GROUP BY用法

mysql> select * from dp_ubadge_rules;
+-----+-----+
| id1 | id2 |
+-----+-----+
|   1 |   1 |
|   2 |   1 |
|   3 |   1 |
|   1 |   2 |
|   1 |   2 |
|   2 |   2 |
+-----+-----+



mysql> select COUNT(DISTINCT(id1)) from dp_ubadge_rules ORDER BY id2;       
+----------------------+
| COUNT(DISTINCT(id1)) |
+----------------------+
|                    3 |
+----------------------+


mysql> select COUNT(DISTINCT(id1)) from dp_ubadge_rules GROUP BY id1;
+----------------------+
| COUNT(DISTINCT(id1)) |
+----------------------+
|                    1 |
|                    1 |
|                    1 |
+----------------------+

注意ORDER BY和GROUP BY的区别，GROUP BY表示将数据分组，每个分组内的数据作为一个数据集，不同的分组的数据互不影响

mysql> select * From dp_ubadge_rules group by id2;    
+-----+-----+
| id1 | id2 |
+-----+-----+
|   1 |   1 |
|   1 |   2 |
+-----+-----+

mysql> select * From dp_ubadge_rules group by id1;
+-----+-----+
| id1 | id2 |
+-----+-----+
|   1 |   1 |
|   2 |   1 |
|   3 |   1 |
+-----+-----+
group by只返回每个group中的第一行数据


mysql> select count(*) From dp_ubadge_rules group by id1;
+----------+
| count(*) |
+----------+
|        3 |
|        2 |
|        1 |
+----------+
mysql> select count(*) From dp_ubadge_rules group by id2;
+----------+
| count(*) |
+----------+
|        3 |
|        3 |
+----------+
配合count()就可以计算每个group中的数据量。