为什么MySQL不建议使用NULL作为列默认值？

吴名氏.2023-08-11 11:26

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<=>.

bash 复制代码 
(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)

bash 复制代码 
(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.

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.

bash 复制代码 
(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().

bash 复制代码 
(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.

bash 复制代码 
(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.

bash 复制代码 
(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:

bash 复制代码 
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)

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。

相关推荐
一 乐2 小时前
民宿|基于java的民宿推荐系统(源码+数据库+文档)
java·前端·数据库·vue.js·论文·源码
鹏码纵横2 小时前
已解决：java.lang.ClassNotFoundException: com.mysql.jdbc.Driver 异常的正确解决方法，亲测有效！！！
java·python·mysql
美林数据Tempodata4 小时前
大模型驱动数据分析革新：美林数据智能问数解决方案破局传统 BI 痛点
数据库·人工智能·数据分析·大模型·智能问数
野槐4 小时前
node.js连接mysql写接口(一)
数据库·mysql
Zzzone6835 小时前
PostgreSQL日常维护
数据库·postgresql
chxii5 小时前
1.13使用 Node.js 操作 SQLite
数据库·sqlite·node.js
冰刀画的圈5 小时前
修改Oracle编码
数据库·oracle
这个胖子不太裤5 小时前
Django(自用)
数据库·django·sqlite
麻辣清汤5 小时前
MySQL 索引类型及其必要性与优点
数据库·mysql
2501_915374356 小时前
Neo4j 图数据库安装教程（2024最新版）—— Windows / Linux / macOS 全平台指南
数据库·windows·neo4j
热门推荐
01Coze扣子平台完整体验和实践（附国内和国际版对比）02【图像处理与机器视觉】XJTU期末考点03KGG转MP3工具|非KGM文件|解密音频04零代码入门 | Coze——让大模型接入自己的数据库05海康Visionmaster-常见问题排查方法-启动阶段06YOLOv8入门 | 重要性能衡量指标、训练结果评价及分析及影响mAP的因素【发论文关注的指标】07扣子（coze）实战|我用扣子搭建了一个自动分析小红薯笔记内容的AI应用|详细步骤拆解08从零安装 LLaMA-Factory 微调 Qwen 大模型成功及所有的坑09【SpeedAI科研小助手】2分钟极速解决知网维普重复率、AIGC率过高，一键全文降！文件格式不变，公式都保留的！10DeepSeek各版本说明与优缺点分析