SQL Beginner question: CASE AS END for multiple columns? - sql

Question

I am making a stored procedure which creates a target data table (#tmp_target_table), does some checking on it, and outputs the results in a resultset table (#tmp_resultset_table). The resultset table needs to have multiple new columns: user_warning_id, user_warning_note, and user_warning_detail in addition to existing columns from #tmp_target_table.
I have a working stored procedure as in the following but this has some issue. I need to write conditionA, conditionB, and conditionB repeatedly but these conditions will need to be changed in the future. How would you write a code that is more extensible?
<Working code>
SELECT existing_col1, existing_col2,
CASE
WHEN conditionA
THEN user_warning_id_A
WHEN conditionB
THEN user_warning_id_B
WHEN conditionC
THEN user_warning_id_C
END AS user_warning_id,
CASE
WHEN conditionA
THEN user_warning_note_A
WHEN conditionB
THEN user_warning_note_B
WHEN conditionC
THEN user_warning_note_C
END AS user_warning_note,
CASE
WHEN conditionA
THEN user_warning_detail_A
WHEN conditionB
THEN user_warning_detail_B
WHEN conditionC
THEN user_warning_detail_C
END AS user_warning_detail,
existing_col3, existing_col4
INTO #tmp_resultset_table
FROM #tmp_target_table
SELECT * FROM #tmp_resultant_table

Answer 1

In SQL Server, you can use a lateral join (i.e., apply) to arrange the data so you can use a reference table:
select tt.*,
v2.user_warning_id, v2.user_warning_note, v2.user_warning_detail
from #tmp_target_table tt cross apply
(values (case when conditionA then 'a'
when conditionA then 'b'
when conditionA then 'c'
end)
) v(cond) left join
(values ('a', user_warning_id_A, user_warning_note_A, user_warning_detail_A),
('b', user_warning_id_B, user_warning_note_B, user_warning_detail_B),
('c', user_warning_id_C, user_warning_note_C, user_warning_detail_C)
) v2(cond, user_warning, user_warning_note, user_warning_detail)
on v2.cond = v.cond;
This also makes it pretty easy to add more levels, if you like.
Note: You could combine v and v2 into a single values list. I separated them, because you might want to consider making v2 an actual reference table.
EDIT:
DB2 supports lateral joins with the lateral keyword. I don't remember if DB2 supports values(). So try this:
select tt.*,
v2.user_warning_id, v2.user_warning_note, v2.user_warning_detail
from #tmp_target_table tt cross join lateral
(select (case when conditionA then 'a'
when conditionA then 'b'
when conditionA then 'c'
end)
from sysibm.sysdummy1
) v(cond) left join
(select 'a' as cond, user_warning_id_A as user_warning_id, user_warning_note_A as user_warning_note, user_warning_detail_A user_warning_detail
from sysibm.sysdummy1
union all
select 'b', user_warning_id_B, user_warning_note_B, user_warning_detail_B
from sysibm.sysdummy1
union all
select 'c', user_warning_id_C, user_warning_note_C, user_warning_detail_C
from sysibm.sysdummy1
) v2(cond, user_warning, user_warning_note, user_warning_detail)
on v2.cond = v.cond;

Answer 2

You could put the messages into a table and the condition logic into a function.
Just using temp tables so you can test it out.
Warnings
select warningID = 1, note = 'note 1', detail = 'notes on warning 1'
into #warning
union
select warningID = 2, note = 'note 2', detail = 'notes on warning 2'
union
select warningID = 3, note = 'note 3', detail = 'notes on warning 3'
union
select warningID = 4, note = 'note 4', detail = 'notes on warning 4'
Data values that have to meet conditions coming from some table ... #conditions
select condID = 1, val1 = 10, val2 = 1
into #conditions
union
select condID = 2, val1 = 20, val2 = 1
union
select condID = 3, val1 = 5, val2 = 2
union
select condID = 4, val1 = 30, val2 = 1
union
select condID = 4, val1 = 12, val2 = 1
Then a function that determines warnings based on conditions in the data. Takes values as input and returns a warningID
create function testWarningF
(
#val1In int
)
returns int
as
begin
declare #retVal int
select #retVal = case when #val1In <= 10 then 1
when #val1In > 10 and #val1In <=20 then 2
else 3
end
return #retVal
end
go
Then, the SQL ...
select *
from #conditions c
inner join #warning w on w.warningID = dbo.warningF(val1)
... returns this result
condID val1 val2 warningID note detail
1 10 1 1 note 1 notes on warning 1
2 20 1 2 note 2 notes on warning 2
3 5 2 1 note 1 notes on warning 1
4 12 1 2 note 2 notes on warning 2
4 30 1 3 note 3 notes on warning 3

Answer 3

Possibly the simplest method it to move the Conditions into a sub-select, then reference a token in the other select. E.g.
SELECT existing_col1
, existing_col2
, CASE CON
WHEN 'A' THEN user_warning_id_A
WHEN 'B' THEN user_warning_id_B
WHEN 'C' THEN user_warning_id_C END AS user_warning_id
, CASE CON
WHEN 'A' THEN user_warning_note_A
WHEN 'B' THEN user_warning_note_B
WHEN 'C' THEN user_warning_note_C END AS user_warning_note
, CASE CON
WHEN 'A' THEN user_warning_detail_A
WHEN 'B' THEN user_warning_detail_B
WHEN 'C' THEN user_warning_detail_C END AS user_warning_detail
, existing_col3
, existing_col4
FROM (
SELECT T.*
, CASE WHEN conditionA THEN 'A'
WHEN conditionB THEN 'B'
WHEN conditionC THEN 'C' END AS CON
FROM
#tmp_target_table T
)
although Gordon's answer is also neat, even though it adds two joins in the access plan. In Db2 Syntax, this works (on Db2 11.1.3.3 anyway)
select tt.*,
v2.user_warning_id, v2.user_warning_note, v2.user_warning_detail
from #tmp_target_table tt
, (values (case when conditionA then 'a'
when conditionB then 'b'
when conditionC then 'c'
end)
) v(cond) left join
(values ('a', 'user_warning_id_A', 'user_warning_note_A', 'user_warning_detail_A'),
('b', 'user_warning_id_B', 'user_warning_note_B', 'user_warning_detail_B'),
('c', 'user_warning_id_C', 'user_warning_note_C', 'user_warning_detail_C')
) v2(cond, user_warning_id, user_warning_note, user_warning_detail)
on v2.cond = v.cond;
testing with
create table #tmp_target_table(i int);
insert into #tmp_target_table(values 1);
create variable conditionA boolean;
create variable conditionB boolean default true;
create variable conditionC boolean;
returns
I USER_WARNING_ID USER_WARNING_NOTE USER_WARNING_DETAIL
- ----------------- ------------------- ---------------------
1 user_warning_id_B user_warning_note_B user_warning_detail_B