Move all the conditions that are not common to all three into CASE expressions - like shown below.SELECT a.NAME,
SUM(
CASE
WHEN TRANSTYPE = 1 AND REF1 = 'invoice' AND
NOT (t.INVNO LIKE 'CR%' OR REF2 LIKE 'T#%') THEN t.AMOUNT
ELSE 0
END) AS LifeTimeInvoice,
ABS(SUM(
CASE
WHEN TRANSTYPE IN (4, 5) --Payment
AND REF1 <> 'Credit' THEN t.AMOUNT
ELSE 0
END )) AS LifeTimePayment,
SUM(
CASE
WHEN (TRANSTYPE IN (4, 5) AND REF1 = 'Credit')
OR (TRANSTYPE = 1 AND (t.INVNO LIKE 'CR%' OR REF2 LIKE 'T#%'))
THEN t.AMOUNT
ELSE 0
END ) AS LifeTimeCredit
FROM dbo.DR_ACCS a
JOIN dbo.DR_TRANS t
ON a.ACCNO = t.ACCNO
WHERE a.CustomerCode = 1326499
GROUP BY
a.NAME