Picking one row over another - sql

Question

I have a table that looks like this:
ID Type Value
A Z01 10
A Z09 20
B Z01 30
C Z01 40
D Z09 50
E Z10 60
For each ID I would like to retrieve a value. Ideally the value should come from the row with type Z01. However, if Z01 is not available I'll pick Z09 instead. If nothing is available I would like to select nothing.
The result would look like this:
Id Type Value
A Z01 10
B Z01 30
C Z01 40
D Z09 50
How can I accomplish this with T-SQL?

Answer 1

This should give you what you want:
select *
from table t
where 1 = case
when t.type = 'Z01'
then 1
when t.type = 'Z09'
and not exists (select 1 from table where id = t.id and type = 'Z01')
then 1
else 0
end
An alternative, with using a more common approach is (re-writing the CASE expression):
select *
from table
where type = 'Z01'
OR (type = 'Z09' and not exists (select 1 from table where id = t.id and type = 'Z01'))
An obvious sargable approach (which will make your query use the appropriate index on your table, if it exists) would be:
select *
from table
where type = `Z01`
union all
select *
from table
where type = `Z09`
and not exists (select 1 from table where id = t.id and type = 'Z01')
And when I'm saying index I'm talking about a non-clustered index on the type column.

Answer 2

You can use query like this
;WITH cte
AS (SELECT
*, ROW_NUMBER() OVER (PARTITION BY id ORDER BY type) AS rn
FROM yourtable
WHERE type IN ('Z01', 'Z09'))
SELECT
id, type, value
FROM cte
WHERE rn = 1

Answer 3

I would use window functions:
select t.*
from (select t.*,
row_number() over (partition by id
order by (case when type = 'Z01' then 1
when type = 'Z09' then 2
end)
) as seqnum
from t
) t
where seqnum = 1;
I don't fully understand teh statement "if nothing is available, then choose nothing". If, by this, you mean you want 'Z01', 'Z09' or nothing at all, then just do:
select t.*
from (select t.*,
row_number() over (partition by id order by type) as seqnum
from t
where type in ('Z01', 'Z09')
) t
where seqnum = 1;
The where clause does the filtering, and you can just order by the type alphabetically.

Answer 4

Something like this:
select distinct
base.Id,
Coalesce(z01.Type, any.Type) Type,
Coalesce(z01.Value, any.Value)
from (select distinct id from [table]) base
left outer join [table] z01
on z01.id = base.id
and z01.Type = 'Z01'
left outer join [table] any
on any.id = base.id
and any.type != 'Z01'

Answer 5

Start with a WHERE clause. Select only entries with Z01 and Z09. Then use ROW_NUMBER to rank your rows and keep the best.
select id, type, value
from
(
select
id, type, value,
row_number()
over (partition by id order by case when type = 'Z01' then 1 else 2 end) as rn
from mytable
where type in ('Z01','Z09')
) ranked
where rn = 1;

Answer 6

Then try this
Select distinct a.id,
coalesce(t1.type, t9.type) type,
case when t1.type is not null
then t1.value else t9.value end value
From table a
left join table t1
on t1.id = a.id
and t1.type = 'Z01'
left join table t9
on t9.id = a.id
and t9.type = 'Z09'
Where a.type in ('Z01', 'Z09') -- < -- this last to eliminate row E