| user
| CustomerUser
| settings
|
| user
| CustomerUser
| password
|
| user
| CustomerUser
| logout
|
| manager
| CustomerManager | dashboard
|
| manager
| CustomerManager | search
|
| manager
| CustomerManager | settings
|
| manager
| CustomerManager | password
|
| manager
| CustomerManager | logout
|
| manager
| CustomerManager | manageUsers
|
| manager
| CustomerManager | manageCompanySubsets |
| admin
| DorgAdmin
| logout
|
| admin
| DorgAdmin
| settings
|
| admin
| DorgAdmin
| api
|
+----------+-----------------+----------------------+
15 rows in set (0.00 sec)
4
mysql>
Summary
Joining multiple tables together can become confusing.
Map out the tables on paper, see which columns map to
each table. In the above example,
Table 1 user Table
id
Table 2 link_role_user Table
id
role_id
user_id
Table 3 role Table
id
name
Table 4 permission Table
id
name
Table 5 link_role_permission Table
id
role_id
permission_id
Once you know the relationships between the tables, you
can start to build the query. This could take several
attempts as I generally have a few typos.
5
In the above example, I join the user.id to the join
link_role_user lru on u.id = lru.user_id.
Next, I join role r on lru.role_id = r.id.
Next, I join link_role_permission lrp on r.id =
lrp.role_id.
And finally, I join permission p on lrp.permission_id =
p.id.
Using this approach should help you to build joins to
multiple tables.
6