Question

我有一个具有id字段的SQL查询 - 将其视为外键。我需要根据这个id字段的值做出一个desicion：

如果该值小于3100，则从表B运行嵌套提取。
如果该值大于3100，则从表C中运行嵌套提取。

声明如下：

Select a.ID, a.SN, a.User_Ident,
    (select b.first_name from b where b.ident = a.User_Ident) as 'First Name',
    (select b.last_name from b where b.ident = a.User_Ident) as 'Last Name',
from a
where ...

我想要完成的是这样的事情：

Select a.ID, a.SN, a.User_Ident,
   when a.User_Ident > 3100 then
       (select b.first_name from b where b.ident = a.User_Ident) as 'First Name',
       (select b.last_name from b where b.ident = a.User_Ident) as 'Last Name'
   else 
       (select c.name from c where c.ident = a.User_Ident) as 'Name'
from a
where ....

这可能吗？

更新：您的回答建议我使用左连接。我的查询已包含几个左外连接，所以我不知道这将如何工作。这是完整的查询：

select    
A.Ident,
A.Serial_Number,
A.Category_Ident,
C.Description as Category,
A.Purchase_Order,
A.Manufacturer_Ident,
M.Description as Manufacturer,    
A.Hardware_Model,
A.Processor_Quantity,
A.Processor_Speed_Hertz,
A.Memory_Installed_Bytes,
A.Memory_Maximum_Bytes,
A.Memory_Slots_Used,
A.Memory_Slots_Total,
A.Storage_Capacity_Bytes,
A.Video_Memory_Bytes,
A.Screen_Size_Diagonal_Inches,    
A.Software_Ident,
S.Software_Title,    
A.Account_Ident,
T.Description as Account,

A.User_Ident,
(select Q.dbo.P.user_name from Q.dbo.P where Q.dbo.P.ident = A.User_Ident) as 'User Name',
(select Q.dbo.P.first_name from Q.dbo.P where Q.dbo.P.ident = A.User_Ident) as 'First Name',
(select Q.dbo.P.last_name from Q.dbo.P where Q.dbo.P.ident = A.User_Ident) as 'Last Name',
(select Q.dbo.R.description from Q.dbo.R where Q.dbo.R.ident =  (select Q.dbo.P.rank from Q.dbo.P where Q.dbo.P.ident = A.User_Ident)) as 'Rank',
(select Q.dbo.P.phone from Q.dbo.P where Q.dbo.P.ident = A.User_Ident) as 'Phone',
(select Q.dbo.P.smtp_address from Q.dbo.P where Q.dbo.P.ident = A.User_Ident) as 'Email',
(select Q.dbo.O.description from Q.dbo.O where Q.dbo.O.ident =  (select Q.dbo.P.organization_ident from Q.dbo.P where Q.dbo.P.ident = A.User_Ident)) as 'Organization',
(select Q.dbo.L.description from Q.dbo.L where Q.dbo.L.ident =  (select Q.dbo.P.location_ident from Q.dbo.P where Q.dbo.P.ident = A.User_Ident)) as 'Location',

A.Disposition_Ident,
D.Description as Disposition,
A.Notes,
A.Updated,
A.UpdatedBy,
A.Label,
A.Scanned,
S.Licensed

FROM Assets 

left outer join C on A.Category_Ident = C.Ident
left outer join M on A.Manufacturer_Ident = M.Ident
left outer join S on A.Software_Ident = S.Ident
left outer join T on A.Account_Ident = T.Ident
left outer join D on A.Disposition_Ident = D.Ident

WHERE ((T.Description like '%' +  @Account + '%') or (A.Account_Ident like '%' +  @Account + '%'))

order by Serial_Number

Answer 1

很多方法可以给猫皮肤，但我认为这种方法值得一试，使用UNION结合2种不同条件的结果（1个查询加入b，ids为> 3100，另一个查询加入c对于ids＆lt; = 3100）。

你必须返回相同的字段（你不能按照你想要的那样），当你为b条件返回2个字段时，查看c时返回1“name”字段。因此，在此示例中，当您加入c时，它将“name”作为First Name返回，并返回一个空白的Last Name值。

Select a.ID, a.SN, a.User_Ident, b.first_name AS 'First Name', b.last_name AS 'Last Name'
FROM a
    JOIN b ON a.User_Ident = b.ident
WHERE (a.User_Ident > 3100)
   AND (......)
UNION ALL
Select a.ID, a.SN, a.User_Ident, c.name AS 'First Name', '' AS 'Last Name'
FROM a
    JOIN c ON a.User_Ident = c.ident
WHERE (a.User_Ident <= 3100)
   AND (......)

Answer 2

我用两个左连接和一个case语句完成了这个：

select  a.field1, 
        case when b.lastname is not null then b.firstname else c.firstname end,
        case when b.lastname is not null then b.lastname else c.larstname end
from table1 a 
left join table2 b 
    on a.id = b.id
left join table3 c 
    on a.id = c.id

注意我使用的lastname在两个案例中都是空的，因为你可能不喜欢; t希望第一个名字形成一个表而最后一个名字形成另一个名字，而lastname不太可能比实际表格中的名字为空。

Answer 3

CASE只能返回一列而不是一行，并且您无法将查询与其关联。无论如何，结果集必须具有静态的列数。

我认为你想要的是一个结果集，如果填写了First Name或Last Name名称为null。

a.ID | a.SN | Name | First Name | Last Name
1    | #    | Name | null       | null
2    | #    | null | John       | Doe

Answer 4

Union会工作（在那里击败我），或者你也可以使用内联函数。就像其他人说的那样，你必须返回相同的字段。

Answer 5

HLGEM的回答略有不同

select  a.field1, 
        case when b.lastname is not null then b.firstname else c.firstname end,
        case when b.lastname is not null then b.lastname else c.larstname end
from table1 a 
left join table2 b 
    on a.User_Ident > 3100 AND a.id = b.id
left join table3 c 
    on a.User_Ident <= 3100 AND a.id = c.id

Answer 6

我对外部联接解决方案的看法是：

SELECT
   a.id
  ,a.ssn
  ,a.user_ident
  ,case when a.user_ident <= 3100 then c.name else b.lastname end   LastName
  ,case when a.user_ident <= 3100 then ''     else b.firstname end  FirstName
 from a
  left outer join b
   on b.ident = a.user_ident
  left outer join c
   on c.ident = a.user_ident

查询不能改变返回的列数（至少在我熟悉的系统中没有），所以如果你有一个“case C”的情况并且只有Name，我会把它作为LastName返回并设置FirstName到空字符串。如果它更适合您的应用程序，则可以将其设置为NULL。

是否可以在SQL中使用if / then / when之类的条件语句？

6 个答案: