Mysql 中的group by

Mysql 中的group by【此博客存在问题！！！待完善】

0.背景

• 建表

create table a
(id int,
title varchar(20),
author varchar(20),
date datetime);

• 插入数据

insert into a values
(1,'study haoop','lawson','2018-01-01'),
(2,'study hbase','lawson','2018-07-01'),
(3,'study opentsdb','stumb','2018-08-01'),
(4,'study kafka','lawson','2018-05-03'),
(5,'study spark','lawson','2018-04-01');

• 查看表数据

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.group by的语法

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;

• 查询结果

+----+----------------+--------+---------------------+
| 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;

• 查询结果

+----+----------------+
| id | title          |
+----+----------------+
|  1 | study hadoop   |
|  5 | study spark    |
|  4 | study kafka    |
|  2 | study hbase    |
|  3 | study opentsdb |
+----+----------------+
5 rows in set (0.00 sec)

2.还有下一种情况

看到表a中的数据是没有是没有任何重复的，但是如果有重复怎么办？比如我们新插入一条数据​​insert into a values(1,'study hive','ting','2018-09-25');​​，这时，就会有id=1的重复，这时再使用sql 1，会是什么样的结果呢？

• sql 1

mysql> select * from a group by date;

• 执行结果

ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause 
and contains nonaggregated column 'insidemysql.a.id' which is not functionally dependent on columns in GROUP BY clause; 
this is incompatible with sql_mode=only_full_group_by

会发现报错，这个原因就是：group by之后