Do all columns in a SELECT list have to appear in a GROUP BY clause

Question

My lecturer stated:

All column names in SELECT list must appear in GROUP BY clause unless name is used only in an aggregate function

I\'m ju

Answer 1

There are exceptions as noted by Sam Saffron but generally what your lecturer said is true.

If I select 3 columns and group by 2 what should the RDBMS do with the 3rd column?

The developers of the RDBMS may make a decision of how to handle the extra colum (as it appears MySQL's developers have) but is it the decision I would have made or the one I want when writing the select? Will the decision always be valid? I certainly prefer the Oracle-like approach of forcing me to explicitly state what should happen.

If I select 3 columns and group by 2 should the RDBS group by all 3, pick a random value from the 3rd, the biggest or littlest, the most common?