MySQL索引对NULL值的处理
# 索引不会包含有NULL值的列
只要列中包含有NULL值都将不会被包含在索引中，复合索引中只要有一列含有NULL值，那么这一列对于此复合索引就是无效的。所以我们在数据库设计时不要让字段的默认值为NULL。
在很多库表设计规范、某某军规的文章中，是不是经常会看到类似这样的内容。小编也经常看到这样的内容，并且在编写规范的时候，准备也把这一条加进去。但在按部就班之余，小编抽空验证了一下，发现事实却并非如此！
小编使用的MySQL版本是社区版 5.7.21
新建测试表 t1，插入不含NULL值得100行数据，然后插入1行带NULL的数据 insert into t1(id) values(101); 表中有主键id，索引a
CREATE TABLE `t1` ( `id` int(11) NOT NULL, `a` int(11) DEFAULT NULL, `b` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `a` (`a`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
测试1，包含NULL单列索引的查询，可以看到即使是查找 IS NULL的行，也是可以用上索引的
测试1：
desc select * from t1 where a > 82; +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | t1 | NULL | range | a | a | 5 | NULL | 18 | 100.00 | Using index condition | +----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+ desc select * from t1 where a is NULL; +----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | t1 | NULL | ref | a | a | 5 | const | 1 | 100.00 | Using index condition | +----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+ desc select * from t1 where a = 20 or a is null; +----+-------------+-------+------------+-------------+---------------+--------+---------+-------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------------+---------------+--------+---------+-------+------+----------+--------------------------+ | 1 | SIMPLE | t1 | NULL | ref_or_null | idx_ab | idx_ab | 5 | const | 2 | 100.00 | Using where; Using index | +----+-------------+-------+------------+-------------+---------------+--------+---------+-------+------+----------+--------------------------+
注意对 NULL 值的检索只能使用 is null / is not null / <=>，不能使用=,<,>这样的运算符（mysql中可以用a <=> NULL 表示查找 a is NULL'的行）
测试2，包含NULL复合索引的查询，首先加一个复合索引 alter table t1 drop index a,add index idx_ab(a,b); 可以看到不管是指定 a is null ，或者指定 b is null ，都可以利用上索引 idx_ab（key_len 可以看出）
测试2：
desc select * from t1 where a=50 and b>20; +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+ | 1 | SIMPLE | t1 | NULL | range | idx_ab | idx_ab | 10 | NULL | 1 | 100.00 | Using where; Using index | +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+ desc select * from t1 where a=50 and b is null; +----+-------------+-------+------------+------+---------------+--------+---------+-------------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+--------+---------+-------------+------+----------+--------------------------+ | 1 | SIMPLE | t1 | NULL | ref | idx_ab | idx_ab | 10 | const,const | 1 | 100.00 | Using where; Using index | +----+-------------+-------+------------+------+---------------+--------+---------+-------------+------+----------+--------------------------+ desc select * from t1 where a is null and b>20; +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+ | 1 | SIMPLE | t1 | NULL | range | idx_ab | idx_ab | 10 | NULL | 1 | 100.00 | Using where; Using index | +----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+ desc select * from t1 where a is null and b is null; +----+-------------+-------+------------+------+---------------+--------+---------+-------------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+--------+---------+-------------+------+----------+--------------------------+ | 1 | SIMPLE | t1 | NULL | ref | idx_ab | idx_ab | 10 | const,const | 1 | 100.00 | Using where; Using index | +----+-------------+-------+------------+------+---------------+--------+---------+-------------+------+----------+--------------------------+
由此，只要列中包含有NULL值都将不会被包含在索引中，复合索引中只要有一列含有NULL值，那么这一列对于此复合索引就是无效的。所以我们在数据库设计时不要让字段的默认值为NULL。这句的前半句是不对的（可参考官网说明： https://dev.mysql.com/doc/refman/5.7/en/is-null-optimization.html），但是后半句的结论确是可以采纳的。
虽然MySQL可以在含有null的列上使用索引，但不代表null和其他数据在索引中是一样的。不建议列上允许为空，最好限制 not null ，并设置一个默认值，比如0和''空字符串等，如果是datetime类型，可以设置成'1970-01-01 00:00:00'这样的值。对MySQL来说，null 是一个特殊的值，Conceptually, NULL means “a missing unknown value” and it is treated somewhat differently from other values。 对null做算术运算的结果都是null，count时不会包括null行，null 比空字符串需要更多的存储空间等。
附：上面说到可用通过 key_len 看出使用了索引列的个数，a,b 都是 int 类型，4 byte，为什么 key_len 是 5 byte 和 10 byte 呢？是因为如果索引列定义时允许NULL，其key_len还需要再加 1 bytes. 参考好友王的文章，可以移步我们的站点查看详情： http://www.fordba.com/spend-10-min-to-understand-how-mysql-use-index.html
