Question

UPDATE Houses 
SET    lStatus = U.codesum
FROM   Houses H
JOIN   (SELECT ref, SUM(code) AS codesum
    FROM   Users
    GROUP  BY ref) AS U ON U.ref = H.ref

The above code gets all users for every house (Houses table). SUMs the code column (Users table) for all users. And finally updates the result in lstatus column of the houses table.

My question is:
I need to rewrite the query which is NOT to sum the code column. Rather I want to create case statements. for example:

tempvar = 0    //local variable might be required

For each user {

If code == 1 then tempvar += 5

else if code == 2 then tempvar += 10

etc

tempvar = 0;
}

Once we have looped through all the users for each house we can now set lStatus = tempvar.
The tempvar should then be reset to 0 for the next house.

Answer 1

You should try to avoid loops and other procedural constructs when coding SQL. A relational database can’t easily optimize such things and they often perform orders of magnitude slower than their declarative counterparts. In this case, it seems simple enough to replace your SUM(code) with the CASE statement that you describe:

UPDATE Houses 
SET    lStatus = U.codesum
FROM   Houses H
JOIN   (SELECT ref, SUM(CASE code WHEN 1 THEN 5 WHEN 2 THEN 10 ELSE 0 END) AS codesum
    FROM   Users
    GROUP  BY ref) AS U ON U.ref = H.ref

In this way, SUM can still handle the duty that you imagine your temp variable would be doing.

Also, if you have many cases, you might think about putting those in a table and simply joining on that to get your sum. This might be better to maintain. I’m using a table variable here, but it could look like the following:

DECLARE @codes TABLE (
    code INT NOT NULL PRIMARY KEY,
    value INT NOT NULL
)
INSERT INTO @codes SELECT 1, 5
INSERT INTO @codes SELECT 2, 10

UPDATE Houses 
SET    lStatus = U.codesum
FROM   Houses H
JOIN   (SELECT a.ref, SUM(b.value) AS codesum
    FROM   Users a
    JOIN   @codes b on a.code = b.code -- Here we get the values dynamically
    GROUP  BY a.ref) AS U ON U.ref = H.ref