Is there any difference between the 2

Question

Is there any difference between the 2

mcampoam

Old Hand

Points: 345
More actions
October 12, 2016 at 5:06 am

#400860

Hi,
Any difference in results or performance between the 2 examples? Names of tables/columns are generic below:
#1
update customer t1
set column_1 = x.value
FROM
(select id
, case
when column_2 is null and column_3 = 1681 then 10338
when column_2 is not null and column_3 = 1681 then 10339
else 10340
end as value
from customer
) x
where t1.id = x.id
;
#2
update customer t1
set column_1 = (select case
when column_2 is null and column_3 = 1681 then 10338
when column_2 is not null and column_3 = 1681 then 10339
else 10340
end as value
from customer t2
where t1.id = t2.id
)
;
thanks in advance!

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ChrisM@Work SSC Guru Points: 186127 More actions · Answer 1

mcampoam (10/12/2016)
Hi,
Any difference in results or performance between the 2 examples? Names of tables/columns are generic below:
#1
update customer t1
set column_1 = x.value
FROM
(select id
, case
when column_2 is null and column_3 = 1681 then 10338
when column_2 is not null and column_3 = 1681 then 10339
else 10340
end as value
from customer
) x
where t1.id = x.id
;
#2
update customer t1
set column_1 = (select case
when column_2 is null and column_3 = 1681 then 10338
when column_2 is not null and column_3 = 1681 then 10339
else 10340
end as value
from customer t2
where t1.id = t2.id
)
;
thanks in advance!

Both methods will fare poorly against the more natural way of writing this statement:

-- read the table only once

UPDATE Customer SET column_1 = CASE

WHEN column_2 IS NULL AND column_3 = 1681 THEN 10338

WHEN column_2 IS NOT NULL AND column_3 = 1681 THEN 10339

ELSE 10340 END

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

mcampoam Old Hand Points: 345 More actions · Answer 2

ChrisM@Work (10/12/2016)
Both methods will fare poorly against the more natural way of writing this statement:
-- read the table only once
UPDATE Customer SET column_1 = CASE
WHEN column_2 IS NULL AND column_3 = 1681 THEN 10338
WHEN column_2 IS NOT NULL AND column_3 = 1681 THEN 10339
ELSE 10340 END

I actually just wrote the same way. Appreciate it!