Question

I have a table with the columns City and ZoneCode. I have a dropdown list where I can select just one City or all. And another dropdownlist for the ZoneCode. Basically they are filters that only filter if I select a value from the dropdown lists.

Is there any recommended technique to do this?

edit: this table is only an example.

Thank you

Answer 1

If you have a composite index on (City, ZoneCode):

SELECT  *
FROM    mytable
WHERE   City = @City
        AND ZoneCode = @ZoneCode
UNION ALL
SELECT  *
FROM    mytable
WHERE   City = @City
        AND @ZoneCode IS NULL
UNION ALL
SELECT  *
FROM    mytable
WHERE   @City IS NULL
        AND @ZoneCode IS NULL

If you don’t have one and cannot create it:

SELECT  *
FROM    mytable
WHERE   City = COALESCE(@City, City)
        AND ZoneCode = COALESCE(@ZoneCode, ZoneCode)

The first query assumes that you cannot filter on ZoneCode without specifying the City.