为什么MySQL不建议使用NULL作为列默认值？

灰太狼 2023-10-14 17:08 21阅读 0赞

## 1 前言 ##

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会默认的为我们添加上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).而这一陋习很容易在使用NULL的场景中得出不确定的查询结果以及引起数据库性能的下降

## 2 介绍 ##

NULL并不意味着什么都没有,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:

*  IS NULL
 *  IS NOT NULL
 *  <=> 太空船操作符,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.怎么使用自己查吧…反正我会了  
    Example

Null never returns true when comparing with any other values except null with “<=>”.

NULL通过任一操作符与其它值比较都会得到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 代表一个不确定的值,就算是两个 NULL,它俩也不一定相等.(像不像 C 中未初始化的局部变量)

任何有返回值的表达式中有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(*) 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.

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 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那么将会使索引失效.

严格来说,这句话对与 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 sbtest1\G
    *************************** 1. row ***************************
           Table: sbtest1
    Create 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=utf8
    1 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.

这个是我自己测试的例子:

## 3 总结 ##

列中使用NULL值容易引发不受控制的事情发生,有时候还会严重托慢系统的性能,例如:

1.  对含有 NULL 值的列进行统计计算,eg. count(),max(),min(),结果并不符合我们的期望值
2.  干扰排序，分组,去重结果
3.  有的时候为了消除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,但是这使程序变得复杂
4.  NULL值并是占用原有的字段空间存储,而是额外申请一个字节去标注,这个字段添加了NULL约束.(就像额外的标志位一样)

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。