Question

Employee Table:
Empid   FNAme           LName      Hair Race
1       john            adam       1     1
2       clift           susanne    2     3

Code Table:
code  type    description
1     hair    black
2     hair    brown
1     race    black
3     race    white

我们需要将输出数据作为empid, Fname, Lname, Hair, Race; 其中hair和race未显示为代码，但描述形成代码表。

我是sql的初学者。您可以帮忙写一个查询我没有得到每个查询以下的预期输出

select E.Empid,E.FName,E.Lname,c.description as Hair,c.description as race
From Employee2 E  inner join code c
on (c.code = E.Hair )

Answer 1

您应该加入代码表两次：

一次得到头发
一次参加比赛

SQL声明

SELECT E.EmpID
       , E.FName
       , E.LName
       , c1.Description AS Hair
       , c2.Description AS Race
FROM   Employee E
       INNER JOIN Code c1 ON c1.Code = e.Hair AND c1.type = 'hair'
       INNER JOIN Code c2 ON c2.Code = e.Race AND c2.type = 'race'

请注意，如果其中任何一个可能是NULL，您就可以将INNER JOIN更改为LEFT OUTER JOIN

说，我建议你为Hair and Race 创建一个单独的代码表。

尝试将所有内容转储到一个代码表中只会让人感到困惑并迫使您最多编写次优查询。您强制自己将类型添加到代码表以及您将要创建的每个联接。

更好的选择是

Employee Table:
Empid   FNAme           LName      Hair Race
1       john            adam       1     1
2       clift           susanne    2     3

HairCode Table:
code  description
1     black
2     brown

RaceCode Table:
1     black
3     white

将您的陈述简化为

SELECT E.EmpID
       , E.FName
       , E.LName
       , c1.Description AS Hair
       , c2.Description AS Race
FROM   Employee E
       INNER JOIN HairCode c1 ON c1.Code = e.Hair 
       INNER JOIN RaceCode c2 ON c2.Code = e.Race

您将获得更好的性能和更小的足迹。

Answer 2

您可以加入两次以获取所需的数据

select E.Empid, E.FName, E.Lname, hair.description as Hair, race.description as race 
From Employee2 E  
inner join code hair on (hair.code = E.Hair and hair.type = 'hair' ) 
inner join code race on (race.code = E.Race and race.type = 'race')

TSQL连接查询

2 个答案: