How does Inner join 3 or more tables works? - sql

Question

I saw some questions about the subject, but frankly i didn't quite understood.
What i want: I have five tables and i want to return a select with all intersections of the main table with the other 4. I'm currently doing 2 selects and then a UNION. It works great, but i guess i could improve the performance doing in a single select.
Here is an example of my code:
SELECT column1 FROM tableA A
INNER JOIN table B ON A.column1 = B.column1
LEFT JOIN table D ON A.column1 = D.column1
LEFT JOIN table E A.column1 = E.column1
WHERE B.column2 IS NOT NULL
AND B.column3 = '7'
AND ( (D.column2 = 'x' OR D.column3 = 'y') OR (E.column2 = 'x' OR E.column3 = 'y') )
UNION
SELECT column1 FROM tableA A
INNER JOIN table C ON A.column1 = C.column1
LEFT JOIN table D ON A.column1 = D.column1
LEFT JOIN table E A.column1 = E.column1
WHERE C.column2 IS NOT NULL
AND C.column3 = '7'
AND ( (D.otherColumn2= 'x' OR D.otherColumn3 = 'y') OR (E.otherColumn2 = 'x' OR E.otherColumn3 = 'y') )
About the tables.
Table A have the column1 shared with all the other tables:
(int) column1
Tables B and C have the same structure as follows:
(int) column1 | (int) column2 | (varchar) column3
And tables D and E also have the same structure:
(int) column1 | (varchar) otherColumn2 | (varchar) otherColumn3
As we can see, both selects are pretty much the same, with the exception of the table B and C.
I asked this question without the full code and #Kevin solved it. But with this extra information, and trying to follow his example, i changed the code to:
SELECT column1 FROM tableA A
LEFT JOIN table B ON (A.column1 = B.column1
AND C.column2 IS NOT NULL
AND C.column3 = '7')
LEFT JOIN table C ON (A.column1 = C.column1
AND C.column2 IS NOT NULL
AND C.column3 = '7')
LEFT JOIN table D ON A.column1 = D.column1
AND (D.otherColumn2 = 'x' OR D.otherColumn3 = 'y')
LEFT JOIN table E A.column1 = E.column1
AND (E.otherColumn2 = 'x' OR E.otherColumn3 = 'y')
WHERE
B.column1 is not null
or c.column1 is not null
or D.column1 is not null
or E.column1 is not null
But i getting a lot of extra results. I hope i'm making myself clear here, and if not, let me know and i will update this question.

Answer 1

You can always use exists:
select a.*
from tablea a
where exists (select 1 from tableb b where a.column1 = b.column1 and b.column3 = 7) or
exists (select 1 from tablec c where a.column1 = c.column1 and c.column3 = 7);
When you use join, you run the risk of getting duplicate rows -- and then you might do extra work to remove the duplicates. All unnecessary with exists.
And, for performance, you want indexes on tableb(column1, column3) and tablec(column1, column3).

Answer 2

This should get you what you want. However, I don't know that performance will be any better.
SELECT A.column1 FROM tableA A
LEFT JOIN tableB B ON A.column1 = B.column1
AND B.column2 IS NOT NULL
AND B.column3 = '7'
LEFT JOIN tableD d ON b.column1 = D.column1
LEFT JOIN tableE e ON B.column1 = E.column1
LEFT JOIN tableC C ON A.column1 = C.column1
AND C.column2 IS NOT NULL
AND C.column3 = '7'
LEFT JOIN tableD d2 ON c.column1 = d2.column1
LEFT JOIN tableE e2 ON c.column1 = e2.column1
WHERE (B.column1 is not null
and (d.othercolumn2 = 'x' or d.othercolumn3='y' or e.othercolumn2 = 'x' or e.othercolumn3='y')
)
or (c.column1 is not null
and (d2.othercolumn2 = 'x' or d2.othercolumn3='y' or e2.othercolumn2 = 'x' or e2.othercolumn3='y')
)
You can check it on sqlfiddle