空間佔用的坑
空’'在存儲過程中是不會佔用空間的，但是NULL會。就像一個杯子，空表示杯子是真空的，NULL表示裝的空氣。
mysql> SELECT length('1'),length(NULL),length('');
+-------------+--------------+------------+
| length('1') | length(NULL) | length('') |
+-------------+--------------+------------+
| 1 | NULL | 0 |
+-------------+--------------+------------+
1 row in set
mysql>
查詢的坑
如果要查詢表的NULL需要使用 is null或is not null，如果直接使用=/!=/in/not inl將查詢不到值
mysql> SELECT * FROM `user`;
+----+------+-----+---------+
| id | name | age | address |
+----+------+-----+---------+
| 1 | wyf | 32 | 合肥 |
| 2 | xx | 31 | 北京 |
| 3 | yy | 30 | 上海 |
| 4 | zz | 11 | |
| 5 | aa | 21 | NULL |
+----+------+-----+---------+
5 rows in set
mysql> SELECT * FROM `user` WHERE address IS NULL;
+----+------+-----+---------+
| id | name | age | address |
+----+------+-----+---------+
| 5 | aa | 21 | NULL |
+----+------+-----+---------+
1 row in set
mysql> SELECT * FROM `user` WHERE address = NULL;
Empty set
mysql>
NULL統計的坑
如果使用count()等統計函數，將不會統計NULL。如下，一共有5條數據，統計address就只返回4。
mysql> SELECT * FROM `user`;
+----+------+-----+---------+
| id | name | age | address |
+----+------+-----+---------+
| 1 | wyf | 32 | 合肥 |
| 2 | xx | 31 | 北京 |
| 3 | yy | 30 | 上海 |
| 4 | zz | 11 | |
| 5 | aa | 21 | NULL |
+----+------+-----+---------+
5 rows in set
mysql> SELECT COUNT(address) FROM `user`;
+----------------+
| COUNT(address) |
+----------------+
| 4 |
+----------------+
1 row in set
mysql>
索引的坑
Mysql的索引會爲NULL值做特殊處理，導致整個索引的查詢效率下降。如果是語句中有 is null會使用索引，如果語句中有is not null則會導致索引失效，如下：
查看索引：
mysql> show index from user;
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| user | 0 | PRIMARY | 1 | id | A | 5 | NULL | NULL | | BTREE | | |
| user | 1 | idx_name | 1 | name | A | 5 | NULL | NULL | YES | BTREE | | |
| user | 1 | idx_address | 1 | address | A | 5 | NULL | NULL | YES | BTREE | | |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set
is null 會使用索引
mysql> explain select * from user where address is null;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | user | NULL | ref | idx_address | idx_address | 1023 | const | 1 | 100 | Using index condition |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-----------------------+
1 row in set
is not null 不會使用索引
mysql> explain select * from user where address is not null;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | ALL | idx_address | NULL | NULL | NULL | 5 | 80 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set
mysql>