Lesson 8 | Aggregate queries |

Objective | Define aggregate queries during SQL Data Selection |

An *aggregate query*^{[1]} is one that contains a specific
Transact-SQL function that performs a calculation on a set of values. This calculation is returned by the query as a single value. For example, if you wanted to know how much you are paying out in salaries, you could ask for the total of all values in the *Salary* column in the *Employees* table. Aggregates are used in conjunction with the GROUP BY clause.

There are ten aggregate functions within the Transact-SQL language. They are:

`AVG`

– Returns the average of all values in a group of values`COUNT`

– Returns the number of rows in the table(s) for which the criteria applies in a group of values`GROUPING`

– Used with On-line Analytical Processing (OLAP) functionality.`MAX`

– Returns the maximum value in a group of values`MIN`

– Returns the minimum value in a group of values`SUM`

– Returns the summarization, or addition, of all values in a group of values`STDEV`

– Returns the statistical standard deviation`STDEVP`

– Returns the statistical standard deviation for a population of values`VAR`

– Returns the statistical standard variance`VARP`

– Returns the statistical standard variance for a population of values

There are a couple of attributes to remember about these functions. Each of the functions ignore null values, except for the

`COUNT`

function. Also, each of the functions uses the following syntax:
In the next lesson, you will learn how to use aggregate functions in your queries.

[1] *Aggregate:* A query or part of a query that performs mathematical summing on one or more columns.