Aggregate Functions

Introduction

Function	Description
MAX()	maximum value of records that are NOT NULL in specified column
MIN()	minimum value of records that are NOT NULL in specified column
COUNT()	count of all records that are NOT NULL in specified column
AVG()	average of all records that are NOT NULL in specified column

• When passing a column or a expression as a parameter, these aggregate functions only work with NOT NULL values.

• To include NULL values, use *, COUNT(*) for example.

• Aggregate functions works for query also, not only tables.

• Aggregate functions accepts expression, when executing, those value from table or query will be calculated first and finally be aggregated.

SELECT [func]([column]|[expr]) AS [alias]
FROM [table]
-- JOIN [table2] ON [condition1]
-- WHERE [condition2]
-- and so on

Using DISTINCT

Use DISTINCT to filter unique records of table or query.

SELECT [func](DISTINCT [column]|[expr]) AS [alias]
FROM [table]
-- JOIN [table2] ON [condition1]
-- WHERE [condition2]
-- and so on

• Aggregate functions always appear with GROUP BY statement, once you see a aggregate functions, think of GROUP BY.