Turning The Tables - MySQL

Before we begin, a quick introduction to the tables I'll be using throughout this tutorial seems to be in order. In order to explain subqueries, I'll be using a fictitious company's accounting database, which consists of the following four tables:

1. The "services" table: The fictitious company under discussion offers customers a number of outsourced services, each of which is associated with a fee and has a unique service ID. This information is stored in a "services" table, which looks like this:

mysql> SELECT * FROM services;

+-----+------------------+---------+

| sid | sname | sfee |

+-----+------------------+---------+

| 1 | Accounting | 1500.00 |

| 2 | Recruitment | 500.00 |

| 3 | Data Management | 300.00 |

| 4 | Administration | 500.00 |

| 5 | Customer Support | 2500.00 |

| 6 | Security | 600.00 |

+-----+------------------+---------+

6 rows in set (0.16 sec)

2. The "clients" table: The company also has a list of its current clients stored in a separate "clients" table. Each client is identified with a unique customer ID.

mysql> SELECT * FROM clients;

+-----+-----------------------------+

| cid | cname |

+-----+-----------------------------+

| 101 | JV Real Estate |

| 102 | ABC Talent Agency |

| 103 | DMW Trading |

| 104 | Rabbit Foods Inc |

| 110 | Sharp Eyes Detective Agency |

+-----+-----------------------------+

5 rows in set (0.00 sec)

3. The "branches" table: Each customer may have one or more branch offices. The "branches" table lists the branch offices per customer, together with each branch's location. Each branch has a description, a unique branch ID, and a foreign key reference to the customer ID.

mysql> SELECT * FROM branches;

+------+-----+--------------------------------+------+

| bid | cid | bdesc | bloc |

+------+-----+--------------------------------+------+

| 1011 | 101 | Corporate HQ | CA |

| 1012 | 101 | Accounting Department | NY |

| 1013 | 101 | Customer Grievances Department | KA |

| 1041 | 104 | Branch Office (East) | MA |

| 1042 | 104 | Branch Office (West) | CA |

| 1101 | 110 | Head Office | CA |

| 1031 | 103 | N Region HO | ME |

| 1032 | 103 | NE Region HO | CT |

| 1033 | 103 | NW Region HO | NY |

+------+-----+--------------------------------+------+

9 rows in set (0.01 sec)

4. The "branches_services" table: Services supplied to each branch office are listed in this table, which contains pairs of branch IDs and service IDs (foreign keys into the "branches" and "services" table respectively).

mysql> SELECT * FROM branches_services;

+------+-----+

| bid | sid |

+------+-----+

| 1011 | 1 |

| 1011 | 2 |

| 1011 | 3 |

| 1011 | 4 |

| 1012 | 1 |

| 1013 | 5 |

| 1041 | 1 |

| 1041 | 4 |

| 1042 | 1 |

| 1042 | 4 |

| 1101 | 1 |

| 1031 | 2 |

| 1031 | 3 |

| 1031 | 4 |

| 1032 | 3 |

| 1033 | 4 |

+------+-----+

16 rows in set (0.11 sec)

In order for you to try out the examples in this tutorial, you should recreate these tables in your development environment. Here are the SQL queries I used:

#

# Table structure for table `branches`

#

CREATE TABLE branches (

bid int(8) NOT NULL default '0',

cid tinyint(4) NOT NULL default '0',

bdesc varchar(255) NOT NULL default '',

bloc char(3) NOT NULL default ''

