sql join scenario - sql

Question

I have 2 tables name like A and B. A have columns say X, Y and Z and Table B have coulmns say P, Q and R. here in my case table have blank data for few rows in all the columns.
I need to join these 2 tables such that If A.X<>'' and B.X<>'' then It should join the table. If A.X='' and B.X='' then
it should check the next columns A.Y<>'' and B.Y<>''. If this is also blank it should join the table on next condition A.Z<>'' and B.Z<>''. If all these 3 condition have blanks It should not join for that row.
How can we achieve this using sql join?
Thanks in advance

Answer 1

You can go for conditional JOINS as given below:
SELECT *
FROM A
LEFT OUTER JOIN B as b1
ON A.X = b1.X AND B1.X <> '' -- JOIN only rows WHERE x is not blank
LEFT OUTER JOIN B as b2
ON A.Y = b2.Y AND b2.Y <> '' -- JOIN only rows WHERE y is not blank
LEFT OUTER JOIN B AS b3
ON A.Z = b3.Z AND b3.Z <> '' -- JOIN only rows WHERE z is not blank
WHERE
b1.X IS NOT NULL OR
b2.Y IS NOT NULL OR
b3.Z IS NOT NULL

Answer 2

Ramu's answer is close (I upvoted it) but it needs to be refined. The important part of the answer that is correct -- the equality conditions in the JOINs make the query easier to optimize.
However, it is better written as:
SELECT a.*,
COALESCE(b1.P, b2.P, b3.P) as P,
COALESCE(b1.Q, b2.Q, b3.Q) as Q,
COALESCE(b1.R, b2.R, b3.R) as R
FROM A LEFT JOIN
B b1
ON A.X = b1.X LEFT JOIN
B b2
ON A.Y = b2.Y AND
b1.X IS NULL LEFT JOIN -- no previous match
B b3
ON A.Z = b3.Z AND
b2.Y IS NULL AND
b1.X IS NULL -- no previous match
WHERE b1.X IS NOT NULL OR
b2.Y IS NOT NULL OR
b3.Z IS NOT NULL ;
The two key changes are:
The LEFT JOIN conditions check that previous columns did not match.
The SELECT uses COALESCE() to fetch columns.
Also, I don't think the condition on empty strings is needed. There will be no match if there are no empty string values in B for that column. If both tables have empty strings, then you apparently do want a match -- and an empty string matches an empty string in SQL Server.
You can also express this using APPLY:
SELECT a.*, b.*
FROM A CROSS APPLY
(SELECT TOP (1) WITH TIES B.*
FROM B
WHERE A.X = b.X OR
A.Y = b.Y OR
A.Z = b.Z
ORDER BY (CASE WHEN A.X = B.X THEN 1
WHEN A.Y = B.Y THEN 2
WHEN A.Z = B.Z THEN 3
)
) B;
However, I would expect the previous version to have much better performance.

Answer 3

Use a CASE expression in the ON clause:
SELECT *
FROM A INNER JOIN B
ON 1 = CASE
WHEN A.X <> '' AND B.P <> '' AND A.X = B.P THEN 1
WHEN A.Y <> '' AND B.Q <> '' AND A.Y = B.Q THEN 1
WHEN A.Z <> '' AND B.R <> '' AND A.Z = B.R THEN 1
END
or:
SELECT *
FROM A INNER JOIN B
ON 1 = CASE
WHEN A.X <> '' AND A.X = B.P THEN 1
WHEN A.Y <> '' AND A.Y = B.Q THEN 1
WHEN A.Z <> '' AND A.Z = B.R THEN 1
END
There is no need for further joins.
The CASE expression makes sure that each condition will be checked in the order that you want it to be checked.
So if the 1st condition is satisfied then the rows of the 2 tables will be joined, if not then the 2nd condition will be checked and so on.