concat() concat_ws() group_concat()
可以实现多个字段使用空字符串拼接为一个字段，如下所示，
mysql> select concat(id, type) from mm_content limit 10;+------------------+| concat(id, type) |+------------------+| 100818image || 100824image || 100825video || 100826video || 100827video || 100828video || 100829video || 100830video || 100831video || 100832video |+------------------+10 rows in set (0.00 sec)
mysql> select concat(id, type, tags) from mm_content limit 10;+------------------------+| concat(id, type, tags) |+------------------------+| NULL || NULL || NULL || NULL || NULL || NULL || NULL || NULL || NULL || NULL |+------------------------+10 rows in set (0.00 sec)
concat()函数如果想要使用分隔符分割，就需要每个字段中间插一个字符串，不是非常便捷，但是通过concat_ws()函数可以一次性地解决分隔符的问题，并且不会因为某个值为NULL，而全部为NULL，如下所示，
mysql> select concat_ws(' ', id, type, tags) from mm_content limit 10;+--------------------------------+| concat_ws(' ', id, type, tags) |+--------------------------------+| 100818 image || 100824 image || 100825 video || 100826 video || 100827 video || 100828 video || 100829 video || 100830 video || 100831 video || 100832 video |+--------------------------------+10 rows in set (0.00 sec)
正常情况下，这个语句会报错，
mysql> select id from test_user group by age;ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'test_user.id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
mysql> select group_concat(name) from test_user group by age;+--------------------+| group_concat(name) |+--------------------+| wen,ning || wxnacy,win |+--------------------+2 rows in set (0.00 sec)
mysql> select group_concat(name separator ' ') from test_user group by age;+----------------------------------+| group_concat(name separator ' ') |+----------------------------------+| wen ning || wxnacy win |+----------------------------------+2 rows in set (0.00 sec)
mysql> select group_concat(name order by id desc separator ' ') from test_user group by age;+---------------------------------------------------+| group_concat(name order by id desc separator ' ') |+---------------------------------------------------+| ning wen || win wxnacy |+---------------------------------------------------+2 rows in set (0.00 sec)
mysql> select group_concat(concat_ws(',', id, name) separator ' ') from test_user group by age;+------------------------------------------------------+| group_concat(concat_ws(',', id, name) separator ' ') |+------------------------------------------------------+| 1,wen 2,ning || 3,wxnacy 4,win |+------------------------------------------------------+2 rows in set (0.00 sec)
参考链接，
https://www.zhuxianfei.com/database/mysql/43287.html
