How to write a select inside case statement

问题

I have a stored procedure that contains a case statement inside a select statement.

select Invoice_ID, 'Unknown' as Invoice_Status, 
case when Invoice_Printed is null then '' else 'Y' end as Invoice_Printed, 
case when Invoice_DeliveryDate is null then '' else 'Y' end as Invoice_Delivered, 
case when Invoice_DeliveryType <> 'USPS' then '' else 'Y' end as Invoice_eDeliver, 
Invoice_ContactLName+', '+Invoice_ContactFName as ContactName, 
from dbo.Invoice
left outer join dbo.fnInvoiceCurrentStatus() on Invoice_ID=CUST_InvoiceID 
where CUST_StatusID= 7 
order by Inv_Created  

At line case when Invoice_DeliveryType <> 'USPS' then '' else 'Y' end as Invoice_eDeliver

I need to check for a valid email address (if email is valid, display Y, else display N).

So the line would read:

if Invoice_DeliveryType <> 'USPS' then '' else ( If ISNULL(Select emailaddr from dbo.Client Where Client_ID = SUBSTRING(Invoice_ID, 1, 6)), 'Y', 'N')

How can I write out this query?

回答1:

You can do this with a case. I think the following is the logic you want:

(case when Invoice_DeliveryType <> 'USPS' then ''
      when exists (Select 1
                   from dbo.Client c
                   Where c.Client_ID = SUBSTRING(i.Invoice_ID, 1, 6) and
                         c.emailaddr is not null
                  )
      then 'Y'
      else 'N'
 end)

来源：https://stackoverflow.com/questions/25767993/how-to-write-a-select-inside-case-statement