=>SELECT * FROM test1; x | y
---+--- a | 3 c | 2 b | 5 a | 1
(4 rows)
second query
=>SELECT x FROM test1 GROUP BY x; x
--- a b c
(3 rows)
In the second query, we could not have written SELECT * FROM test1 GROUP BY x, because there is no single value for the column y that could be associated with each group. The grouped-by columns can be referenced in the select list since they have a single value in each group.
24 лип. 2024 р. о 19:17 Tom Lane <tgl@sss.pgh.pa.us> пише:
"David G. Johnston" <david.g.johnston@gmail.com> writes:
I don't know that the existing wording is the most clear, but it is correct.
Perhaps it'd be better to write "... we could not have written SELECT x, y FROM test1 GROUP BY x, because ..." ? The first half of the example uses "SELECT *", and this bit was meant to be consistent with that; but if the reader is confused about how "y" got into the discussion then writing it out explicitly should help.
