Summarizing Data Results from a SQL Query
In this hour, you learn about SQL's aggregate functions. You can perform a variety of useful functions with aggregate functions.
The highlights of this hour include

What functions are

How functions are used

When to use functions

Using aggregate functions

Summarizing data with aggregate functions

Results from using functions
What Are Aggregate Functions?
Functions are keywords in SQL used to manipulate values within columns for output purposes. A function is a command always used in conjunction with a column name or expression. There are several types of functions in SQL. This hour covers aggregate functions. An aggregate function is used to provide summarization information for an SQL statement, such as counts, totals, and averages.
The aggregate functions discussed in this hour are
COUNT
SUM
MAX
MIN
AVG
The following queries show the data used for most of this hour's examples:
SELECT * FROM PRODUCTS_TBL;
PROD_ID PROD_DESC COST    11235 WITCHES COSTUME 29.99 222 PLASTIC PUMPKIN 18 INCH 7.75 13 FALSE PARAFFIN TEETH 1.1 90 LIGHTED LANTERNS 14.5 15 ASSORTED COSTUMES 10 9 CANDY CORN 1.35 6 PUMPKIN CANDY 1.45 87 PLASTIC SPIDERS 1.05 119 ASSORTED MASKS 4.95 1234 KEY CHAIN 5.95 2345 OAK BOOKSHELF 59.99 11 rows selected.
Some employees do not have a pager number in the results of the following query:
SELECT EMP_ID, LAST_NAME, FIRST_NAME, PAGER FROM EMPLOYEE_TBL;
EMP_ID LAST_NAM FIRST_NA PAGER     311549902 STEPHENS TINA 442346889 PLEW LINDA 213764555 GLASS BRANDON 3175709980 313782439 GLASS JACOB 8887345678 220984332 WALLACE MARIAH 443679012 SPURGEON TIFFANY 6 rows selected.
The COUNT Function
The COUNT function is used to count rows or values of a column that do not contain a NULL value. When used with a query, the COUNT function returns a numeric value. When the COUNT function is used with the DISTINCT command, only the distinct rows are counted. ALL (opposite of DISTINCT) is the default; it is not necessary to include ALL in the syntax. Duplicate rows are counted if DISTINCT is not specified. One other option with the COUNT function is to use COUNT with an asterisk. COUNT, when used with an asterisk (COUNT(*)), counts all the rows of a table including duplicates, whether a NULL value is contained in a column or not.
The syntax for the COUNT function is as follows:
COUNT [ (*)  (DISTINCT  ALL) ] (COLUMN NAME)
NOTE
The DISTINCT command cannot be used with COUNT(*), only with the COUNT(column_name) .
Example 
Meaning 
SELECT COUNT(EMPLOYEE_ID) FROM EMPLOYEE_PAY_ID 
Counts all employee IDs 
SELECT COUNT(DISTINCT SALARY)FROM EMPLOYEE_PAY_TBL 
Counts only the distinct rows 
SELECT COUNT(ALL SALARY)FROM EMPLOYEE_PAY_TBL 
Counts all rows for SALARY 
SELECT COUNT(*) FROM EMPLOYEE_TBL  Counts all rows of the EMPLOYEE table 
COUNT(*) is used in the following example to get a count of all records in the EMPLOYEE_TBL table. There are six employees.
SELECT COUNT(*) FROM EMPLOYEE_TBL;
COUNT(*)  6
COUNT(EMP_ID) is used in the next example to get a count of all the employee identifications that exist in the table. The returned count is the same as the last query because all employees have an identification number.
SELECT COUNT(EMP_ID) FROM EMPLOYEE_TBL;
COUNT(EMP_ID)  6
COUNT(PAGER) is used in the following example to get a count of all of the employee records that have a pager number. Only two employees had pager numbers.
SELECT COUNT(PAGER) FROM EMPLOYEE_TBL;
COUNT(PAGER)  2
The ORDERS_TBL table, shown next, is used in the following COUNT example:
SELECT * FROM ORDERS_TBL;
ORD_NUM CUST_ID PROD_ID QTY ORD_DATE_      56A901 232 11235 1 22OCT99 56A917 12 907 100 30SEP99 32A132 43 222 25 10OCT99 16C17 090 222 2 17OCT99 18D778 287 90 10 17OCT99 23E934 432 13 20 15OCT99 90C461 560 1234 2 7 rows selected.
This last example obtains a count of all distinct product identifications in the ORDERS_TBL table.
SELECT COUNT(DISTINCT(PROD_ID)) FROM ORDERS_TBL;
COUNT(DISTINCT(PROD_ID))  6
The PROD_ID 222 has two entries in the table, thus reducing the distinct values from 7 to 6.
NOTE
Because the COUNT function counts the rows, data types do not play a part. The rows can contain columns with any data type.
