为什么MySQL不建议使用NULL作为列默认值

今天来分享这个高频面试题，5分钟搞懂“为什么MySQL不建议使用NULL作为列默认值？ ”。

对于这个问题，通常能听到的答案是“使用了 NULL 值的列会使索引失效”，但是如果实际测试过一下，你就知道 IS NULL 会使用索引，所以上述说法存在缺陷。

着急的人拉到最下边看结论。

前言

Null 是列的特殊约束。如果在创建表时没有明确定义带有“not null”关键字的列，那么表中的列就会被添加空约束。很多程序员喜欢默认定义列，因为这样方便（减少为空的判断代码）从而导致查询存在一定的不确定性，数据库性能较差。

NULL值是一种对列的特殊约束，我们创建一个新列时，如果没有明确的使用关键字not null声明该数据列，Mysql会默认为我们添加上NULL约束。有些开发人员在创建数据表时，由于懒惰直接使用Mysql的默认推荐设置（即允许字段使用NULL值）。而这种陋习很容易在使用NULL的场景中指定不确定的查询结果以及导致数据库性能的崩溃。

介绍

null就是null意味着什么都没有，我们不能认为null等于''，它们是完全不同的。MySQL提供了三个操作符来处理空值：“IS NULL”、“IS NOT NULL”、“<=>”和一个函数ifnull()。IS NULL：如果列值为空，则返回 true。IS NOT NULL：如果列值不为空，则返回 true。<=>：它是一个比较运算符，与“=”类似但不相同。即使对于两个空值也返回 true。（例如，null <=> null 是合法的） IFNULL()：指定两个输入参数，如果第一个为空值则返回第二个。它与 Oracle 的 NVL() 函数类似。

NULL并不代表什么都没有，我们要注意NULL跟''(空值)是两个完全不一样的值。MySQL中可以操作NULL值操作符主要有三个。

• 一片空白

• 不为空

• <=>太空船符操作，这个操作符很像=，select NULL<=>NULL可以返回true，但是select NULL=NULL返回false。

• IFNULL 一个函数。怎么用自己查吧……其实我会了

例子

当与除 null 和“<=>”之外的任何其他值进行比较时，Null 永远不会返回 true。

NULL通过任一操作符与其他值比较都会得到NULL，除了<=>。

(root@localhost mysql3306.sock)[zlm]>create table test_null(    -> id int not null,    -> name varchar(10)    -> );Query OK, 0 rows affected (0.02 sec)(root@localhost mysql3306.sock)[zlm]>insert into test_null values(1,'zlm');Query OK, 1 row affected (0.00 sec)(root@localhost mysql3306.sock)[zlm]>insert into test_null values(2,null);Query OK, 1 row affected (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select * from test_null;+----+------+| id | name |+----+------+|  1 | zlm  ||  2 | NULL |+----+------+2 rows in set (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select * from test_null where name=null;Empty set (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select * from test_null where name is null;+----+------+| id | name |+----+------+|  2 | NULL |+----+------+1 row in set (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select * from test_null where name is not null;+----+------+| id | name |+----+------+|  1 | zlm  |+----+------+1 row in set (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select * from test_null where null=null;Empty set (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select * from test_null where null<>null;Empty set (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select * from test_null where null<=>null;+----+------+| id | name |+----+------+|  1 | zlm  ||  2 | NULL |+----+------+2 rows in set (0.00 sec)//null<=>null always return true,it's equal to "where 1=1".

Null 的意思是“缺失且未知的值”。让我们看看下面的详细信息。

NULL代表一个不确定的值，即使是两个NULL，它俩也不一定能够。（不像C中未初始化的局部变量）

(root@localhost mysql3306.sock)[zlm]>SELECT 0 IS NULL, 0 IS NOT NULL, '' IS NULL, '' IS NOT NULL;+-----------+---------------+------------+----------------+| 0 IS NULL | 0 IS NOT NULL | '' IS NULL | '' IS NOT NULL |+-----------+---------------+------------+----------------+|         0 |             1 |          0 |              1 |+-----------+---------------+------------+----------------+1 row in set (0.00 sec)//It's not equal to zero number or vacant string.//In MySQL,0 means fasle,1 means true.(root@localhost mysql3306.sock)[zlm]>SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;+----------+-----------+----------+----------+| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |+----------+-----------+----------+----------+|     NULL |      NULL |     NULL |     NULL |+----------+-----------+----------+----------+1 row in set (0.00 sec)//It cannot be compared with number.//In MySQL,null means false,too.

