1.enum欄位與int欄位有null情況
2.null欄位在timestamp，datetime中的情況
3.select count在欄位有null的情況
4.not in 和not exists在有null值存在的情況

舉例：

1.enum欄位與int欄位有null情況
因為在MySQL中有三種狀態：True、False、Unknown，任何NULL的比較操作都是Unknown狀態，NULL跟任何比較操作返回結果都是unknown狀態，而且所有查詢條件都是unknown狀態當做false處理。所以不會返回null這個組。

mysql> select default_group_type,count(*) from emailTemplateGroup where default_group_type <>'takedownTool' group by 1;
+--------------------+----------+
| default_group_type | count(*) |
+--------------------+----------+
| p2p                |      147 |
| Ad network         |      154 |
| searchEngine       |      154 |
| linkingsite        |      154 |
+--------------------+----------+
4 rows in set (0.00 sec)

mysql> select default_group_type,count(*) from emailTemplateGroup where default_group_type is null;
+--------------------+----------+
| default_group_type | count(*) |
+--------------------+----------+
| NULL               |      386 |
+--------------------+----------+
1 row in set (0.02 sec)

mysql> select 'p2p'<>'takedownTool','Ad network'<>'takedownTool','searchEngine' <>'takedownTool','linkingsite'<>'takedownTool','takedownTool'<>'takedownTool',null <>'takedownTool';
+-----------------------+------------------------------+---------------------------------+-------------------------------+--------------------------------+-----------------------+
| 'p2p'<>'takedownTool' | 'Ad network'<>'takedownTool' | 'searchEngine' <>'takedownTool' | 'linkingsite'<>'takedownTool' | 'takedownTool'<>'takedownTool' | null <>'takedownTool' |
+-----------------------+------------------------------+---------------------------------+-------------------------------+--------------------------------+-----------------------+
|                     1 |                            1 |                               1 |                             1 |                              0 |                  NULL |
+-----------------------+------------------------------+---------------------------------+-------------------------------+--------------------------------+-----------------------+

mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;   
+----------+-----------+----------+----------+   
| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |   
+----------+-----------+----------+----------+   
| NULL | NULL | NULL | NULL |   
+----------+-----------+----------+----------+   

因為在MySQL中有三種狀態：True、False、Unknown，任何NULL的比較操作都是Unknown狀態，NULL跟任何比較操作返回結果都是unknown狀態，而且所有查詢條件都是unknown狀態當做false處理。所以不會返回null這個組。
由此可見，我們不能對null進行數學運算，如要查詢Null值資料必須在where後面加is null；

2. null欄位在timestamp，datetime中的情況

timestamp 預設允許為 “非空”（not null by default）， 如果你在定義“ts TIMESTAMP DEFAULT NULL” 是非法的。 可以指定為空 null，當在timestamp中插入Null值，不管我們給timestamp定義default為何值，DB會預設記錄當前時間

[email protected] 03:47:34>create table xbb4(
    -> id  int(10) unsigned NOT NULL AUTO_INCREMENT,
    -> col1 datetime,
    -> col2 timestamp,
    -> col3 date,
    -> PRIMARY KEY (`id`)
    -> )ENGINE=InnoDB  DEFAULT CHARSET=latin1;
Query OK, 0 rows affected (0.33 sec)

[email protected] 03:57:29>insert into xbb4(col1,col2,col3) values(null,null,null);
Query OK, 1 row affected (0.00 sec)

[email protected] 03:57:36>select * from xbb4;
+----+------+---------------------+------+
| id | col1 | col2                | col3 |
+----+------+---------------------+------+
|  1 | NULL | 2013-05-23 07:57:36 | NULL |
+----+------+---------------------+------+
1 row in set (0.00 sec)

[email protected] 09:31:54>show create table xbb6\G
*************************** 1. row ***************************
       Table: xbb6
Create Table: CREATE TABLE `xbb6` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `col1` datetime DEFAULT '0000-00-00 00:00:00',
  `col2` timestamp DEFAULT '0000-00-00 00:00:00',
  `col3` date DEFAULT '0000-00-00',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

[email protected] 09:32:25>insert into xbb6(col1,col2,col3) values(null,null,null);
Query OK, 1 row affected (0.00 sec)

[email protected] 09:32:51>select * from xbb6;
+----+------+---------------------+------+
| id | col1 | col2                | col3 |
+----+------+---------------------+------+
|  2 | NULL | 2013-05-24 01:32:51 | NULL |
+----+------+---------------------+------+
1 row in set (0.01 sec)


[email protected] 11:17:08>show create table xbb7\G
*************************** 1. row ***************************
       Table: xbb7
Create Table: CREATE TABLE `xbb7` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `col1` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `col2` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `col3` date NOT NULL DEFAULT '0000-00-00',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=latin1


[email protected] 11:16:59>insert into xbb7(col2) values(null);
Query OK, 1 row affected (0.00 sec)

[email protected] 11:17:04>select * from xbb7;
+----+---------------------+---------------------+------------+
| id | col1                | col2                | col3       |
+----+---------------------+---------------------+------------+
|  5 | 0000-00-00 00:00:00 | 2013-05-24 03:17:04 | 0000-00-00 |
+----+---------------------+---------------------+------------+
2 rows in set (0.00 sec)

3.count在欄位有null的情況下

COUNT(*)通常是對主鍵進行索引掃描，而COUNT(COL)就不一定了，另外前者是統計表中的所有符合的紀錄總數，而後者是計算表中所有符合的COL的紀錄數。null不被計數

[email protected] 11:27:01>select * from xbb6;
+----+------+---------------------+------+
| id | col1 | col2                | col3 |
+----+------+---------------------+------+
|  2 | NULL | 2013-05-24 03:20:57 | NULL |
+----+------+---------------------+------+
1 row in set (0.00 sec)

[email protected] 11:27:06>select count(*) from xbb6;
+----------+
| count(*) |
+----------+
|        1 |
+----------+
1 row in set (0.00 sec)

[email protected] 11:27:13>select count(col1) from xbb6;
+-------------+
| count(col1) |
+-------------+
|           0 |
+-------------+
1 row in set (0.00 sec)

[email protected] 11:27:19>select count(col2) from xbb6;
+-------------+
| count(col2) |
+-------------+
|           1 |
+-------------+
1 row in set (0.00 sec)

也就是說，欄位中有null值的，count(col) 將不被計數

4.not in 和not exists在有null值存在的情況
in 是子表驅動外表，把外表和那表作hash join，而exists是對外表作loop，外表驅動子表，每次loop再對那表進行查詢。
由此可見使用not exists和not in要小心，當返回值中包含null時，not in返回false，not exists返回true。

[email protected] 11:36:28>select * from xbb;
+----+------+------+
| id | col1 | col2 |

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