Select and update; SQL Server - sql-server

Question

I need to loop table tab_A entry by entry and update table tab_B according to content in each returned entry:
SELECT item, regBy, MAX(regTime) AS latestUpdateTime
FROM tab_A
GROUP BY item, regBy;
The result must be looped entry by entry and the following UPDATE must be performed for each entry:
UPDATE tab_B
SET lastUpdated = "data from latestUpdateTime in SQL above"
lastUpdBy = "data from regBy in SQL above"
WHERE item = "data from item in SQL above"
I'm not familiar with Transact SQL, so any help would be appreciated.

Answer 1

You can turn a regular select statement into an update statement quite easily by replacing the SELECT * with UPDATE table...
This allows you to first verify what will get updated before actually applying the update.
Select statement
SELECT *
FROM tab_B b
INNER JOIN (
SELECT item
, regBy
, MAX(regTime) AS latestUpdateTime
FROM tab_A
GROUP BY
item
, regBy
) a ON a.item = b.item
Update statement
UPDATE tab_B
SET lastUpdated = a.latestUpdateTime
, lastUpdBy = a.regBy
FROM tab_B b
INNER JOIN (
SELECT item
, regBy
, MAX(regTime) AS latestUpdateTime
FROM tab_A
GROUP BY
item
, regBy
) a ON a.item = b.item
Another way to verify the update and depending on your version would be by starting a transaction and using the OUTPUT clause.
BEGIN TRAN
UPDATE tab_B
SET lastUpdated = a.latestUpdateTime
, lastUpdBy = a.regBy
OUTPUT INSERTED.*
FROM tab_B b
INNER JOIN (
SELECT item
, regBy
, MAX(regTime) AS latestUpdateTime
FROM tab_A
GROUP BY
item
, regBy
) a ON a.item = b.item
ROLLBACK TRAN

Answer 2

If you're using SQL Server 2005 or newer, you can do something like this:
;WITH TableAData AS
(
SELECT
item, regBy, regTime,
RowNum = ROW_NUMBER() OVER(PARTITION BY item ORDER BY regTime DESC)
FROM dbo.tab_A
)
UPDATE dbo.tab_B
SET
lastUpdated = a.regTime,
lastUpdBy = a.regby
FROM TableAData a
WHERE
tab_B.item = a.item
AND a.RowNum = 1
Basically, this CTE (Common Table Expression) orders your data in such a way that for each item, a RowNum is calculated (with the most recent one getting RowNum = 1).
With this, you can easily update your tab_B in a single statement - no row-by-agonizing-row looping needed!