Case having subquery for Oracle bot working - sql

Question

I am facing some problem in CASE query in Oracle
select
case
when substr(object_subtype,0,1) = '8'
then
'Planatias'
when substr(object_subtype,0,1) = '1'
then
'Licence'
when substr(object_subtype,0,1) = '4'
then
'PMA'
when substr(object_subtype,0,1) = '7'
then
'Location'
else
'no'
end objectType,
id ,substr(object_subtype,0,1)
from amatia_logtask order by 1
Now my problem is I have 4 different tables for each number from case
select * from amatia_licencias ;
select * from amatia_locacion ;
select * from amatia_pma;
select id_plantilla from amatia_plantillas;
And I want specific field from these 4 table with respect to their Id in CASE statment
but query like this
select
case
when substr(object_subtype,0,1) = '8'
then
select id_pma from amatia_plantillas where id_plantilla = substr(object_subtype,3)
when substr(object_subtype,0,1) = '1'
then
'Licence'
when substr(object_subtype,0,1) = '4'
then
'PMA'
when substr(object_subtype,0,1) = '7'
then
'Location'
else
'no'
end objectType,
id ,substr(object_subtype,0,1)
from amatia_logtask order by 1
is not working for me
giving This error
ORA-00936: missing expression
00936. 00000 - "missing expression"
*Cause:
*Action:
Error at Line: 5 Column: 15

Answer 1

CASE statements return expressions: SELECT statements don't count as expressions in this context. So don't use CASE here, use outer joins.
You haven't provided enough details for us to guarantee working SQL so you'll have to pick the bones out of this:
with logtask as ( select id
, substr(object_subtype,0,1) as st_1
, substr(object_subtype,3) as st_3
from amatia_logtask
)
select logtask.id
, logtask.st_1
, coalesce ( apla.id_pma
, alic.id_blah
, aloc.id_meh
, apma.id_etc
, 'no' ) as whatever
from logtask
left join amatia_plantillas pla
on logtask.st_1 = apla.id_plantilla
left join amatia_licencias alic
on logtask.st_1 = alic.id_licencia
left join amatia_locacion aloc
on logtask.st_1 = aloc.id_locacion
left join amatia_pma apma
on logtask.st_1 = apma.id_pma
order by 3, 1
/

Answer 2

It is possible to use subqueries - BUT they MUST only have one row and one column. Think about the result of a query in columns/rows :
| COL1 | COL2 |
|------|------|
| a | x |
| b | y |
each "cell" holds one value (and one value only)
| COL1 | COL2 |
|------|----------------------------------------------------------------------|
| a | cannot be a "select *" subquery because that is more than one column |
| b | y |
So, IF you use subqueries in a select clause, they can only return one value (one row, one column) so the subquery must be carefully written
CREATE TABLE A_TABLE
("COL1" varchar2(1), "COL2" varchar2(1))
;
INSERT ALL
INTO A_TABLE ("COL1", "COL2")
VALUES ('a', 'x')
INTO A_TABLE ("COL1", "COL2")
VALUES ('b', 'y')
SELECT * FROM dual
;
**Query 1**:
select
case
when col1 = 'a' then (select 'subquery 1' from dual)
when col1 = 'b' then (select 'subquery 2' from dual)
else (select 'one value' from dual)
end as col1_case
from a_table
**[Results][2]**:
| COL1_CASE |
|------------|
| subquery 1 |
| subquery 2 |
http://sqlfiddle.com/#!4/76041/3
I'm NOT recommending it, I'm merely showing the method. I would use joins in preference if it is feasible.