first query => 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 <t...@sss.pgh.pa.us> пише: > > "David G. Johnston" <david.g.johns...@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. > > regards, tom lane
