为什么 MySQL 中 GROUP_CONCAT 函数返回 BLOB 大对象类型？（Why GROUP_CONCAT returns BLOB?）
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我的问题是 CONCAT(字段名, '')，这个用法，就是为了将字段名所代表的数值字段转换成字符串结果。
那么，从以下三楼回复中，我们可以摘出：CAST(user_id AS CHAR)
用 CAST 函数来将某个字段的值转换成字符型。
问题出现的原因是，CONCAT 的函数计算结果以字节形式返回，就造成被误认为是 BLOB 二进制的大对象。
所以，转换成字符串时，慎用该函数，而使用 CAST 进行转换。
使用 WorkBench 这类工具时，会有一个设置选项，可以把 二进制或可变二进制返回结果当成非二进制的字符串来看待，这样这类工具执行类似SQL语句，返回的函数计算字段值就不会被当成 BLOB 看待，而是直接显示其字符串值了。
Why GROUP_CONCAT returns BLOB?
Last week, when using the GROUP_CONCAT() function on a MySQL database, I got an unexpected result. 
Indeed, instead of getting my result as VARCHAR types, I got it as BLOB types! For information, a BLOB is a binary large object that can hold a variable amount of data:
http://dev.mysql.com/doc/refman/5.0/en/blob.html
Because BLOB values are treated as binary strings, it is not easy to use. This is why we would prefer to haveVARCHAR values.
So the question is how to get around this frustrating problem?
The answer is, for once, very simple! 
You simply need to:
- Open your my.ini or my.cnf file;
- Change the value of the
group_concat_max_lensystem variable to 512 (no ‘k’ suffix); - Restart the mysql service
To verify if the value has been successfully updated, execute the following command in your mysql client:
mysql> show variables like "%concat%"; |
+----------------------+-------+ |
| Variable_name | Value | |
+----------------------+-------+ |
| group_concat_max_len | 512 | |
+----------------------+-------+ |
1 row in set (0.00 sec) |
Note that you cannot set the value of group_concat_max_len to less than 1Kb using the MySQL Administrator GUI. Which means that the only way to set this system variable to 512 (which is less than 1Kb) is to edit your MySQL configuration file as described above.
#1 by Gerald Mengisen on 15 Sep 2010 - 20:34
The solution above doesn’t quite work for shared hosting. However, the solution in the link below worked for me (I’m just adding this here because your entry was one of the first ones in Google about this problem):
http://stackoverflow.com/questions/2133936/using-group-concat-in-phpmyadmin-will-show-the-result-as-blob-3b
#2 by smoreau on 15 Sep 2010 - 22:52
Thank you very much for sharing this link.
Indeed, it wouldn’t be possible to change the value of a system variable on a shared hosting.
It is actually a nice trick to convert the column value to a string !
#3 by mart on 13 Apr 2011 - 11:25
Had the same problem, found answer
SELECT rec_id, GROUP_CONCAT(CAST(user_id AS CHAR))
FROM t1
GROUP BY rec_id
@ http://stackoverflow.com/questions/2133936/using-group-concat-in-phpmyadmin-will-show-the-result-as-blob-3b
#4 by RakonDark on 13 Jul 2012 - 17:46
Dear mart
I found a problem with your CAST on MySQL
when i have INT and i Gdo this
SELECT rec_id, GROUP_CONCAT(CAST(user_id AS CHAR))
FROM t1
GROUP BY rec_id
then i have always only 1 char space for the int value , this may work for 0-9 but not when your INT grow
for me it was fine to CAST as CHAR(7) to hold the
biggest value 9999999 value from my INT:)
SELECT rec_id, GROUP_CONCAT(CAST(user_id AS CHAR(7)))
FROM t1
GROUP BY rec_id