MySQL can create indexes on multiple columns. An index may consist of up to 15 columns. (On CHAR and VARCHAR columns you can also use a prefix of the column as a part of an index.)
A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.
MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a WHERE clause, even if you don't specify values for the other columns.
Suppose a table has the following specification:
mysql> CREATE TABLE test ( -> id INT NOT NULL, -> last_name CHAR(30) NOT NULL, -> first_name CHAR(30) NOT NULL, -> PRIMARY KEY (id), -> INDEX name (last_name,first_name));
Then the index name is an index over last_name and first_name. The index can used for queries that specify values in a known range for last_name, or for both last_name and first_name. Therefore, the name index will be used in the following queries:
mysql> SELECT * FROM test WHERE last_name="Widenius"; mysql> SELECT * FROM test WHERE last_name="Widenius" -> AND first_name="Michael"; mysql> SELECT * FROM test WHERE last_name="Widenius" -> AND (first_name="Michael" OR first_name="Monty"); mysql> SELECT * FROM test WHERE last_name="Widenius" -> AND first_name >="M" AND first_name < "N";
However, the name index will not be used in the following queries:
mysql> SELECT * FROM test WHERE first_name="Michael"; mysql> SELECT * FROM test WHERE last_name="Widenius" -> OR first_name="Michael";
For more information on the manner in which MySQL uses indexes to improve query performance, see MySQL indexes.