如果任何表达式包含 null 值，则结果为 null。

任何有返回值的表达式有NULL参与时，都会得到另外一个NULL值。

(root@localhost mysql3306.sock)[zlm]>select ifnull(null,'First is null'),ifnull(null+10,'First is null'),ifnull(concat('abc',null),'First is null');+------------------------------+---------------------------------+--------------------------------------------+| ifnull(null,'First is null') | ifnull(null+10,'First is null') | ifnull(concat('abc',null),'First is null') |+------------------------------+---------------------------------+--------------------------------------------+| First is null                | First is null                   | First is null                              |+------------------------------+---------------------------------+--------------------------------------------+1 row in set (0.00 sec)//null value needs to be disposed with ifnull() function,what usually causes sql statement more complex.//As we all know,MySQL does not support funcion index.Therefore,indexes on the column may not be used.That's really worse.

使用 count(*) 和 count(null column) 时有所不同。

使用count(*)或者count(null column)结果不同，count(null column)<=count(*)。

(root@localhost mysql3306.sock)[zlm]>select count(*),count(name) from test_null;+----------+-------------+| count(*) | count(name) |+----------+-------------+|        2 |           1 |+----------+-------------+1 row in set (0.00 sec)//count(*) returns all rows ignore the null while count(name) returns the non-null rows in column "name".//This will also leads to uncertainty if someone is unaware of the details above.

当使用distinct、group by、order by时，所有空值都被视为相同的值。

虽然select NULL=NULL的结果为false，但是在我们使用distinct、group by、order by时，NULL又被认为是相同的值。

(root@localhost mysql3306.sock)[zlm]>insert into test_null values(3,null);Query OK, 1 row affected (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select distinct name from test_null;+------+| name |+------+| zlm  || NULL |+------+2 rows in set (0.00 sec)//Two rows of null value returned one and the result became two.(root@localhost mysql3306.sock)[zlm]>select name from test_null group by name;+------+| name |+------+| NULL || zlm  |+------+2 rows in set (0.00 sec)//Two rows of null value were put into the same group.//By default,group by will also sort the result(null row showed first).(root@localhost mysql3306.sock)[zlm]>select id,name from test_null order by name;+----+------+| id | name |+----+------+|  2 | NULL ||  3 | NULL ||  1 | zlm  |+----+------+3 rows in set (0.00 sec)//Three rows were sorted(two null rows showed first).

MySQL支持在包含空值的列上使用索引（与oracle不同）。

MySQL中支持在含有NULL值的列上使用索引，但是Oracle不支持。这就是我们平时所说的如果列上含有NULL那么就会使索引失效。

严格来说，这句话对于 MySQL 来说是不准确的。

(root@localhost mysql3306.sock)[sysbench]>show tables;+--------------------+| Tables_in_sysbench |+--------------------+| sbtest1            || sbtest10           || sbtest2            || sbtest3            || sbtest4            || sbtest5            || sbtest6            || sbtest7            || sbtest8            || sbtest9            |+--------------------+10 rows in set (0.00 sec)(root@localhost mysql3306.sock)[sysbench]>show create table sbtest1G*************************** 1. row ***************************       Table: sbtest1Create Table: CREATE TABLE `sbtest1` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `k` int(11) NOT NULL DEFAULT '0',  `c` char(120) NOT NULL DEFAULT '',  `pad` char(60) NOT NULL DEFAULT '',  PRIMARY KEY (`id`),  KEY `k_1` (`k`)) ENGINE=InnoDB AUTO_INCREMENT=100001 DEFAULT CHARSET=utf81 row in set (0.00 sec)(root@localhost mysql3306.sock)[sysbench]>alter table sbtest1 modify k int null,modify c char(120) null,modify pad char(60) null;Query OK, 0 rows affected (4.14 sec)Records: 0  Duplicates: 0  Warnings: 0(root@localhost mysql3306.sock)[sysbench]>insert into sbtest1 values(100001,null,null,null);Query OK, 1 row affected (0.00 sec)(root@localhost mysql3306.sock)[sysbench]>explain select id,k from sbtest1 where id=100001;+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+|  1 | SIMPLE      | sbtest1 | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+1 row in set, 1 warning (0.00 sec)(root@localhost mysql3306.sock)[sysbench]>explain select id,k from sbtest1 where k is null;+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra                    |+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+|  1 | SIMPLE      | sbtest1 | NULL       | ref  | k_1           | k_1  | 5       | const |    1 |   100.00 | Using where; Using index |+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+1 row in set, 1 warning (0.00 sec)//In the first query,the newly added row is retrieved by primary key.//In the second query,the newly added row is retrieved by secondary key "k_1"//It has been proved that indexes can be used on the columns which contain null value.//column "k" is int datatype which occupies 4 bytes,but the value of "key_len" turn out to be 5.what's happed?Because null value needs 1 byte to store the null flag in the rows.

