LEFT JOIN with CASE WHEN - sql-server

Question

I am trying to implement a way to replace "dynamic SQL" of WHERE clause.
That is, to create a temp table, "#FilterTable", which contains my WHERE's name and value.
Then, I use "left join" to map my target table #Hotel with #FilterTable.
However, I got an error when using the following code:
DECLARE #filterName varchar(50)='Id',
#filterValue int =13
CREATE TABLE #Hotel (Id varchar(50), DisplayName varchar(100))
CREATE TABLE #FilterTable (Name varchar(50), Value varchar(100))
INSERT INTO #Hotel(Id,DisplayName) VALUES ('1','California Inn'), ('13','Hilton Hotel')
INSERT INTO #FilterTable(Name,Value) VALUES ('Id','13'),('DisplayName','Hotel')
SELECT a.*
FROM #Hotel a WITH(NOLOCK)
LEFT JOIN #FilterTable b WITH(NOLOCK)
ON #filterName= b.Name
AND
CASE b.Name
WHEN 'Id' THEN CONVERT(varchar(10), a.Id) = b.Value
WHEN 'DisplayName' THEN a.DisplayName like b.Value END
DROP Table #Hotel
DROP Table #FilterTable
It always shows error in
WHEN 'Id' THEN CONVERT(varchar(10), a.Id) = b.Value
WHEN 'DisplayName' THEN a.DisplayName like b.Value END
I know if I change to
WHEN 'DisplayName' THEN CONVERT(varchar(10), a.Id) END = b.Value
, it will pass, but there is no way to add the second condition in "WHEN" clause under this situation.
Does anyone know what is the correct syntax to put those two "WHEN" clause togather?
My db version is SQL SERVER 2014 Enterprise.
Thank you.

Answer 1

Your syntax for CASE is incorrect. You're using it like a switch statement in C, Java, etc. You don't say case against a value, then switch on the value. Rather, it's like an if statement where each condition is independent.
SELECT a.*
FROM #Hotel a WITH(NOLOCK)
LEFT JOIN #FilterTable b WITH(NOLOCK)
ON #filterName= b.Name
AND
CASE
WHEN b.Name = 'Id' AND CONVERT(varchar(10), a.Id) = b.Value THEN 1
WHEN b.Name = 'DisplayName' AND a.DisplayName like b.Value THEN 1
ELSE 0
END = 1
This says, if b.Name is ID and the convert pattern matches, the row matches, or if b.Name is DisplayName and the display name is like b.Value, the row matches.
Give that a try.
I do have to point out that this is likely to not scale well given multiple rows. You should really have a separate query for each case so that SQL Server can use the appropriate indexes and statistics.

Answer 2

In your code:
WHEN 'Id' THEN CONVERT(varchar(10), a.Id) = b.Value
WHEN 'DisplayName' THEN a.DisplayName like b.Value END
= b.Value and like b.Value equals = b.Value. If you want to use LIKE you have to add: like '%' + b.Value + '%'
But if it doesn't mean then you can use:
b.Value = CASE b.Name
WHEN 'Id' THEN CONVERT(varchar(10), a.Id)
WHEN 'DisplayName' THEN a.DisplayName END