MySQL 中NULL和空值的区别

平时我们在使用MySQL的时候，对于MySQL中的NULL值和空值区别不能很好的理解。注意到NULL值是未知的，且占用空间，不走索引，DBA建议建表的时候最好设置字段是NOT NULL 来避免这种低效率的事情的发生。
   问题 1： 首先，我们需要搞清楚 "空值" 和"NULL"的概念：
    1：空值('')是不占用空间的
    2: MySQL中的NULL其实是占用空间的。官方文档说明:
“NULL columns require additional space in the row to record whether their values are NULL. For MyISAM tables, each NULL column takes one bit extra, rounded up to the nearest byte.”
长度验证：注意空值的''之间是没有空格的。
mysql> select length(''),length(null),length('  ');
+------------+--------------+--------------+
| length('') | length(null) | length('  ') |
+------------+--------------+--------------+
|          0 |         NULL |            2 |
+------------+--------------+--------------+

   问题2：
判断字段不为空的时候，查询语句到底是用 select * from  tablename  where columnname <> '' 还是用
select * from tablename where column is not null，2个查询语句有啥不同。
eg:
mysql> show create table testaa;
+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                         |
+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| testaa | CREATE TABLE `testaa` (
  `a` int(11) NOT NULL,
  `b` varchar(20) DEFAULT NULL,
  `c` varchar(20) NOT NULL,
  PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ;

插入测试数据:
mysql> insert testaa  values (1,'aa','');
Query OK, 1 row affected (0.00 sec)
mysql> insert testaa  values (2,'','');
Query OK, 1 row affected (0.00 sec)
mysql> insert testaa  values (3,null,'');
Query OK, 1 row affected (0.00 sec)
mysql> insert testaa  values (4,NULL,'');
Query OK, 1 row affected (0.00 sec)
mysql> insert testaa  values (5,'aafa','fa');
Query OK, 1 row affected (0.00 sec)

mysql> insert testaa values (6,'',NULL);
ERROR 1048 (23000): Column 'c' cannot be null

mysql> select * from testaa;
+---+------+----+
| a | b    | c  |
+---+------+----+
| 1 | aa   |    |
| 2 |      |    |
| 3 | NULL |    |
| 4 | NULL |    |
| 5 | aafa | fa |
+---+------+----+

查询验证过程:

mysql> select * from testaa where c is not null;
+---+------+----+
| a | b    | c  |
+---+------+----+
| 1 | aa   |    |
| 2 |      |    |
| 3 | NULL |    |
| 4 | NULL |    |
| 5 | aafa | fa |
+---+------+----+
5 rows in set (0.00 sec)

mysql> select * from testaa where c <> '';
+---+------+----+
| a | b    | c  |
+---+------+----+
| 5 | aafa | fa |
+---+------+----+
1 row in set (0.00 sec)
mysql> select * from testaa  where c = '';
+---+------+---+
| a | b    | c |
+---+------+---+
| 1 | aa   |   |
| 2 |      |   |
| 3 | NULL |   |
| 4 | NULL |   |
+---+------+---+
4 rows in set (0.00 sec)

mysql> select * from testaa where  c is null;
Empty set (0.00 sec)


mysql> select * from testaa where b is not null;
+---+------+----+
| a | b    | c  |
+---+------+----+
| 1 | aa   |    |
| 2 |      |    |
| 5 | aafa | fa |
+---+------+----+
3 rows in set (0.00 sec)

mysql> select * from testaa where b <> '';
+---+------+----+
| a | b    | c  |
+---+------+----+
| 1 | aa   |    |
| 5 | aafa | fa |
+---+------+----+
2 rows in set (0.00 sec)

mysql> select * from testaa where b ='';
+---+------+---+
| a | b    | c |
+---+------+---+
| 2 |      |   |
+---+------+---+
1 row in set (0.00 sec)
mysql> select * from testaa where  b is null;
+---+------+---+
| a | b    | c |
+---+------+---+
| 3 | NULL |   |
| 4 | NULL |   |
+---+------+---+

mysql> select length(b),length(c) from testaa;
+-----------+-----------+
| length(b) | length(c) |
+-----------+-----------+
| 2 | 0 |
| 0 | 0 |
| NULL | 0 |
| NULL | 0 |
| 4 | 2 |
+-----------+-----------+
5 rows in set (0.00 sec)


mysql> select count(b),count(c) from testaa;
+----------+----------+
| count(b) | count(c) |
+----------+----------+
| 3 | 5 |
+----------+----------+
1 row in set (0.00 sec)

mysql> create table testbb ( a int primary key , b timestamp);
Query OK, 0 rows affected (0.07 sec)
mysql> show create table testbb;
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| testbb | CREATE TABLE `testbb` (
`a` int(11) NOT NULL,
`b` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

mysql> insert into testbb vales (1,null) ;
mysql> insert into testbb values (2,'');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> show warnings;
+---------+------+----------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------+
| Warning | 1265 | Data truncated for column 'b' at row 1 |
+---------+------+----------------------------------------+
mysql> select * from testbb;
+---+---------------------+
| a | b |
+---+---------------------+
| 1 | 2014-08-15 14:32:10 |
| 2 | 0000-00-00 00:00:00 |
+---+---------------------+
2 rows in set (0.00 sec)


注意事项：
1：在进行count()统计某列的记录数的时候，如果采用的NULL值，会别系统自动忽略掉，但是空值是会进行统计到其中的。
2： 判断NULL 用IS NULL 或者 is not null,SQL 语句函数中可以使用ifnull()函数来进行处理，判断空字符用 =''或者 <>''来进行处理
3: 对于MySQL特殊的注意事项，对于timestamp数据类型，如果往这个数据类型插入的列插入NULL值，则出现的值是当前系统时间。插入空值，则会出现 '0000-00-00 00:00:00'
4：对于空值的判断到底是使用is null 还是 =''要根据实际业务来进行区分。

转自：http://blog.sina.com.cn/s/blog_3f2a82610102v4dn.html