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.