sql - Example for GROUP BY restriction -


in postgresql documentation, says:

when group present, not valid select list expressions refer ungrouped columns except within aggregate functions or if ungrouped column functionally dependent on grouped columns, since there otherwise more 1 possible value return ungrouped column

i don't understand part "there otherwise more 1 possible value return ungrouped column".

can please give me example of this? how can there more 1 possible value return ungroup column?

consider table:

col_1 | col_2 | col_3       10      10       11      20 b       20      40 c       40      60

and try running query sums on values of col_3

select col_1, col_2, sum(col_3)   t1 group col_1

the above query can have 2 possible outputs:

output 1: here, col_2 = 10 --------------------------- col_1   |   col_2   |   sum(col_3)           10          30       b           20          40 c           40          60  output 2: here, col_2 = 11 --------------------------- col_1   |   col_2   |   sum(col_3)           11          30  b           20          40 c           40          60

and because col_2 not included in group clause. summation on col_3 happens expected based on grouping col_1 sql engine not know whether wanted row having col_2 11 or 11. , hence, "there otherwise more 1 possible value return ungrouped column"

the above query work fine mysql, randomly return 1 of above 2 outputs whereas oracle/sql server throw error mentioned


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