Optimizing SELECT COUNT to EXISTS

Question

I have a query to find certain customers from a table.

SELECT COUNT(*)   FROM CUSTOMER  WHERE amount <> 0    AND customerid = 22 

There is an index on customerid, so the DB scans all rows with customerid = 22.

Since the result is processed by checking whether the count returns zero or more than zero, how can I optimize the query? I.e. such that at the first customer row with amount <> 0 the query returns 0 else if all rows are = 0, then return 1.

Answer 1

select case          when exists (select *                       from   customer                       where  amount <> 0                              and customerid = 22) then 1          else 0        end  as non_zero_exists 

Answer 2

First index on customerid and amount

CREATE INDEX customer_idx ON customer(customerid, amount);  

then rewrite your query as

IF EXISTS (SELECT customerid     FROM customer     WHERE amount > 0 -- I am assuming here that amount cannot be a negative number.     AND customerid = 22)    SELECT 1 ELSE    SELECT 0 

This should result in an index seek on customer_idx. Otherwise you'll need to scan all rows for that customer (which your question seems to imply could be a lot).