Question

I have this SQL statement and SQL Server is giving me the following error:

An aggregate may not appear in the WHERE clause unless it is in a
subquery contained in a HAVING clause or a select list.

SELECT 
  SUM(M1.InvoiceTotal)-SUM(M1.AmountApplied) as PastDueAmount
  , M1.BillingID
  , M2.Name
  , M2.DelinquentDaysThreshold
  , M2.DelinquentAmountThreshold
  , DATEDIFF(d, MIN(BillingDate),GETDATE()) as DaysLate
FROM Invoices M1 
LEFT JOIN ClientAccounts M2 ON M1.BillingID = M2.ID
WHERE 
  InvoiceTotal <> AmountApplied
  AND M2.DelinquentDaysThreshold > DATEDIFF(d, MIN(BillingDate),GETDATE())
  OR (SUM(M1.InvoiceTotal)-SUM(M1.AmountApplied)) > M2.DelinquentAmountThreshold
GROUP BY 
  M1.BillingID
  , M2.Name
  , M2.DelinquentDaysThreshold
  , M2.DelinquentAmountThreshold

In the where clause, I only want to pull records where the oldest unpaid Billing Invoice Date is greater than the DelinquentDaysThreshhold (in days), OR the PastDueAmount (a calculated value) is greater than the DelinquentAmountThreshold.
For some reason SQL Server does not like aggregated amounts.

Answer 1

Use the HAVING as hinted by the error message, which requires a GROUP BY:

SELECT
    SUM(M1.InvoiceTotal)-SUM(M1.AmountApplied) as PastDueAmount, 
    M1.BillingID, M2.Name, 
    M2.DelinquentDaysThreshold, M2.DelinquentAmountThreshold,
    DATEDIFF(d, MIN(BillingDate),GETDATE()) as DaysLate
FROM
    Invoices M1
    LEFT JOIN
    ClientAccounts M2 ON M1.BillingID = M2.ID
WHERE
    InvoiceTotal <> AmountApplied
    AND
    M2.DelinquentDaysThreshold > DATEDIFF(d, MIN(BillingDate),GETDATE())
GROUP BY
    M1.BillingID, M2.Name, 
    M2.DelinquentDaysThreshold, M2.DelinquentAmountThreshold,
    DATEDIFF(d, MIN(BillingDate),GETDATE())
HAVING
    (SUM(M1.InvoiceTotal)-SUM(M1.AmountApplied)) > M2.DelinquentAmountThreshold