Using Case in a select statement - sql

Question

Consider the following table
create table temp (id int, attribute varchar(25), value varchar(25))
And values into the table
insert into temp select 100, 'First', 234
insert into temp select 100, 'Second', 512
insert into temp select 100, 'Third', 320
insert into temp select 101, 'Second', 512
insert into temp select 101, 'Third', 320
I have to deduce a column EndResult which is dependent on 'attribute' column. For each id, I have to parse through attribute values in the order
First, Second, Third and choose the very 1st value which is available i.e. for id = 100, EndResult should be 234 for the 1st three records.
Expected result:
| id | EndResult |
|-----|-----------|
| 100 | 234 |
| 100 | 234 |
| 100 | 234 |
| 101 | 512 |
| 101 | 512 |
I tried with the following query in vain:
select id, case when isnull(attribute,'') = 'First'
then value
when isnull(attribute,'') = 'Second'
then value
when isnull(attribute,'') = 'Third'
then value
else '' end as EndResult
from
temp
Result
| id | EndResult |
|-----|-----------|
| 100 | 234 |
| 100 | 512 |
| 100 | 320 |
| 101 | 512 |
| 101 | 320 |
Please suggest if there's a way to get the expected result.

Answer 1

You can use analytical function like dense_rank to generate a numbering, and then select those rows that have the number '1':
select
x.id,
x.attribute,
x.value
from
(select
t.id,
t.attribute,
t.value,
dense_rank() over (partition by t.id order by t.attribute) as priority
from
Temp t) x
where
x.priority = 1
In your case, you can conveniently order by t.attribute, since their alphabetical order happens to be the right order. In other situations you could convert the attribute to a number using a case, like:
order by
case t.attribute
when 'One' then 1
when 'Two' then 2
when 'Three' then 3
end

Answer 2

In case the attribute column have different values which are not in alphabetical order as is the case above you can write as:
with cte as
(
select id,
attribute,
value,
case attribute when 'First' then 1
when 'Second' then 2
when 'Third' then 3 end as seq_no
from temp
)
, cte2 as
(
select id,
attribute,
value,
row_number() over ( partition by id order by seq_no asc) as rownum
from cte
)
select T.id,C.value as EndResult
from temp T
join cte2 C on T.id = C.id and C.rownum = 1
DEMO

Answer 3

Here is how you can achieve this using ROW_NUMBER():
WITH t
AS (
SELECT *
,ROW_NUMBER() OVER (
PARTITION BY id ORDER BY (CASE attribute WHEN 'First' THEN 1
WHEN 'Second' THEN 2
WHEN 'Third' THEN 3
ELSE 0 END)
) rownum
FROM TEMP
)
SELECT id
,(
SELECT value
FROM t t1
WHERE t1.id = t.id
AND rownum = 1
) end_result
FROM t;
For testing purpose, please see SQL Fiddle demo here:
SQL Fiddle Example

Answer 4

keep it simple
;with cte as
(
select row_number() over (partition by id order by (select 1)) row_num, id, value
from temp
)
select t1.id, t2.value
from temp t1
left join cte t2
on t1.Id = t2.id
where t2.row_num = 1
Result
id value
100 234
100 234
100 234
101 512
101 512