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 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 mysql3306.sock)[sysbench]>show create table sbtest1\G*************************** 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 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 mysql3306.sock)[sysbench]>insert into sbtest1 values(100001,null,null,null);Query OK, 1 row affected (0.00 sec)(root 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 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）
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