How to return values from same table? - sql

Question

I've two tables A and B. I want to return all records from A and only matching from B. I can use left join for this. But after joining, I want to return records based on a flag in the same table.
Table A:
| Col1 | Col2 |
|------|------|
| 123 | 12 |
| 456 | 34 |
| 789 | 56 |
Table B:
| Col1 | Col2 | Col3 | Col4 | Col5 |
|------|------|------|------|------|
| 123 | 12 | NULL | I | 1 |
| 456 | 34 | NULL | E | 1 |
| 111 | 98 | NULL | I | 1 |
| 222 | 99 | NULL | E | 1 |
| 123 | 12 | AB | NULL | 2 |
| 456 | 34 | CD | NULL | 2 |
| 123 | 12 | EF | NULL | 2 |
| 111 | 98 | GH | NULL | 2 |
| 222 | 99 | IJ | NULL | 2 |
After left joining A and B this how the result will look like:
| Col1 | Col2 | Col3 | Col4 | Col5 |
|------|------|------|------|------|
| 123 | 12 | NULL | I | 1 |
| 456 | 34 | NULL | E | 1 |
| 123 | 12 | AB | NULL | 2 |
| 456 | 34 | CD | NULL | 2 |
| 123 | 12 | EF | NULL | 2 |
| 789 | 56 | NULL | NULL | NULL |
1 and 2 values in Col5 tells if Col4 should be populated or Col3. 1 for Col4 and 2 for Col3.
I want to return all the records for 'I'(but excluding the record which has 'I') in Col4 which will look like this:
| Col1 | Col2 | Col3 | Col4 | Col5 |
|------|------|------|--------|------|
| 123 | 12 | AB | (null) | 2 |
| 123 | 12 | EF | (null) | 2 |
I also want to return records for 'E' (again excluding the record which has 'E') in col4 but for all the values other than one in Col3. In this case CD. Which would look like this:
| Col1 | Col2 | Col3 | Col4 | Col5 |
|------|------|------|--------|------|
| 456 | 34 | AB | (null) | 2 |
| 456 | 34 | EF | (null) | 2 |
| 456 | 34 | GH | (null) | 2 |
| 456 | 34 | IJ | (null) | 2 |
Can someone suggest how to handle this in SQL?

Answer 1

Ok I believe the following two queries achieve your desired results. You can see all the sample code via the following SQL Fiddle.
Existence Rule:
select A.*
, B.Col3
, B.Col4
, B.Col5
from TableA A
JOIN TableB B
on A.Col1 = B.Col1
and A.Col2 = B.Col2
and B.Col5 = 2
where exists (select 1 from TableB C
where C.col1 = B.col1 and C.col2 = B.col2
and c.col4 = 'I' AND C.col5 = 1)
Results:
| Col1 | Col2 | Col3 | Col4 | Col5 |
|------|------|------|--------|------|
| 123 | 12 | AB | (null) | 2 |
| 123 | 12 | EF | (null) | 2 |
Exclusion Rule:
select A.*
, B.Col3
, B.Col4
, B.Col5
from TableA A
CROSS JOIN TableB B
where b.col5 = 2
and exists (select 1 from TableB C
where C.col1 = a.col1 and C.col2 = a.col2
and c.col4 = 'E' AND C.col5 = 1)
and b.col3 not in (select col3 from TableB b
where b.col1 = a.col1 and b.col2 = a.col2 and b.col5 = 2)
Results:
| Col1 | Col2 | Col3 | Col4 | Col5 |
|------|------|------|--------|------|
| 456 | 34 | AB | (null) | 2 |
| 456 | 34 | EF | (null) | 2 |
| 456 | 34 | GH | (null) | 2 |
| 456 | 34 | IJ | (null) | 2 |

Answer 2

Result for I:-
;with cte1 As(select a.col1,a.col2 from A a left join B b on a.col1 =b.col2 and a.col2=b.col2 where b.col4 = 'I'),cte2 As(select b.col3,b.col4,b.col5 from from A a left join B b on a.col1 =b.col2 and a.col2=b.col2 where b.col4 <> 'I')
Result for E:-
select a.col1,a.col2,b.col3,b.col4,b.col5 from cte1 a cross join cte2 b
;with cte1 As(select a.col1,a.col2 from A a left join B b on a.col1 =b.col2 and a.col2=b.col2 where b.col4 = 'E'),cte2 As(select b.col3,b.col4,b.col5 from from A a left join B b on a.col1 =b.col2 and a.col2=b.col2 where b.col4 <> 'E')
select a.col1,a.col2,b.col3,b.col4,b.col5 from cte1 a cross join cte2 b

Answer 3

select c.col1, c.col2
from
(select a.col1, a.col2, b.col3 from a inner join table b on a.id = b.id
where "condition" ) c
where c.col1 = "condition"
This is the script. The explanation is:
Inside the () i wrote the first select. There, you will do the select with your joins and your conditions. At the end of the select i wrote "c" which is the name of the table generated from the sub-select.
Then, you'll select some values from the generated table and filter them with a where that will act on the results generated by the table created with the sub-select
EDIT: I used your question's names to make it easier