Mysql 中的group by

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Mysql 中的group by

0.背景

mysql> select * from a;
+----+----------------+--------+---------------------+
| id | title          | author | date                |
+----+----------------+--------+---------------------+
|  1 | study hadoop   | lawson | 2018-01-01 00:00:00 |
|  2 | study hbase    | lawson | 2018-07-01 00:00:00 |
|  3 | study opentsdb | stumb  | 2018-08-01 00:00:00 |
|  4 | study kafka    | lawson | 2018-05-03 00:00:00 |
|  5 | study spark    | lawson | 2018-04-01 00:00:00 |
+----+----------------+--------+---------------------+
5 rows in set (0.00 sec)

1.执行查询

在mysql中，group by 中的字段，是不需要在select中选择。即，使用下面的sql1，和sql2得到的结果都是一样的排序。

• sql 1

select * 
from a
group by date;
+----+----------------+--------+---------------------+
| id | title          | author | date                |
+----+----------------+--------+---------------------+
|  1 | study hadoop   | lawson | 2018-01-01 00:00:00 |
|  5 | study spark    | lawson | 2018-04-01 00:00:00 |
|  4 | study kafka    | lawson | 2018-05-03 00:00:00 |
|  2 | study hbase    | lawson | 2018-07-01 00:00:00 |
|  3 | study opentsdb | stumb  | 2018-08-01 00:00:00 |
+----+----------------+--------+---------------------+
5 rows in set (0.00 sec)

• sql 2

select id,title
from a
group by date;
+----+----------------+
| id | title          |
+----+----------------+
|  1 | study hadoop   |
|  5 | study spark    |
|  4 | study kafka    |
|  2 | study hbase    |
|  3 | study opentsdb |
+----+----------------+
5 rows in set (0.00 sec)