MySQL Table Joins

One of the great characteristics of SQL (Structured Query Language) is the fact that one can develop, access and modify data across various tables. There are several benefits to this, including greater ease of manipulation, increased speed of access, and reduced data redundancy. In MySQL (as well as many other SQL languages), this is accomplished via the join command.

One of the great characteristics of SQL (Structured Query Language) is the fact that one can develop access and modify data across various tables. There are several benefits to this, including greater ease of manipulation, increased speed of access, and reduced data redundancy. In MySQL (as well as many other SQL languages), this is accomplished via the join command.

So what does a join do, anyway? In short, a join command unites some or all of the data from two or more tables into one comprehensive structure. If this capability was not available, then the administrator would be forced to create ever-expanding, all-encompassing tables that would result in the reduction of overall efficiency and speed of data access and manipulation, even eventually possibly causing the database structure to fail entirely. The joining capability allows the administrator to create small, compact relational tables that can greatly influence database performance. This is especially useful among larger organizations, given the fact that they have the opportunity to divide their database into several departmental sections, allowing each department to effectively manage their relevant data. The alternative to this would be to force one administrator to manage all departmental sections, something that probably does not bode well both in terms of overall organization and efficiency.

For sake of illustration, let’s assume a computer manufacturer is in need of a database in which it can efficiently mine data. This data will be divided into various compact tables, each specifying data relative to a certain concept, such as orders, clients and products. After the tables are created, various examples with be illustrated, showing how this manufacturer can use joins to make the most of the database.

(Note to reader: While the concepts to follow could be applied to practically all SQL-based databases, it should be noted that the examples are illustrated using MySQL syntax. Therefore, there could be minor differences between this syntax and other sql-based languages.)

The first table will hold the various types of PCs that the manufacturer distributes:


mysql> create table pcs (     -> pid INT, // product id     -> spec char(3),     -> os char(10),     -> ram INT,     -> hd char(4)     -> );
The second table will contain data regarding the various clients of the manufacturer:


mysql> create table clients (     -> name char(25),     -> cid char(8), // client id     -> email char(25),     -> tel char(10)     -> );

The third table will contain data regarding order information:


mysql> create table orders (     -> order_date date,     -> pid INT,     -> cid char(8)     -> );

Data Insertion

We next insert data into each table:

pcs
 
pid spec os ram hd
1 386 linux 64 3.1
2 386 linux 128 4.2
3 486 WinNT 64 3.1
4 586 Linux 128 4.2
5 586 Win98 128 6.4

clients
 
name cid email tel
acme, inc. acm-042 acme@acme.com 123-456-7890
widgets-r-us, inc. wig-043 widgets@rus.com 421-555-3434
italimp, inc. imp-042 italian@imports.com 459-555-3212
fedey, inc. fed-043 fed@ey.com 439-555-8899

orders
 
date pid cid
1999-12-05 2 acm-042
1999-12-04 3 wig-043
1999-12-04 1 acm-042
1999-12-05 2 acm-042
1999-12-12 5 fed-043
1999-12-05 5 imp-042


So, how does one go about pulling useful data from these tables? What if the boss wanted a report listing the email addresses of all clients who ordered a particular pc on a particular date? Or perhaps the amount of RAM used in filling orders placed by a particular client? This is easily accomplished via the various joins available to us. Let’s use the tables we have created and construct our first join statement. {mospagebreak title=The Cross Join} The cross-join is a very basic type of join that simply matches each row from one table to every row from another table. While not very efficient, it does illustrate the common characteristic of all joins: table unification.

An example of a cross-join would be as follows:


mysql> SELECT * FROM pcs, clients;

So why are there so many rows? Remember that a cross-join matches up each row of one table with every row of the second table. Therefore, a join involving two tables in which one table has 3 rows and the second table has 4 rows would result in 12 rows.

Perhaps it is easy to remember the cross-join by putting the result in the perspective that each row in table1 crosses every row in table 2 once.

VoilĂ ! You have successfully performed your first join! Not so hard after all, is it?

Now try the following and see what happens:


mysql> select c.name, o.cid from orders o, clients c where o.cid = “acm-042″;

name cid
acme, Inc. acm-042
acme, Inc. acm-042
acme, Inc. acm-042
widgets-r-us, inc. acm-042
widgets-r-us, inc. acm-042
widgets-r-us, inc. acm-042
italimp, inc. acm-042
italimp, inc. acm-042
italimp, inc. acm-042
fedey, inc. acm-042
fedey, inc. acm-042
fedey, inc. acm-042

So what did you think about the result? Was it what you expected it to be? This time, each name in table clients was matched to every row in orders containing “acm-042″. It still fits our above explanation of a cross-join, yet it is more selective in which rows are “crossed”. Try playing around with the order of the query, and see what happens. Notice that when the table order is switched, the output will change accordingly.

Note: Why are aliases used when naming tables? Aliases are used as a method of saving repetitious keystrokes when entering queries. Thus, instead of repeatedly spelling out ‘clients’ when specifying columns, we could use ‘c’ and then state ‘from clients c’ within our query.

Although the cross-join does in fact bring our tables together, it is not very efficient. So how can we sparingly choose the data we want from each table? Read on to find out more. {mospagebreak title=The Equi-join} The equi-join is performed when certain values contained in one table are matched with values contained within a second (or multiple) table(s).

Let’s assume that we want a list of clients who ordered a pc with the product id 1:


mysql> select p.os, c.name from orders o, pcs p, clients c
where p.pid=o.pid and o.pid = 1 and o.cid=c.cid”;

Google+ Comments

Google+ Comments

os name
linux acme, Inc.