RIGHT JOIN in place of subselect - a genuine use case? - sql

Question

I have avoided RIGHT OUTER JOIN, since the same can be achieved using LEFT OUTER JOIN if you reorder the tables.
However, recently I have been working with the need to have large numbers of joins, and I often encounter a pattern where a series of INNER JOINs are LEFT JOINed to a sub select which itself contains many INNER JOINs:
SELECT *
FROM Tab_1 INNER JOIN Tab_2 INNER JOIN Tab_3...
LEFT JOIN (SELECT *
FROM Tab_4 INNER JOIN Tab_5 INNER JOIN Tab_6....
)...
The script is hard to read. I often encounter sub sub selects. Some are correlated sub-selects and performance across the board is not good (probably not only because of the way the scripts are written).
I could of tidy it up in several ways, such as using common table expressions, views, staging tables etc, but a single RIGHT JOIN could remove the need for the sub selects. In many cases, doing so would improve performance.
In the example below, is there a way to replicate the result given by the first two SELECT statements, but using only INNER and LEFT joins?
DECLARE #A TABLE (Id INT)
DECLARE #B TABLE (Id_A INT, Id_C INT)
DECLARE #C TABLE (Id INT)
INSERT #A VALUES (1),(2)
INSERT #B VALUES (1,10),(2,20),(1,20)
INSERT #C VALUES (10),(30)
-- Although we want to see all the rows in A, we only want to see rows in C that have a match in B, which must itself match A
SELECT A.Id, T.Id
FROM
#A AS A
LEFT JOIN ( SELECT *
FROM #B AS B
INNER JOIN #C AS C ON B.Id_C = C.Id) AS T ON A.Id = T.Id_A;
-- NB Right join as although B and C MUST match, we only want to see them if they also have a row in A - otherwise null.
SELECT A.Id, C.Id
FROM
#B AS B
INNER JOIN #C AS C ON B.Id_C = C.Id
RIGHT JOIN #A AS A ON B.Id_A = A.Id;
Would you rather see the long-winded sub-selects, or a RIGHT JOIN, assuming decent comments in each case?
All the articles I have ever read have said pretty much what I think about RIGHT JOINS, that they are unecessary and confusing. Is this case strong enough to break the cultural aversion?

Answer 1

As #jarlh wrote most people think LEFT to RIGHT as much more intuitive, so it's very confusing to see RIGHT joins in the code.
In this cases sometimes I found that SQL Server creates better query plans when I use OUTER APPLY in combination with WHERE EXISTS clauses, over your LEFT JOINs and inner INNER JOIN with WHERE EXISTS
The result is not much different of what you have in your first example:
SELECT A.Id, T.Id
FROM
#A AS A
OUTER APPLY (
SELECT C.Id FROM #C AS C
WHERE EXISTS (SELECT 1 FROM #B AS B WHERE A.Id = B.Id_a AND B.Id_C = C.Id) )T;

Answer 2

I have found an answer to this question in the old scripts that I was going through - I came across this syntax which performs the same function as the RIGHT JOIN example, using LEFT JOINs (or at least I think it does - it certainly gives the correct results in the example):
DECLARE #A TABLE (Id INT)
DECLARE #B TABLE (Id_A INT, Id_C INT)
DECLARE #C TABLE (Id INT)
INSERT #A VALUES (1),(2)
INSERT #B VALUES (1,10),(2,20),(1,20)
INSERT #C VALUES (10),(30)
SELECT
A.Id, C.Id
FROM
#A AS A
LEFT JOIN #B AS B
INNER JOIN #C AS C
ON C.Id = B.Id_C
ON B.Id_A = A.Id
I don't know if there is a name for this pattern, which I have not seen before in other places of work, but it seems to work like a "nested" join, allowing the LEFT JOIN to preserve rows from the later INNER JOIN.
EDIT: I have done some more research and apparently this is an ANSI SQL syntax for nesting joins, but... it does not seem to be very popular!
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