Update top n number of columns where n comes from another table

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I have one table which is giving count of items to be updated in another table both having one column in common
for eg

#table1

Type
Purpose
Itemstoupdate

#table2
Name
Type
Purpose
Flag

I need to update flag to 1 in #table2
for those no of records which is equal to Itemstoupdate from table1 where type and purpose match

logic is something like the below one ..but the syntax i dont know. Please help me

What I have tried:

update top (Itemstoupdate)#table2
set Flag=1
from #table1 
where #table1.type=#table2.type and #table1.Purpose=#table2.purpose

Posted 16-Oct-18 2:29am

Member 10112611

Updated 16-Oct-18 2:47am

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CHill60 · Accepted Answer · 2018-10-16T02:48:00

Unfortunately your logic won't work - where you have Itemstoupdate you need a constant value. You can achieve it using a Common Table Expression (CTE)

e.g.

SQL

;with CTE AS
(
	SELECT ROW_NUMBER() OVER (ORDER BY NAME) AS RN,
	name, type, purpose, flag
	FROM #table2
)
Update A SET FLAG = 1
from CTE A
INNER JOIN #table1 B on  A.type = B.type and A.purpose = B.purpose
WHERE A.RN <= B.itemstoupdate

The key point here is the ORDER BY clause on the ROW_NUMBER function - TOP doesn't have much context without ORDER BY