The GROUP BY Clause

Introduction

The GROUP BY clause separates records base on given condition into small tables, and finally UNION these small tables into the result table. The following example shows that GROUP BY separates records from a WHERE clause base on a column, each part of that can be considered as a temporary table will be passed to SELECT [func]([column]|[expr]) FROM [table], and generates multiple tables, finally UNIONthese table as result table.

SELECT [func]([column]|[expr])
FROM [table]
WHERE [condition]
GROUP BY [column1], [column3]
-- ORDER BY [column2]

• GROUP BY accepts multiple columns to group, basically means create multiple matches to a same value. The order of grouping follows how columns are placed after GROUP BY.

• Every column/expr selected will separately calculate the final value base on every group unit.