Group Functions
Sheila has learned that there are several group functions that are available for use. All are ANSI-standard SQL functions. 
- You can find the average of a group of values by using the AVG function.
- You can find the total of a group of values by using the SUM function.
- MAX finds a largest value. MIN returns a smallest value.
- STDDEV returns a standard deviation. VARIANCE returns the statistical variance.
- AVG , SUM , STDDEV , and VARIANCE can be used with numerical data only. You can use MAX and MIN on character, numeric, and date data types.
- You can use the COUNT function to return the number of values in a column.
Sheila will use the SQL GROUP functions in her SELECT statements to:
- Find the number of rows in a table
- Add the salaries
- Find the average, maximum, and minimum salaries
- Analyze salary information by department
To specify a group function, you write the name of the function followed by an argument in parentheses. The argument can be a column name, an expression, or a constant.
The COUNT function returns the number of non-null values in a column. If you count the values of a primary key column or specify COUNT(*), you will find the number of rows in a table because a primary key column cannot contain nulls.