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MYSQL

MySQL Table Joins
By: W.J. Gilmore
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    1999-07-06

    Table of Contents:
  • MySQL Table Joins
  • The Cross Join
  • The Equi-join
  • The Left Join
  • Self-joins

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    MySQL Table Joins - The Equi-join


    (Page 3 of 5 )

    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";

    osname
    linuxacme, Inc.

    The Non-Equi-Join

    The equi-join works great when we want to match corresponding data between multiple tables. But what if there was a need to display data according to their characteristic of inequality? For example, the boss requires a list of all operating systems (os) and their corresponding order id numbers where the order pid is greater than the product pid. The solution? The aptly named non-equi join.


    mysql> SELECT p.os, o.pid from orders o, pcs p where o.pid > p.pid;

    ospid
    linux2
    linux3
    linux2
    linux5
    linux5
    linux3
    linux5
    linux5
    WinNT5
    WinNT5
    linux5
    linux5

    This matches all rows where the order table pid is greater than the product table pid. Look carefully, as you will realize that this is simply a cross-join with various restrictions placed upon it. While this may not be exceptionally useful to the boss, it does set the stage for a very useful function, the left join. Let's move on, concentrating upon the various options available when using the left join.

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