mysqlingroupconcat_将GROUP_CONCAT中的MYSQL字符
串拆分成。。。
这个问题从
MYSQL join results set wiped results during IN () in where clause?开始
所以，短版的问题.如何将GROUP_CONCAT返回的字符串转换为逗号分隔的表达式列表,IN()将被视为要循环的多个项⽬的列表？
注： MySQL⽂档看起来是将IN()⽤作“表达式列表”的“(逗号，分隔的列表)”，有趣的是,IN()中的页⾯似乎或多或少是MySQL⽂档中的唯⼀页⾯曾经提到表达式列表.所以我不知道⽤于制作数组或临时表的函数是否可以在这⾥使⽤.
长基于⽰例的版本的问题：从这样的2表数据库：
SELECT id, name, GROUP_CONCAT(tag_id) FROM person INNER JOIN tag ON person.id = tag.person_id GROUP BY person.id;
+----+------+----------------------+
| id | name | GROUP_CONCAT(tag_id) |
+----+------+----------------------+
| 1 | Bob | 1,2 |
| 2 | Jill | 2,3 |
+----+------+----------------------+
我怎么能这样做，因为它使⽤⼀个字符串被视为逻辑等价于(1 = X)AND(2 = X)…
SELECT name, GROUP_CONCAT(tag.tag_id) FROM person LEFT JOIN tag ON person.id = tag.person_id
GROUP BY person.id HAVING ( ( 1 IN (GROUP_CONCAT(tag.tag_id) ) ) AND ( 2 IN (GROUP_CONCAT(tag.tag_id) ) ) );
Empty set (0.01 sec)
…到GROUP_CONCAT结果被视为⼀个列表的东西，所以对于Bob，它将相当于：
SELECT name, GROUP_CONCAT(tag.tag_id) FROM person INNER JOIN tag ON person.id = tag.person_id AND person.id = 1
GROUP BY person.id HAVING ( ( 1 IN (1,2) ) AND ( 2 IN (1,2) ) );
+------+--------------------------+
| name | GROUP_CONCAT(tag.tag_id) |
+------+--------------------------+
| Bob | 1,2 |
mysql帮助文档+------+--------------------------+
1 row in set (0.00 sec)
…和吉尔，这将相当于：
SELECT name, GROUP_CONCAT(tag.tag_id) FROM person INNER JOIN tag ON person.id = tag.person_id AND person.id = 2
GROUP BY person.id HAVING ( ( 1 IN (2,3) ) AND ( 2 IN (2,3) ) );
Empty set (0.00 sec)
…所以整体结果将是排他性搜索条款，要求所有列出的标签不使⽤HAVING COUNT(DISTINCT …)？
(注意：该逻辑在没有AND的情况下⼯作，适⽤于字符串的第⼀个字符.
SELECT name, GROUP_CONCAT(tag.tag_id) FROM person LEFT JOIN tag ON person.id = tag.person_id GROUP BY person.id HAVING ( ( 2 IN (GROUP_CONCAT(tag.tag_id) ) ) );
+------+--------------------------+
| name | GROUP_CONCAT(tag.tag_id) |
+------+--------------------------+
| Jill | 2,3 |
+------+--------------------------+
1 row in set (0.00 sec)