MySQL报错分析

1、

Error Code: 1175. You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column.  To disable safe mode, toggle the option in Preferences -> SQL Editor and reconnect.

在使用mysql执行update的时候，如果不是用主键当where语句，会报以上错误，使用主键用于where语句中正常。

2、
分组条件下必须通过having对分组进行限制

3、distinct必须放在要查询字段的开头
错误：select Id,distinct Email from person

4、leetcode 原题：
地址：https://leetcode.com/problems/trips-and-users/

select t2.Request_at as Day,t2.cancel_count/t3.status_sum as 'Cancellation Rate'
from
(select t1.Request_at ,count(*)as cancel_count
from 
(select Trips.*
from Trips
left join Users 
on Trips.Client_Id = Users.Users_Id
where Users.Banned = 'No') as t1 #建立嵌套表1，两个表join
where t1.Status like 'cancelled%'
group by t1.Request_at) as t2,# 计算指定时间取消的单数
(select Trips.Request_at , count(*) as status_sum
from Trips
left join Users 
on Trips.Client_Id = Users.Users_Id
where Users.Banned = 'No'
group by Trips.Request_at) as t3计算指定时间内的总单数
where t2.Request_at = t3.Request_at

错误分析：1、输出无2013-10-2的取消率，自动筛选掉无取消订单的日期
2、嵌套表太多且复杂

另一解决方案

select Trips.Request_at as Day,
round(sum(if (Trips.Status = 'completed',0,1))/count(*) ,2)as 'Cancellation Rate'#使用sum、if函数计算取消的订单数
from Trips
where Trips.Client_Id not in (select Users_Id from Users where Banned = 'yes')#建立筛选表
        and Trips.Request_at between '2013-10-01' and '2013-10-03'
group by Trips.Request_at

SELECT ROUND(column_name,decimals) FROM table_name;

• column_name需要四舍五入的字段
• decimals保留的小数点位数