Question

我试图对几张桌子进行INNER JOIN，但有一定的条件：

我有CONTACTS表：

code                contact_type                  name               email
----------------------------------------------------------------------------
1                       P                       AAAAAAAA            SSSS@DDD.COM
2                       P                        BBBBBBB             SDFSDF@DDD.COM
3                       P                         CCCCC              SDSDF@DD.COM
1                       C                        AAAABBB             DDDD@DDD.COM
2                       C                        BBBBCCC             DSDF@DD.COM
3                       C                        CCCCDDD             ASSAD@DD.COM

PROVIDERS表：

code                               name
----------------------------------------------------
1                                  SIEMENS
2                                  FUJITSU
3                                  ASUS

和CLIENTS表：

CODE                                     NAME
---------------------------------------------------------
1                                       SMITH
2                                       PETER
3                                       MICHAEL

我想从联系人中选择所有行，但当CONTACT_TYPE为C时，此代码会显示相应的CLIENT.name，并且当CONTACT_TYPE为P时，会显示相应的PROVIDER.NAME }，

我试试：

SELECT code,contact_type,con.name,con.EMAIL 
FROM contacts con 
  inner join CLIENTS cli 
    on cli.codcli=con.CODE 
   and con.CONTACT_TYPE='C'
  inner join provIDERS p 
     on con.CODE=p.codpro 
    and con.CONTACT_TYPE='P'

但是没有用，

我也尝试： [SQL Fiddle Demo]

我希望看到这样：

PROVIDER/CLIENT NAME            CONTACT_TYPE                      EMAIL
---------------------------------------------------------------------------
SIEMENS                             P                          SSSS@DDD.COM
FUJITSU                             P                          SDFSDF@DDD.COM 
ASUS                                P                          SDSDF@DD.COM
SMITH                               C                          DDDD@DDD.COM
PETER                               C                          DSDF@DD.COM
MICHAEL                             C                          ASSAD@DD.COM

[SQL Fiddle Demo]

Answer 1

您可以尝试使用union来执行此操作：

SELECT  p.Name AS [Provider/Client Name] ,
        c.Contact_Type ,
        c.Email
FROM    contacts c
        INNER JOIN providers p
        ON c.code = p.code
WHERE  c.Contact_Type = 'P'
UNION
SELECT  cl.Name AS [Provider/Client Name] ,
        c.Contact_Type ,
        c.Email
FROM    contacts c
        INNER JOIN client cl
        ON  cl.code = p.code
WHERE   c.Contact_Type = 'C'

或用例陈述：

SELECT  CASE
        WHEN c.Contact_Type = 'P' THEN 
            (SELECT p.Name FROM providers p WHERE p.code = c.code)
        ELSE 
            (SELECT cl.Name FROM client cl WHERE cl.code = c.code) END
        AS [Provider/Client Name] ,
        c.Contact_Type ,
        c.Email
FROM    contacts c

Answer 2

尝试使用left join：

SELECT code, contact_type, con.name, con.EMAIL 
FROM contacts con left join
     CLIENTS cli
     on cli.codcli =c on.CODE and
        con.CONTACT_TYPE = 'C' left join
     providers p 
     on con.CODE = p.codpro and
        con.CONTACT_TYPE = 'P' ;

inner join要求每一行同时匹配两个表。鉴于条件，这显然是不可能的。因此，请使用left join。

对于您的实际结果：

SELECT coalesce(con.name, p.name) as name, cli.contact_type, cli.email 
FROM contacts con left join
     CLIENTS cli
     on cli.codcli = con.CODE and
        con.CONTACT_TYPE = 'C' left join
     providers p 
     on con.CODE = p.codpro and
        con.CONTACT_TYPE = 'P' ;

Answer 3

我建议你使用这个查询：

SELECT 
    CASE 
        WHEN CONTACT_TYPE = 'C' THEN
            (SELECT Name FROM CLIENTS c WHERE c.Code = co.Code)
        WHEN CONTACT_TYPE = 'P' THEN
            (SELECT Name FROM PROVIDERS p WHERE p.Code = co.Code)
    END AS [PROVIDER/CLIENT NAME],
    CONTACT_TYPE, EMAIL
FROM
    CONTACTS co;

[SQL Fiddle Demo]

Answer 4

你想要的是这样的：

SELECT CASE WHEN C.ContactType = 'P' THEN P.Name,
        WHEN C.ContactType = 'C' THEN C.Name,
        ELSE 'Unknown' END AS ProviderClientName,
        C.ContactType,
        C.Name
FROM CONTACTS C WITH(NOLOCK)
LEFT JOIN CLIENTS CL WITH(NOLOCK) ON C.Code = CL.Code
LEFT JOIN PROVIDERS P WITH(NOLOCK) ON C.Code = P.Code

但请检查您的数据，因为您示例中的Contacts和Clients表之间的连接不会返回匹配项。

如何在同一列中使用具有2个条件的内部联接

4 个答案: