MySQL null值字段是否使用索引的总结

null和not null索引失效与否主要与表中字段的设立有关系，分为相应的两种情况，当对不能是null的字段使用索引时，条件无论是null或者not null 索引都失效，当对能是null的字段使用索引时，条件无论是null或者not null 索引都生效. 以下是null字段走索引的一个例子： (root@localhost)-[09:51:01]-[(none)]>create database test; Query OK, 1 row affected (0.02 sec) (root@localhost)-[09:51:09]-[(none)]>CREATE TABLE `test_null` (     ->   `id` int(11) DEFAULT NULL,     ->   `mark` varchar(20) DEFAULT NULL     -> ) ENGINE=InnoDB DEFAULT CHARSET=utf8; (root@localhost)-[09:51:26]-[(none)]>use test Database changed (root@localhost)-[09:51:27]-[test]>CREATE TABLE `test_null` (     ->   `id` int(11) DEFAULT NULL,     ->   `mark` varchar(20) DEFAULT NULL     -> ) ENGINE=InnoDB DEFAULT CHARSET=utf8; Query OK, 0 rows affected (0.37 sec) (root@localhost)-[09:51:29]-[test]>delimiter // (root@localhost)-[09:51:37]-[test]>DROP PROCEDURE IF EXISTS test_null;     -> create procedure test_null(in num int)     -> BEGIN     -> DECLARE i int;       -> set i=1;       -> while (i<num)      -> DO      ->   if mod(i,10)!=0 then      ->      insert into test_null values (i,concat('aaa',i));     ->    else     ->      insert into test_null values (null,concat('aaa',i));     ->    end if;     -> set i=i+1;       -> END while;       -> END;     -> // Query OK, 0 rows affected, 1 warning (0.00 sec) Query OK, 0 rows affected (0.00 sec) (root@localhost)-[09:51:38]-[test]>delimiter ; (root@localhost)-[09:51:44]-[test]>call test_null(10000); Query OK, 1 row affected (12.34 sec) (root@localhost)-[09:52:03]-[test]> (root@localhost)-[09:52:03]-[test]> (root@localhost)-[09:52:03]-[test]> (root@localhost)-[09:52:03]-[test]> (root@localhost)-[09:52:03]-[test]> (root@localhost)-[09:52:03]-[test]>select count(*) from test_null; +----------+ | count(*) | +----------+ |     9999 | +----------+ 1 row in set (0.00 sec) (root@localhost)-[09:52:24]-[test]>explain SELECT * from test_null WHERE id is null; +----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+ | id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       | +----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+ |  1 | SIMPLE      | test_null | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 10003 |    10.00 | Using where | +----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) (root@localhost)-[09:52:34]-[test]>create index idx_test_null on test_null(id); Query OK, 0 rows affected (0.13 sec) Records: 0  Duplicates: 0  Warnings: 0 (root@localhost)-[09:52:46]-[test]>explain SELECT * from test_null WHERE id is null; +----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | id | select_type | table     | partitions | type | possible_keys | key           | key_len | ref   | rows | filtered | Extra                 | +----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ |  1 | SIMPLE      | test_null | NULL       | ref  | idx_test_null | idx_test_null | 5       | const |  999 |   100.00 | Using index condition | +----+-------------+-----------+------------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (root@localhost)-[09:52:54]-[test]> (root@localhost)-[09:52:54]-[test]>explain SELECT * from test_null WHERE id is not null; +----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+ | id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       | +----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+ |  1 | SIMPLE      | test_null | NULL       | ALL  | idx_test_null | NULL | NULL    | NULL | 10003 |    89.97 | Using where | +----+-------------+-----------+------------+------+---------------+------+---------+------+-------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) 建议: MySQL列中尽量避免NULL,应该指定列为NOT NULL，除非你想存储NULL。在MySQL中，含有空值的列很难进行查询优化。因为它们使得索引、索引的统计信息以及比较运算更加复杂。你应该用0、一个特殊的值或者一个空串代替空值。