The SUM Function
The SUM function is used to return a total on the values of a column for a group of rows. The SUM function can also be used in conjunction with DISTINCT. When SUM is used with DISTINCT, only the distinct rows are totaled, which may not have much purpose. Your total is not accurate in that case because rows of data are omitted.
The syntax for the SUM function is as follows:
SUM ([ DISTINCT ] COLUMN NAME)
NOTE
The value of an argument must be numeric to use the SUM function. The SUM function cannot be used on columns having a data type other than numeric, such as character or date.
Example 
Meaning 
SELECT SUM(SALARY) FROM EMPLOYEE_PAY_TBL 
Totals the salaries 
SELECT SUM(DISTINCT SALARY) FROM EMPLOYEE_PAY_TBL  Totals the distinct salaries 
In the following query, the sum, or total amount, of all cost values is being retrieved from the PRODUCTS_TBL table:
SELECT SUM(COST) FROM PRODUCTS_TBL;
SUM(COST)  163.07
The AVG Function
The AVG function is used to find averages for a group of rows. When used with the DISTINCT command, the AVG function returns the average of the distinct rows. The syntax for the AVG function is as follows:
AVG ([ DISTINCT ] COLUMN NAME)
NOTE
The value of the argument must be numeric for the AVG function to work.
Example 
Meaning 
SELECT AVG(SALARY) FROM EMPLOYEE_PAY_TBL 
Returns the average salary 
SELECT AVG(DISTINCT SALARY) EMPLOYEE_PAY_TBL  Returns the distinct FROM average salary 
The average value for all values in the PRODUCTS_TBL table's COST column is being retrieved in the following example:
SELECT AVG(COST) FROM PRODUCTS_TBL;
AVG(COST)  13.5891667
NOTE
In some implementations, the results of your query may be truncated to the precision of the data type.
The next example uses two aggregate functions in the same query. Because some employees are paid hourly and others paid a salary, you want to retrieve the average value for both PAY_RATE and SALARY.
SELECT AVG(PAY_RATE), AVG(SALARY) FROM EMPLOYEE_PAY_TBL;
AVG(PAY_RATE) AVG(SALARY)   13.5833333 30000
The MAX Function
The MAX function is used to return the maximum value for the values of a column in a group of rows. NULL values are ignored when using the MAX function. The DISTINCT command is an option. However, because the maximum value for all the rows is the same as the distinct maximum value, DISTINCT is useless.
MAX([ DISTINCT ] COLUMN NAME)
Example 
Meaning 
SELECT MAX(SALARY) FROM EMPLOYEE_PAY_TBL 
Returns the highest salary 
SELECT MAX(DISTINCT SALARY) FROM EMPLOYEE_PAY_TBL  Returns the highest distinct salary 
The following example returns the maximum value for the COST column in the PRODUCTS_TBL table:
SELECT MAX(COST) FROM PRODUCTS_TBL;
MAX(COST)  59.99
The MIN Function
The MIN function returns the minimum value of a column for a group of rows. NULL values are ignored when using the MIN function. The DISTINCT command is an option. However, because the minimum value for all rows is the same as the minimum value for distinct rows, DISTINCT is useless.
MIN([ DISTINCT ] COLUMN NAME)
Example 
Meaning 
SELECT MIN(SALARY) FROM EMPLOYEE_PAY_TBL 
Returns the lowest salary 
SELECT MIN(DISTINCT SALARY) FROM EMPLOYEE_PAY_TBL  Returns the lowest distinct salary 
The following example returns the minimum value for the COST column in the PRODUCTS_TBL table:
SELECT MIN(COST) FROM PRODUCTS_TBL;
MIN(COST)  1.05
CAUTION
One very important thing to keep in mind when using aggregate functions with the DISTINCT command is that your query may not return the desired results. The purpose of aggregate functions is to return summarized data based on all rows of data in a table.
The final example combines aggregate functions with the use of arithmetic operators:
SELECT COUNT(ORD_NUM), SUM(QTY), SUM(QTY) / COUNT(ORD_NUM) AVG_QTY FROM ORDERS_TBL;
COUNT(ORD_NUM) SUM(QTY) AVG_QTY    7 160 22.857143
You have performed a count on all order numbers, figured the sum of all quantities ordered, and, by dividing the two figures, have derived the average quantity of an item per order. You also created a column alias for the computation—AVG_QTY.