How Can I Update Table Records with Calculated Values from a CTE?
Efficiently Updating Table Records Using CTEs
This article explains how to correctly update table records using a Common Table Expression (CTE) to calculate new values. The goal is to update the DocTotal
column in the PEDI_InvoiceDetail
table with a recalculated total. A previous attempt failed because updates to the CTE didn't propagate to the base table.
The Challenge
Directly updating a table based on a CTE's calculated values often leads to errors. The CTE acts as a temporary result set; changes within it don't automatically update the underlying table.
The Solution
The solution involves a two-step process using a CTE:
;WITH T AS ( SELECT InvoiceNumber, DocTotal, SUM(Sale + VAT) OVER (PARTITION BY InvoiceNumber) AS NewDocTotal FROM PEDI_InvoiceDetail ) UPDATE T SET DocTotal = NewDocTotal;
This revised query:
-
Creates a CTE: The CTE
T
selectsInvoiceNumber
, the originalDocTotal
, and calculates a newDocTotal
(NewDocTotal
) using theSUM() OVER (PARTITION BY InvoiceNumber)
function. This efficiently aggregatesSale VAT
for each uniqueInvoiceNumber
. -
Updates the Base Table: The
UPDATE
statement then directly targets the CTET
, setting the originalDocTotal
to the newly calculatedNewDocTotal
. This ensures the changes are applied to thePEDI_InvoiceDetail
table.
Key Concepts
-
OVER (PARTITION BY ...)
: This crucial clause partitions the data byInvoiceNumber
, allowing theSUM()
function to calculate the total for each invoice separately.
This approach provides a clean and efficient method for updating records with calculated values derived from a CTE, avoiding the common pitfalls of trying to modify the CTE itself.
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