多列索引
多列索引，顾名思义由多个列构成的索引，mysql多列索引的列最多可包含16列，并且对于索引的查询满足最左原则，例子如下：
现在有这样一个表
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)
);
其中有个name的索引，是建立在（last_name,first_name）,以下查询会使用索引
SELECT * FROM test WHERE last_name='Jones';
SELECT * FROM test
WHERE last_name='Jones' AND first_name='John';
SELECT * FROM test
WHERE last_name='Jones'
AND (first_name='John' OR first_name='Jon');
SELECT * FROM test
WHERE last_name='Jones'
AND first_name >='M' AND first_name < 'N';
但以下方式就不会使用索引
SELECT * FROM test WHERE first_name='John';
SELECT * FROM test
WHERE last_name='Jones' OR first_name='John';
