MySQL CONCAT()与GROUP_CONCAT()的使用

1 . MySQL CONCAT(str1,str2, ...)

--返回连接的字符串

mysql> SELECT CONCAT('My', 'S', 'QL');

        -> 'MySQL'

mysql> SELECT CONCAT('My', NULL, 'QL'); --若有一个为null,结果为null

        -> NULL

mysql> SELECT CONCAT(14.3);

        -> '14.3'

mysql> SELECT 'My' 'S' 'QL'; --相邻字符会相连接

-> 'MySQL'

2 . MySQL CONCAT_WS(separater,str1,str2, ...)

concat_wr()是用分隔符连接的特殊的concat()，第一个参数是分隔符，将其余的参数分隔。

mysql> SELECT CONCAT_WS('--','First name','Second name','Last Name');

        -> 'First name--Second name--Last Name'

mysql> SELECT CONCAT_WS(',','First name',NULL,'Last Name'); --连接的遇到null,直接忽略

        -> 'First name,Last Name'

mysql> SELECT CONCAT_WS(NULL, 'First name',NULL,'Last Name'); --分隔符为null,结果为null 
       -> NULL

CONCAT_WS() does not skip empty strings. However, it does skip any NULL values after the separator argument.

MySQL GROUP_CONCAT()

1. MySQL GROUP_CONCAT()

mysql> SELECT pub_id,GROUP_CONCAT(cate_id)

-> FROM book_mast

-> GROUP BY pub_id;

返回结果：（默认逗号分隔）

+--------+-----------------------+
| pub_id | GROUP_CONCAT(cate_id) |
+--------+-----------------------+
| P001 | CA002,CA004 |
| P002 | CA003,CA003 |
| P003 | CA001,CA003 |
| P004 | CA005,CA002 |
| P005 | CA001,CA004 |
| P006 | CA005,CA001 |
| P007 | CA005,CA002 |
| P008 | CA005,CA004 |
+--------+-----------------------+

2.MySQL GROUP_CONCAT() with order by and distinct

语句：（DISTINCT：消重，看结果的第四行CA003，只有一个。ASC：升序）
mysql> SELECT pub_id,GROUP_CONCAT(DISTINCT cate_id)

-> FROM book_mast

-> GROUP BY pub_id

-> ORDER BY GROUP_CONCAT(DISTINCT cate_id) ASC;

返回结果：
+--------+--------------------------------+
| pub_id | GROUP_CONCAT(DISTINCT cate_id) |
+--------+--------------------------------+
| P003 | CA001,CA003 |
| P005 | CA001,CA004 |
| P001 | CA002,CA004 |
| P002 | CA003 |
| P006 | CA005,CA001 |
| P004 | CA005,CA002 |
| P007 | CA005,CA002 |
| P008 | CA005,CA004 |
+--------+--------------------------------

3. MySQL GROUP_CONCAT() with separator

语句：

mysql> SELECT pub_id,GROUP_CONCAT(DISTINCT cate_id

-> ORDER BY cate_id ASC SEPARATOR ' ') --去除逗号

-> FROM book_mast

-> GROUP BY pub_id ;

+--------+----------------
返回结果：
+--------+--------------------------------------------------------------------+
| pub_id | GROUP_CONCAT(DISTINCT cate_id ORDER BY cate_id ASC SEPARATOR ' ') |
+--------+--------------------------------------------------------------------+
| P001 | CA002 CA004 |
| P002 | CA003 |
| P003 | CA001 CA003 |
| P004 | CA002 CA005 |
| P005 | CA001 CA004 |
| P006 | CA001 CA005 |
| P007 | CA002 CA005 |
| P008 | CA004 CA005 |
+--------+--------------------------------------------------------------------+

参考文章：http://www.w3resource.com/mysql/aggregate-functions-and-grouping/aggregate-functions-and-grouping-group_concat.php#

https://dev.mysql.com/doc/refman/5.7/en/string-functions.html