mysql 关于命令行下，字符串间少逗号分隔符的坑

遇到一个 mysql 的坑，关于字符串连接的问题，分享一下

mysql> select * from my_table;
+----+--------+------------+
| id | mod_id | mod_name   |
+----+--------+------------+
|  1 |     20 | red        |
|  2 |     20 | blue       |
|  3 |     20 | pink       |
|  4 |     21 | yellow     |
|  5 |     21 | green      |
|  6 |     21 | white      |
|  7 |     21 | black      |
|  8 |     30 | gray       |
|  9 |     30 | purple     |
| 10 |     30 | pinkpurple |
| 11 |     30 | red purple |
+----+--------+------------+
11 rows in set (0.00 sec)

原始表数据。

mysql> select * from my_table where mod_name in ("red", ‘blue‘, ‘pink‘, ‘purple‘);
+----+--------+----------+
| id | mod_id | mod_name |
+----+--------+----------+
|  1 |     20 | red      |
|  2 |     20 | blue     |
|  3 |     20 | pink     |
|  9 |     30 | purple   |
+----+--------+----------+
4 rows in set (0.00 sec)

使用 where 条件查询匹配列表中的列。（正常查询）

mysql> select * from my_table where mod_name in ("red", ‘blue‘, ‘pink‘ ‘purple‘);
+----+--------+------------+
| id | mod_id | mod_name   |
+----+--------+------------+
|  1 |     20 | red        |
|  2 |     20 | blue       |
| 10 |     30 | pinkpurple |
+----+--------+------------+
3 rows in set (0.00 sec)

注意最后一组值，(‘pink‘ ‘purple‘) 中间少了个逗号，那么查询出来的结果，默认会解释为 ‘pinkpurple‘ 字符串。

mysql> select * from my_table where mod_name = (‘pink‘ ‘purple‘);
+----+--------+------------+
| id | mod_id | mod_name   |
+----+--------+------------+
| 10 |     30 | pinkpurple |
+----+--------+------------+
1 row in set (0.00 sec)
使用 = 符号条件精确查询，仍然解释为 两个字符串的连接结果。

mysql> select * from my_table where mod_name = ‘pink‘ ‘purple‘;
+----+--------+------------+
| id | mod_id | mod_name   |
+----+--------+------------+
| 10 |     30 | pinkpurple |
+----+--------+------------+
1 row in set (0.00 sec)
使用 = 符号条件精确查询，取消掉括弧，仍然解释为 两个字符串的连接结果。

mysql> select * from my_table where mod_name = ‘‘ ‘purple‘;
+----+--------+----------+
| id | mod_id | mod_name |
+----+--------+----------+
|  9 |     30 | purple   |
+----+--------+----------+
1 row in set (0.00 sec)
头一个字符串为0长度字符串，那么合并即等于 purple 字符串。

mysql> select * from my_table where mod_name = ‘red‘‘ ‘ ‘purple‘;
Empty set (0.00 sec)
写三个字符串，其中第一个字符串的单引号和第二个字符串的单引号中间没有空格，那么解释为未知字符。。。。。

mysql> select * from my_table where mod_name = ‘red‘ ‘ ‘ ‘purple‘;
+----+--------+------------+
| id | mod_id | mod_name   |
+----+--------+------------+
| 11 |     30 | red purple |
+----+--------+------------+
1 row in set (0.00 sec)
 ‘red‘ ‘ ‘ ‘purple‘ ，每一组字符串用空格隔开，那么就合并为   red purple 字符串。

本来，这应该报语法错误的，但是，，，，，
唉，被坑了，，，，

原文地址：http://blog.51cto.com/professor/2093120