SQL Aggregate Functions

SQL Aggregate functions perform calculations in an SQL query.  Aggregates are most often used in combination with a Group By clause.

Below are the SQL aggregate functions available in an MS Access query:

Aggregate Function	Description
Select Sum	Total (count) of the field values
Select Avg	Average of the field values
Select Min	Lowest (minimum) field value
Select Max	Highest (maximum) field value
Select Count	Count of the values other than nulls
Select StDev	Standard deviation of the field values including date/time fields
Select Var	Variance of the field values including date/time

Here are some examples of aggregate function usage:

Select Count Aggregate Function Query

Select Count(Emp_ID) From M_Employees;

The query above simply counts the autonumber field in the M_Employees table.

Select Average Aggregate Function Example

Select Avg(Emp_Salary) From M_Employees Where Emp_Age<50;

The above aggregate query determines the average salary for employees under 50 years of age.

The SQL aggregate query below gets a little interesting by showing you how to answer more complex question of your data:

M_Employees

ID	Emp_Name	Emp_Salary	Emp_Age
1	Joe	$18.00	51
2	billy	$17.00	52
3	Molly	$16.00	53
4	bobby	$15.00	41
5	robert	$14.00	42
6	milly	$13.00	43
7	harry	$12.00	44
8	ed	$11.00	45
9	sally	$10.00	46

SELECT avg(iif(emp_age>=50,Emp_Salary,null)) as Over_50_Salary,avg(iif(emp_age<50,emp_salary,null)) as Under_50_Salary
FROM M_Employees;

Over_50_Salary	Under_50_Salary
$17.00	$12.50

Note the use of the immediate if (iif) to bracket the results and return two columns where the would normally be only one.

Note there is no Group By which is normally associated with an SQL aggregate function query.

Note we are using aliases (as) to generate our own column names.

Note nulls are skipped in the average aggregate and that is what makes this query work.

Select StDev Query

Using the Employee table shown above now we get the standard deviation of the salary column:

SELECT StDev([Emp_Salary]) AS Salary_Standard_Deviation FROM M_Employees;

Results are pretty simple:

SQL Select Var Aggregate Variance Query

SELECT Var([Emp_Salary]) AS Salary_Variance FROM M_Employees;

The results are shown below:

Here are some additional techniques to consider...

When creating a report it is often useful to have the Count Average and Standard Deviation listed at the bottom of each column of numbers.  As fairly simple technique is to use a series of union queries to build the result rows.

The first query would retrieve all the raw numbers in a multi row list.

The second union query would append the aggregate count function.

The third union query would add the aggregate average function.  Finally the last union select would add the standard deviation.

Another technique to try out is to use the aggregate functions in the scalar query.

More Aggregate Function Information:
 

Domain Aggregate Functions in MS Access

SQL Aggregate Function Examples: Dlookup, Dmin, Dmax, Dlast, Dfirst, DAvg, DSum, Dcount, DStdev, DstdevP,Dvar, DvarP. www.blueclaw-db.com/domain_aggregate_function/

Advanced Union Query Download

In this example we selecting the individual data records from the table and at the same time using the union query function to select SQL Aggregate functions ... www.blueclaw-db.com/download/union_query_advanced.htm