oracle SQL select/join with many-to-one and nulls - sql

Question

I'm having trouble crafting an Oracle SQL query that returns the results I seek. It's possible that what I'm trying to do isn't possible.
For a given code in table one, if the code exists in table two and ANY of the flags are "1", then the status should be "1" in the query results. Otherwise, the status should be "0". If the code doesn't exist at all in table 2, then the status should be null.
tab1
------------
id,code
------------
1,ABC
2,DEF
3,GHI
4,JKL
5,MNO
6,PQR
7,STU
tab2
------------
id,code,flag
------------
1,ABC,0
2,ABC,0
3,DEF,1
4,DEF,1
5,GHI,0
6,GHI,1
7,JKL,1
8,JKL,0
9,MNO,0
10,PQR,1
(query?)
result
------------
id,code,status
------------
1,ABC,0
2,DEF,1
3,GHI,1
4,JKL,1
5,MNO,0
6,PQR,1
7,STU,null
So far, the only query I've been able to come up with is this, which doesn't give the right results in the status column...
select tab1.*, (select * from (
select flag from tab2 where tab2.code = code order by flag desc)
where rownum <=1) as status from tab1;
... status is always "1", which is incorrect.
I'm thinking that instead of using order by and selecting the first result, it might be possible to instead count the number of "1" flags for each code, but I'm not sure if that would work.

Answer 1

My first inclination is to use a subselect:
select t1.*,
(select max(t2.flag)
from table2 t2
where t2.code = t1.code
) as t2flag
from table1 t1;
You can also phrase this as a left join with an aggregation:
select t1.*, t2.flag
(select max(t2.flag)
from table2 t2
where t2.code = t1.code
) as t2flag
from table1 t1 left join
(select t2.code, max(t2.flag) as flag
from table2 t2
group by t2.code
) t2
on t2.code = t1.code;
Both these methods are assuming that flag is either 0 or 1, as in your question.

Answer 2

I'd use a LEFT JOIN:
select t1.id, max(t1.code) code, max(t2.flag) status
from table1 t1
left join table2 t2 on t1.code=t2.code
group by t1.id

Answer 3

Assuming that 0 and 1 are the only two flags, what about something like:
with tab1 as (select 1 id, 'ABC' code from dual union all
select 2 id, 'DEF' code from dual union all
select 3 id, 'GHI' code from dual union all
select 4 id, 'JKL' code from dual union all
select 5 id, 'MNO' code from dual union all
select 6 id, 'PQR' code from dual union all
select 7 id, 'STU' code from dual),
tab2 as (select 1 id, 'ABC' code, 0 flag from dual union all
select 2 id, 'ABC' code, 0 flag from dual union all
select 3 id, 'DEF' code, 1 flag from dual union all
select 4 id, 'DEF' code, 1 flag from dual union all
select 5 id, 'GHI' code, 0 flag from dual union all
select 6 id, 'GHI' code, 1 flag from dual union all
select 7 id, 'JKL' code, 1 flag from dual union all
select 8 id, 'JKL' code, 0 flag from dual union all
select 9 id, 'MNO' code, 0 flag from dual union all
select 10 id, 'PQR' code, 1 flag from dual)
select t1.id,
t1.code,
t2.flag status
from tab1 t1
left join (select code,
max(flag) flag
from tab2
group by code) t2
on (t1.code = t2.code);
ID CODE STATUS
---------- ---- ----------
5 MNO 0
6 PQR 1
2 DEF 1
1 ABC 0
3 GHI 1
4 JKL 1
7 STU

Answer 4

Maybe I'm missing something, but it seems to be as simple as this:
select tab1.id, tab1.code, max(tab2.flag) status
from tab1
left join tab2 on tab1.code = tab2.code
group by tab1.id, tab1.code
order by tab1.id;
A sample SQL Fiddle gives the desired result:
| id | code | status |
|----|------|--------|
| 1 | ABC | 0 |
| 2 | DEF | 1 |
| 3 | GHI | 1 |
| 4 | JKL | 1 |
| 5 | MNO | 0 |
| 6 | PQR | 1 |
| 7 | STU | (null) |

Answer 5

SELECT ID,
CODE,
MAX( STATUS)
FROM
(SELECT DISTINCT T1.ID,
T1.CODE,
FLAG AS STATUS
FROM T1
LEFT JOIN T2
ON T1.CODE = T2.CODE
)
GROUP BY ID,
CODE;

Answer 6

This gives result as you asked:
SELECT tab1.id,tab1.code,max(tab2.flag) as Status
FROM tab1 LEFT JOIN tab2
ON tab1.code=tab2.code
GROUP BY tab2.code
ORDER BY tab1.id;