MySQL 中NULL和空值的區別

平時咱們在使用MySQL的時候，對於MySQL中的NULL值和空值區別不能很好的理解。注意到NULL值是未知的，且佔用空間，不走索引，DBA建議建表的時候最好設置字段是NOT NULL 來避免這種低效率的事情的發生。

   問題 1： 首先，咱們須要搞清楚 "空值" 和"NULL"的概念：

    1：空值('')是不佔用空間的

    2: MySQL中的NULL實際上是佔用空間的。官方文檔說明:

「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.」 

長度驗證：注意空值的''之間是沒有空格的。

mysql> select length(''),length(null),length('  ');

+------------+--------------+--------------+

| length('') | length(null) | length('  ') |

+------------+--------------+--------------+

|          0 |         NULL |            2 |

+------------+--------------+--------------+

 

   問題2：

判斷字段不爲空的時候，查詢語句究竟是用 select * from  tablename  where columnname <> '' 仍是用

select * from tablename where column is not null，2個查詢語句有啥不一樣。

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 ;

 

插入測試數據:

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 |

+---+------+----+

 

查詢驗證過程:

 

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)

 

 

注意事項：

1：在進行count()統計某列的記錄數的時候，若是採用的NULL值，會別系統自動忽略掉，可是空值是會進行統計到其中的。

2： 判斷NULL 用IS NULL 或者 is not null,SQL 語句函數中能夠使用ifnull()函數來進行處理，判斷空字符用 =''或者 <>''來進行處理

3: 對於MySQL特殊的注意事項，對於timestamp數據類型，若是往這個數據類型插入的列插入NULL值，則出現的值是當前系統時間。插入空值，則會出現 '0000-00-00 00:00:00'

4：對於空值的判斷究竟是使用is null 仍是 =''要根據實際業務來進行區分。