Referencing other columns in queries using group by

SQL standards state that the group by clause must contain items from the select list. However, Transact-SQL allows you to specify any valid column name in either the group by or select list, whether they employ aggregates or not.

Sybase lifts restrictions on what you can include or omit in the select list of a query that includes group by:

• The columns in the select list are not limited to the grouping columns and columns used with the vector aggregates.

• The columns specified by group by are not limited to nonaggregate columns in the select list.

A vector aggregate must be accompanied by a group by clause. The SQL standards require nonaggregate columns in the select list to match the group by columns. However, the first bulleted item above allows you to specify additional, extended columns in the select list of the query.

For example, many versions of SQL do not allow the inclusion of the extended title_id column in the select list, but Transact-SQL: allows this:

select type, title_id, avg(price), avg(advance)
from titles
group by type
type title_id

------------ -------- ----- -------
business BU1032 13.73 6,281.25
business BU1111 13.73 6,281.25
business BU2075 13.73 6,281.25
business BU7832 13.73 6,281.25
mod_cook MC2222 11.49 7,500.00
mod_cook MC3021 11.49 7,500.00
UNDECIDED MC3026 NULL NULL
popular_comp PC1035 21.48 7,500.00
popular_comp PC8888 21.48 7,500.00
popular_comp PC9999 21.48 7,500.00
psychology PS1372 13.50 4,255.00
psychology PS2091 13.50 4,255.00
psychology PS2106 13.50 4,255.00
psychology PS3333 13.50 4,255.00
psychology PS7777 13.50 4,255.00
trad_cook TC3218 15.96 6,333.33
trad_cook TC4203 15.96 6,333.33
trad_cook TC7777 15.96 6,333.33
(18 rows affected)

The above example still aggregates the price and advance columns based on the type column, but its results also display the title_id for the books included in each group.

The second bullet item described above allows you to group columns that are not specified as columns in the select list of the query. These columns do not appear in the results, but the vector aggregates still compute their summary values. For example:

select state, count(au_id)
from authors
group by state, city

state
----- --------
CA 2
CA 1
CA 5
CA 5
CA 2
CA 1
CA 1
CA 1
CA 1
IN 1
KS 1
MD 1
MI 1
OR 1
TN 1
UT 2
(16 rows affected)

This example groups the vector aggregate results by both state and city, even though it does not display which city belongs to each group. Therefore, results are potentially misleading.

You may think the following query should produce similar results to the previous query, since only the vector aggregate seems to tally the number of each city for each row:

select state, count(au_id)
from authors
group by city

However, its results are much different. By not using group by with both the state and city columns, the query tallies the number of each city, but it displays the tally for each row of that city in authors rather than grouping them into one result row per city.

state
----- -----------
CA 1
CA 5
CA 2
CA 1
CA 5
KS 1
CA 2
CA 2
CA 1
CA 1
TN 1
OR 1
CA 1
MI 1
IN 1
CA 5
CA 5
CA 5
MD 1
CA 2
CA 1
UT 2
UT 2

(23 rows affected)