Using distinct on a column and doing order by on another column gives an error

Question

I have a table: abc_test with columns n_num, k_str.

This query doesnt work:

    select distinct(n_num) from abc_test order by(k_str)

B

Answer 1

According to SQL Standards, a SELECT clause may refer either to as clauses ("aliases") in the top level SELECT clause or columns of the resultset by ordinal position, and therefore nether of your queries would be compliant.

It seems Oracle, in common with other SQL implemetations, allows you to refer to columns that existed (logically) immediately prior to being projected away in the SELECT clause. I'm not sure whether such flexibility is such a good thing: IMO it is good practice to expose the sort order to the calling application by including the column/expressions etc in the SELECT clause.

As ever, you need to apply dsicpline to get meaningful results. For your first query, the definition of order is potentially entirely arbitrary.You should be grateful for the error ;)