Is having an 'OR' in an INNER JOIN condition a bad idea?

This kind of JOIN is not optimizable to a HASH JOIN or a MERGE JOIN.

It can be expressed as a concatenation of two resultsets:

SELECT  *
FROM    maintable m
JOIN    othertable o
ON      o.parentId = m.id
UNION
SELECT  *
FROM    maintable m
JOIN    othertable o
ON      o.id = m.parentId

, each of them being an equijoin, however, SQL Server's optimizer is not smart enough to see it in the query you wrote (though they are logically equivalent).

I use following code for get different result from condition That worked for me.

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Select A.column, B.column
FROM TABLE1 A
INNER JOIN
TABLE2 B
ON A.Id = (case when (your condition) then b.Id else (something) END)