On 2017/11/20 5:33 PM, x wrote:
Only if ColB, ColC and ColD are unique in their tables. Otherwise each join has
the potential of returning multiple rows, which will carry over to the next
joins.
Thanks David, I did say ColB, ColC & ColD were primary keys.
Any kind of Query (especially of the SELECT variety) can have multiple
redundancies or indeed omit-able specified values. An optimization to
ensure non-used left-joins on Primary keys are not used is probably a
negligible improvement.
To answer your question: You are correct to assume the same output -
There is no difference in the SET-Theory/Algebra producing those
specific rows whether you omit the left joins or not, BUT that only
holds true while the Indexes are indeed all PRIMARY (or at least Unique)
and the joins are of the "LEFT" variety. Change any of those and the
query output may look very different.
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