## Aggregate functions in SQL

In database management, an aggregate function is a function where the values of multiple rows are grouped together as input on certain criteria to form a single value of more significant meaning.

An aggregate function performs a calculation on a set of values and returns a single value. Except for COUNT(*), aggregate functions ignore null values. Aggregate functions are often used with the GROUP BY clause of the SELECT statement.

## Various Aggregate Functions

1. AVG – calculates the average of a set of values.

2. COUNT – counts rows in a specified table or view.

3. MIN – gets the minimum value in a set of values.

4. MAX – gets the maximum value in a set of values.

5. SUM – calculates the sum of values.

Notice that all aggregate functions above ignore NULL values except for the COUNT function.

SQL aggregate functions syntax

aggregate_function (DISTINCT | ALL expression)

AVG() Aggregate Function

The AVG() aggregate function returns the average value in a column. For instance, to find the average salary for the employees who have less than 5 years of experience, the given query can be used.

 SELECT AVG(salary)
 FROM employees**
 WHERE experience < 5;

COUNT() Aggregate Function

The COUNT() aggregate function returns the total number of rows that match the specified criteria. For instance, to find the total number of employees who have less than 5 years of experience, the given query can be used.

 SELECT COUNT(*)
 FROM employees
 WHERE experience < 5;

MIN() Aggregate Function

The MIN() aggregate function returns the smallest value in a column. For instance, to find the smallest value of the amount column from the table named transactions, the given query can be used.

 SELECT MIN(amount) 
 FROM transactions;

MAX() Aggregate Function

The MAX() aggregate function takes the name of a column as an argument and returns the largest value in a column. The given query will return the largest value from the amount column.

 SELECT MAX(amount) 
 FROM transactions;

SUM() Aggregate Function

The SUM() aggregate function takes the name of a column as an argument and returns the sum of all the values in that column.

 SELECT SUM(salary)
 FROM salary_disbursement;

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