Modify select query with multi condition cases - sql

Question

I have the following table dbo.Product:
Name Amount
----------
A 10
B 4
C 6
D 3
A 7
I have condition: if name is equal to 'A', then show code 'X' and discount '10%', otherwise show code 'Y' and discount '5%'
I want to modify the following query, could you advice the better solution :
select
case p.Name
when 'A' then 'X' else 'Y' end as Code,
case p.Name
when 'A' then (p.Amount * 0.1) else (p.Amount * 0.05) end as Discount,
p.Name,
p.Amount
from
dbo.Product as P

Answer 1

There are other ways of writing this query - but using a case expression in the select clause seems to be the best option.
I've taken your sample data and multiplied it to 840 rows so that I can get some insight on performance. Then I've tested three different approaches to get the desired results:
First, the query from the question - it had the best performance of all three.
Second, the union all query suggested by Gauravsa, and third, a cross apply version.
Results (on my server) are these:
Case expressions: 16%
Union all: 36%
Cross Apply: 48%
You can clearly see from the results that the case expressions version is by far the winner - Mainly because of the fact that computing scalars is a very cheap operation compared to table or index scans and even index seek.
Please note that I've tested on a temporary table with no indexing whatsoever so a test with real live data might yield different results - however I don't imagine the order of the results to change - I mean, the case expression version will probably still win.
Than being said, Starting from 2012 version, there is a shorter way to write simple case expressions: The IFF function.
select
IIF(p.Name = 'A', 'X', 'Y') as Code,
IIF(p.Name = 'A', 0.1, 0.05) * p.Amount as Discount,
p.Name,
p.Amount
from
#Product as P
This query is an exact equivalent to the query from the question - just with slightly less code.
Here's the full script from my tests so that anyone can repeat it or find problems in the way I've tested: (the result execution plan can be found here)
-- Create sample data
create table #Product
(
Name char(1),
Amount int
);
insert into #Product (Name, Amount) values
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),
('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7),('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7), ('A', 10), ('B', 4), ('C', 6), ('D', 3), ('A', 7);
-- Cross apply version: nice trick, too slow.
select
Code,
Discount,
p.Name,
p.Amount
from
#Product as P
cross apply
(
select 'X' as code, p.Amount * 0.1 As Discount
where p.Name = 'A'
union all
select 'Y', p.Amount * 0.5
where p.Name <> 'A'
) x
-- Case expression: OP's query
select
case p.Name
when 'A' then 'X' else 'Y' end as Code,
case p.Name
when 'A' then (p.Amount * 0.1) else (p.Amount * 0.05) end as Discount,
p.Name,
p.Amount
from
#Product as P
-- Union all - Gauravsa's query
select
p.Name,
p.Amount,
'X' as Code,
p.Amount * 0.1 as Discount
from
#Product as P
where p.Name = 'A'
union all
select
p.Name,
p.Amount,
'Y' as Code,
p.Amount * 0.05 as Discount
from
#Product as P
where p.Name != 'A'
-- Clean up
drop table #Product

Answer 2

One way is:
Select
p.Name,
p.Amount,
'X' as Code,
p.Amount * 0.1 as Discount
from
dbo.Product as P
where p.Name = 'A'
Union All
Select
p.Name,
p.Amount,
'Y' as Code,
p.Amount * 0.05 as Discount
from
dbo.Product as P
where p.Name != 'A';
!= in sql server:
https://learn.microsoft.com/en-us/sql/t-sql/language-elements/not-equal-to-transact-sql-exclamation?view=sql-server-2017
DB Fiddle:
Select ##version;
Create Table Product (Name varchar(1), Amount money);
Insert into Product values ('A', 10);
Insert into Product values ('B', 4);
Insert into Product values ('C', 6);
Insert into Product values ('D', 3);
Insert into Product values ('A', 7);
Select
case p.Name
when 'A' then 'X' else 'Y' end as Code,
case p.Name
when 'A' then (p.Amount * 0.1) else (p.Amount * 0.05) end as Discount,
p.Name,
p.Amount
from
Product as P;
Select
'X' as Code,
p.Amount * 0.1 as Discount,
p.Name,
p.Amount
from
dbo.Product as P
where p.Name = 'A'
Union all
Select
'Y' as Code,
p.Amount * 0.05 as Discount,
p.Name,
p.Amount
from
dbo.Product as P
where p.Name != 'A';

Answer 3

Try this in a single query like below
Select
p.Name,
p.Amount,
CASE WHEN p.Name = 'A' THEN 'X' ELSE 'Y' END as Code,
CASE WHEN p.Name <> 'A' THEN (p.Amount * 0.1) ELSE (p.Amount * 0.05) END as Discount,
from dbo.Product as P