如何计算SQL SUM函数中求和的行数？

Question

Here's a certain part of my SQL query:

SELECT pir.paymentInstrumentItemID AS id,
       SUM(pir.priceChange) AS priceSum,
       pir.clientInfoID AS clientID,
       IFNULL(SUM(pir.costChange), 0) AS selfPrice,
       pir.paymentInstrumentItemID
FROM paymentInstrumentRegister as pir

GROUP BY pir.paymentInstrumentItemID 

What I need to do is to count the amount of rows that got grouped by inside the first SUM function and make a CASE statement out of it, so something like that:

SUM(CASE WHEN amtRows = 1 THEN 0 ELSE pir.priceChange) as priceSum

So basically whenever there is a single row I don't need the value, but as long as there's two or more of those, I need their sum.

Will appreciate feedback.

Answer 1

I think the best option is to add COUNT() to query, and let programming language to do the rest, but if you really need do that by MySQL:

SELECT
    id,
    (CASE WHEN amtRows = 1 THEN 0 ELSE priceSum) as priceSum,
    amtRows,
    clientID,
    selfPrice,
    paymentInstrumentItemID
FROM (

    SELECT
        pir.paymentInstrumentItemID AS id,
        SUM(pir.priceChange) AS priceSum,
        COUNT(pir.priceChange) as as amtRows,
        pir.clientInfoID AS clientID,
        IFNULL(SUM(pir.costChange),0) AS selfPrice,
        pir.paymentInstrumentItemID
    FROM 
        paymentInstrumentRegister as pir
    GROUP BY 
        pir.paymentInstrumentItemID 

) tmp