EXPERT RESPONSE
Great question. No, it selects the same
column from all the rows it finds!
I would be surprised to
see a "SELECT 1" query all by itself as a stand-alone query.
By itself, it's useless. Like all SELECT statements,
it returns a result set, and in this case, the result set consists
of a number of rows, where each row has exactly one column, and
where the value of that column is an integer 1.
In other words, there's a 1 in each row, and nothing else!
The result set has as many rows as there are in the
table being selected from, subject to WHERE conditions.
Hence the result set, by itself, a table of 1s, is useless.
The only meaningful
information you can get from it is the number of rows, and that
can be obtained a lot more efficiently by using COUNT(*).
More likely, you saw it in a subselect.
SELECT 1 or SELECT * or SELECT NULL
are constructions commonly used in an EXISTS subselect.
In an EXISTS subselect, the database
does not actually "retrieve" rows,
and it does not always need to scan the entire result set
for the subselect, because just one row will provide an
answer. That answer is either TRUE or FALSE.
There's a somewhat contrived but illustrative example of
an EXISTS subselect in one of my previous answers,
How does WHERE EXISTS ( SELECT NULL... ) work? (22 February 2002).
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