These are functions that can be used in the "columns" argument for select statements:

soql_cop_avg(): return the averge value for the given column when grouping
soql_cop_max(): return the maximum value for the given column when grouping
soql_cop_min(): return the minimum value for the given column when grouping
soql_cop_sum(): return the sum of all values for the given column when grouping
soql_cop_count(): use the COUNT() function in the query
soql_cop_count_distinct(): use the COUNT_DISTINCT() function in the query
soql_cop_format(): use the FORMAT() on the given column
soql_cop_convert_currency(): return the given column value in the default currency for the calling user
soql_cop_calendar_month(): return the calendar month of a date or date/time value
soql_cop_calendar_quarter(): return the calendar quarter of a date or date/time value
soql_cop_calendar_year(): return the calendar year of a date or date/time value
soql_cop_day_in_month(): return the day in a month of a date or date/time value
soql_cop_day_in_week(): return the day in the week of a date or date/time value (1 = Sunday, 7 = Saturday)
soql_cop_day_in_year(): return the day in the year of a date or date/time value
soql_cop_day_only(): return the date from a date/time value
soql_cop_fiscal_month(): return the fiscal month of a date or date/time value
soql_cop_fiscal_quarter(): return the fiscal quarter of a date or date/time value
soql_cop_fiscal_year(): return the fiscal year of a date or date/time value
soql_cop_hour_in_day(): return the hour in the day of a date/time value
soql_cop_week_in_month(): return the week in the month of a date or date/time value
soql_cop_week_in_year(): return the week in the year of a date or date/time value