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Grouped Set Functions - the GROUP BY Clause

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Normally, set functions return a single value, calculated from the set of all values in the column or expression.

If the SELECT statement includes a GROUP BY clause, set functions will be applied to groups of values. Columns used for GROUP BY do not have to be included in the SELECT list.

Find the most expensive NO SMOKING single room in each hotel.

 SELECT  HOTELCODE, MAX(PRICE) AS EXPENSIVE
 FROM    ROOM_PRICES
 WHERE   ROOMTYPE = 'NSSGLB'
 OR      ROOMTYPE = 'NSSGLS'
 GROUP BY HOTELCODE;

Result:

HOTELCODE	EXPENSIVE
LAP	800
SKY	870
STG	680
WIND	1410
WINS	1370

Restrictions when Using GROUP BY

Using a GROUP BY clause places some restrictions on the SELECT statement:

• Only constants, columns used in the GROUP BY clause, and columns used in set functions may be included in the SELECT list
• A column used in the GROUP BY clause may not be used in a set function.

How many hotels are there in each city?

 SELECT  CITY, COUNT(HOTELCODE)
 FROM    HOTEL
 GROUP BY CITY;
 

Result:

CITY
COPENHAGEN	1
GOTHENBURG	1
London	1
STOCKHOLM	2
UPPSALA	1

Column References

In a statement with column references in the SELECT list, all columns not used in set functions must be used as grouping columns.

Null Values

For grouping purposes, NULL values are regarded as equivalent. Thus for the example table:

SAMPLE
1.0
2.0
2.0
2.0
3.0
3.0
4.0
5.0
-
-

The following statement:

 SELECT  SAMPLE, COUNT(*) AS NUMBER
 ...
 GROUP BY SAMPLE;
 

Results in:

SAMPLE	NUMBER
1.0	1
2.0	3
3.0	2
4.0	1
5.0	1
-	2

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