) TYPE=MyISAM CHARSET=latin1;
#

# Dumping data for table `branches`

#
INSERT INTO branches (bid, cid, bdesc, bloc) VALUES (1011, 101, 'Corporate 
HQ', 'CA'); INSERT INTO branches (bid, cid, bdesc, bloc) VALUES (1012, 101, 
'Accounting Department', 'NY'); INSERT INTO branches (bid, cid, bdesc, bloc) 
VALUES (1013, 101, 'Customer Grievances Department', 'KA'); INSERT INTO branches 
(bid, cid, bdesc, bloc) VALUES (1041, 104, 'Branch Office (East)', 'MA'); INSERT 
INTO branches (bid, cid, bdesc, bloc) VALUES (1042, 104, 'Branch Office (West)', 
'CA'); INSERT INTO branches (bid, cid, bdesc, bloc) VALUES (1101, 110, 'Head 
Office', 'CA'); INSERT INTO branches (bid, cid, bdesc, bloc) VALUES (1031, 103, 
'N Region HO', 'ME'); INSERT INTO branches (bid, cid, bdesc, bloc) VALUES (1032, 
103, 'NE Region HO', 'CT'); INSERT INTO branches (bid, cid, bdesc, bloc) VALUES 
(1033, 103, 'NW Region HO', 'NY');
#

# Table structure for table `branches_services`

#
CREATE TABLE branches_services (

bid int(8) NOT NULL default '0',

sid tinyint(4) NOT NULL default '0'

) TYPE=MyISAM CHARSET=latin1;
#

# Dumping data for table `branches_services`

#
INSERT INTO branches_services (bid, sid) VALUES (1011, 1); INSERT INTO branches_services 
(bid, sid) VALUES (1011, 2); INSERT INTO branches_services (bid, sid) VALUES 
(1011, 3); INSERT INTO branches_services (bid, sid) VALUES (1011, 4); INSERT 
INTO branches_services (bid, sid) VALUES (1012, 1); INSERT INTO branches_services 
(bid, sid) VALUES (1013, 5); INSERT INTO branches_services (bid, sid) VALUES 
(1041, 1); INSERT INTO branches_services (bid, sid) VALUES (1041, 4); INSERT 
INTO branches_services (bid, sid) VALUES (1042, 1); INSERT INTO branches_services 
(bid, sid) VALUES (1042, 4); INSERT INTO branches_services (bid, sid) VALUES 
(1101, 1); INSERT INTO branches_services (bid, sid) VALUES (1031, 2); INSERT 
INTO branches_services (bid, sid) VALUES (1031, 3); INSERT INTO branches_services 
(bid, sid) VALUES (1031, 4); INSERT INTO branches_services (bid, sid) VALUES 
(1032, 3); INSERT INTO branches_services (bid, sid) VALUES (1033, 4);
#

# Table structure for table `clients`

#
CREATE TABLE clients (

cid tinyint(4) NOT NULL default '0',

cname varchar(255) NOT NULL default '',

PRIMARY KEY (cid)

) TYPE=MyISAM CHARSET=latin1;
#

# Dumping data for table `clients`

#
INSERT INTO clients (cid, cname) VALUES (101, 'JV Real Estate'); INSERT INTO 
clients (cid, cname) VALUES (102, 'ABC Talent Agency'); INSERT INTO clients 
(cid, cname) VALUES (103, 'DMW Trading'); INSERT INTO clients (cid, cname) VALUES 
(104, 'Rabbit Foods Inc'); INSERT INTO clients (cid, cname) VALUES (110, 'Sharp 
Eyes Detective Agency');
#

# Table structure for table `services`

#
CREATE TABLE services (

sid tinyint(4) NOT NULL default '0',

sname varchar(255) NOT NULL default '',

sfee float(6,2) NOT NULL default '0.00',

PRIMARY KEY (sid)

) TYPE=MyISAM CHARSET=latin1;
#

# Dumping data for table `services`

#
INSERT INTO services (sid, sname, sfee) VALUES (1, 'Accounting', '1500.00'); 
INSERT INTO services (sid, sname, sfee) VALUES (2, 'Recruitment', '500.00'); 
INSERT INTO services (sid, sname, sfee) VALUES (3, 'Data Management', '300.00'); 
INSERT INTO services (sid, sname, sfee) VALUES (4, 'Administration', '500.00'); 
INSERT INTO services (sid, sname, sfee) VALUES (5, 'Customer Support', '2500.00'); 
INSERT INTO services (sid, sname, sfee) VALUES (6, 'Security', '600.00'); 

Once you've got your tables created, flip the page and let's start subquerying!