Not understanding CASE function in WHERE clause

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Not understanding CASE function in WHERE clause

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Rock_query
Yak Posting Veteran

55 Posts

Posted - 2013-05-22 : 16:56:35

I copied a CASE function as a part of a WHERE function, then modified some column names to test this on the AdventureWorks2012 database.

Here is the code:

select top 5 ProductID, ActualCost
from Production.TransactionHistory
where
case
when ActualCost < 30.00 then 'Cheap'
when ActualCost < 12.00 then 'Really Cheap'
when ActualCost > 30.00 and ActualCost < 100.00 then 'Average'
else 'Expensive'
end = 'Average'

The results are two columns with these values:

Product ID: 894, 907, 916, 941, 945
ActualCost: 89.88, 78.81, 38.95, 59.93 and 67.70

I don't understand how this is working.

1. Is SQL selecting the first 5 ProductIDs, then looking at their ActualCost? Or is SQL grouping the ActualCosts, then extracting the top 5 ProductIDs?

2. Also, END = 'Average' implies that there should be a column displayed called Average right?

chadmat
The Chadinator

1974 Posts

Posted - 2013-05-22 : 17:08:26

No, columns come from the Select list.

-Chad

Lamprey
Master Smack Fu Yak Hacker

4614 Posts

Posted - 2013-05-22 : 17:21:22

Using a CASE expression in the WHERE clasue like that works, but it is a bit superfluous and makes the intent less clear (IMHO).

1. SQL is getting 5 random rows that statisfy the predicate.
2. No. That that is just a predicate.

Again, no need for the case expression:

select top 5 
	ProductID, 
	ActualCost
from 
	Production.TransactionHistory
where 
	ActualCost > 30.00 
	and ActualCost < 100.00

Rock_query
Yak Posting Veteran

55 Posts

Posted - 2013-05-22 : 20:01:39

quote:
Originally posted by Lamprey

Using a CASE expression in the WHERE clasue like that works, but it is a bit superfluous and makes the intent less clear (IMHO).

1. SQL is getting 5 random rows that statisfy the predicate.
2. No. That that is just a predicate.

Again, no need for the case expression:
select top 5
ProductID,
ActualCost
from
Production.TransactionHistory
where
ActualCost > 30.00
and ActualCost < 100.00

Thank you Lamprey.

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