带case语句的条件WHERE子句

Question

I am working with the following query and I'm stuck

SELECT 
    @siteID = Site.siteID, 
    @customerID = Customer.customerID 
FROM 
    Customer
    inner join Site on Customer.siteID = Site.siteID
WHERE 
    Site.phoneNumber = @vaultID 
    and Customer.clientID = @clientID

Some of the Customer.clientID fields are being updated from a four character field to an eight character field, ex: '1234' is becoming '1234 01'. The customer will always be passing in a four character ID. Depending on what siteID they're record has, I need to modify the query so that it reads only the first four characters of the ID. So what I have been trying to work on is adding this in the where clause:

    AND
CASE
    WHEN Site.siteID IN ('1T','1Q') 
    THEN RTRIM(LTRIM(SUBSTRING(Customer.clientID ,1,6)))= @clientID
ELSE
    Customer.clientID = @clientID
END

but I'm not having much luck. The research I've done so far leads me to believe case statements in the where clause aren't a great idea to begin with but I feel like I'm close here. I know I can make a totally separate call to the site table before making this call but I really want to get this in one statement, hoping that's possbile.

Answer 1

The case statement should look like this:

CASE
  WHEN Site.siteID IN ('1T','1Q') 
    THEN RTRIM(LTRIM(SUBSTRING(Customer.clientID ,1,6)))
  ELSE Customer.clientID
END = @clientID

You don't do comparisson inside the CASE, rather use the CASE to get you the value which you can compare.