Question

Table: Unit
ID NAME  VALUE
1  Kilo  1000
2  Mega  1000000
3  Giga  1000000000

Table: Storage
ID Title   Drive_value DriveUnitID Cache_value CacheUnitID Status_ID error error_unit area
1  Seagate  100         3          400         1           2         1     1          1
2  Scansoft 250         3          80          2           1         1     2          2

Table: manufac
    ID   area 
    1    US
    2    CHINA 

Table: Status
ID   Description 
1    Blah.. Blah
2    Durka Durka

期望的目标是有一个连接三个表的子选择像这样的东西（如下），但这种语法不起作用

select 
s.title, 
t.description,
x.area,
u1.value * s.cache_value as Cache,
u2.value * s.drive_value as Drive,
u3.value * s.error_value as Error
((u4.value * s.error_value)+(u4.value * s.error_value)) as ErrorHigh
((u5.value * s.error_value)-(u5.value * s.error_value)) as ErrorHigh
from storage s
join status t on t.id = s.status_id
join manufac x on x.id = s.manufac_id 
join unit u1 on s.cache_unit_id = u1.id
join unit u2 on s.drive_unit_id = u2.id
join unit u3 on s.error_unit_id = u3.id
join unit u4 on s.error_unit_id = u4.id
join unit u5 on s.error_unit_id = u5.id

Answer 1

检查您的查询，第三个内部联接没有条件。如果您要转发Status，则无需在FROM条款中加入INNER JOIN！

SELECT 
s.Title,
t.Description, 
u1.VALUE * s.Drive_value AS Drive, 
u2.VALUE * s.Cache_value AS Cache
FROM Storage s
INNER JOIN Unit u1 ON u1.ID = s.DriveUnitID
INNER JOIN Unit u2 ON u2.ID = s.CacheUnitID
INNER JOIN Status t ON t.ID = s.Status_ID;

Answer 2

此查询在语义上是相同的。但请记住，要么将表放在连接中，要么放在from子句中：

使用显式连接：

select s.title, t.description,
u1.value * s.cache_value as Cache,
u2.value * s.drive_value as Drive
from storage s
join status t on t.id = s.status_id
join unit u1 on s.cache_unit_id = u1.id
join unit u2 on s.drive_unit_id = u2.id

使用隐式连接：

select s.title, t.description,
u1.value * s.cache_value as Cache,
u2.value * s.drive_value as Drive
from storage s, status t, unit u1, unit u2
where t.id = s.status_id and s.cache_unit_id = u1.id and s.drive_unit_id = u2.id

最终用户的最终结果是相同的，DBMS也以相同的方式处理两个查询，因此使用其中一个没有性能增加。

修改

在要求更改后，这是我认为您正在寻找的查询：

select s.title, t.description, u1.value * s.cache_value as Cache, u2.value * s.drive_value as Drive, u3.value * s.error_value as Error, u3.value * s.error_value * 2 as ErrorHigh, u3.value * s.error_value - u3.value * s.error_value as ErrorLow, m.area from storage s join status t on t.id = s.status_id join manufac m on m.id = s.area join unit u1 on s.cache_unit_id = u1.id join unit u2 on s.drive_unit_id = u2.id join unit u3 on s.error_unit_id = u3.id

Example

结果：

+----------+-------------+----------+--------------+---------+-----------+----------+-------+ | TITLE | DESCRIPTION | CACHE | DRIVE | ERROR | ERRORHIGH | ERRORLOW | AREA | +----------+-------------+----------+--------------+---------+-----------+----------+-------+ | Seagate | Durka Durka | 400000 | 100000000000 | 1000 | 2000 | 0 | US | | Scansoft | Blah.. Blah | 80000000 | 250000000000 | 1000000 | 2000000 | 0 | CHINA | +----------+-------------+----------+--------------+---------+-----------+----------+-------+

请注意，在您的问题中有两列名为ErrorHigh。

数学“错误”x + x = 2 * x（其中x = u4.value * s.error_value）

可能是最差的数学错误:) x - x = 0（其中x = u5.value * s.error_value）

所以我猜这个查询不是你真正需要的，但应该足以引导你朝着正确的方向前进。祝你好运！

在mysql中如何在子查询中添加另一个表连接？

2 个答案: