Mysql implements three functions for field splicing

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)

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)

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)