这是我自己测试的例子。

mysql> select * from test_1;+-----------+------+------+| name      | code | id   |+-----------+------+------+| gaoyi     | wo   |    1 || gaoyi     | w    |    2 || chuzhong  | wo   |    3 || chuzhong  | w    |    4 || xiaoxue   | dd   |    5 || xiaoxue   | dfdf |    6 || sujianhui | su   |   99 || sujianhui | NULL |   99 |+-----------+------+------+8 rows in set (0.00 sec)mysql> explain select * from test_1 where code is NULL;+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+| id | select_type | table  | partitions | type | possible_keys | key        | key_len | ref   | rows | filtered | Extra                 |+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+|  1 | SIMPLE      | test_1 | NULL       | ref  | index_code    | index_code | 161     | const |    1 |   100.00 | Using index condition |+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+1 row in set, 1 warning (0.00 sec)mysql> explain select * from test_1 where code is not NULL;+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+| id | select_type | table  | partitions | type  | possible_keys | key        | key_len | ref  | rows | filtered | Extra                 |+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+|  1 | SIMPLE      | test_1 | NULL       | range | index_code    | index_code | 161     | NULL |    7 |   100.00 | Using index condition |+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+1 row in set, 1 warning (0.00 sec)mysql> explain select * from test_1 where code='dd';+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+| id | select_type | table  | partitions | type | possible_keys | key        | key_len | ref   | rows | filtered | Extra                 |+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+|  1 | SIMPLE      | test_1 | NULL       | ref  | index_code    | index_code | 161     | const |    1 |   100.00 | Using index condition |+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+1 row in set, 1 warning (0.00 sec)mysql> explain select * from test_1 where code like "dd%";+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+| id | select_type | table  | partitions | type  | possible_keys | key        | key_len | ref  | rows | filtered | Extra                 |+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+|  1 | SIMPLE      | test_1 | NULL       | range | index_code    | index_code | 161     | NULL |    1 |   100.00 | Using index condition |+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+1 row in set, 1 warning (0.00 sec)

总结

空值在处理sql语句时总是会带来很多不确定性，一不小心可能会导致性能下降。

列中使用NULL值很容易引发不受控制的事情发生，有时还会严重拖慢系统的性能。

例如：

聚合函数()中不会估计空值，这可能会导致结果不准确。

对含有NULL值的列进行统计计算，eg. count()、max()、min()，结果没有达到我们的期望值。

空值会影响“distinct”、“group by”、“order by”等操作的行为，从而导致错误排序。

任务排序，分组，去重结果。

Null值需要ifnull()函数进行判断，使得程序代码更加复杂。

有时为了消除 NULL 带来的技术债务，我们需要在 SQL 中使用 IFNULL() 来确保结果可控，但这会使程序变得复杂。

Null 值需要额外的 1 个字节来存储行中的 Null 信息。

NULL值并占用了原有的字段空间存储，另外申请了一个字节去标注，这个字段添加了NULL约束（就像附加的标志位一样）。

由于上述缺点，不建议定义默认为 null 的列。我们建议在所有列上定义“not null”，并使用零数字和空字符串来替换相关的null数据类型。

根据以上缺点，我们并不推荐在列中设置 NULL 作为列的默认值，你可以使用 NOT NULL 消除默认设置，使用 0 或者''空字符串来代替 NULL。

译者丨广州来源丨公众号：JavaGuide（ID：JavaGuide）