Overview

• GROUP BY groups a selected set of rows into a set of summary rows by the values of one or more columns or expressions.
  • One row is returned for each group.
  • Rows that do not meet the conditions in the WHERE clause are removed before any grouping operation is performed.
• Aggregate functions provide information about each group instead of individual rows.
• The GROUP BY clause does not order the result set.
• If a grouping column contains null values, all null values are considered equal, and they are put into a single group.
• A GROUP BY clause can be described as general or simple:
  • A general GROUP BY clause includes GROUPING SETS, CUBE, ROLLUP.

SELECT 
	DATEPART(yyyy,OrderDate) AS N'Year',
	SUM(TotalDue) AS N'Total Order Amount'
FROM Sales.SalesOrderHeader
GROUP BY DATEPART(yyyy,OrderDate) 
ORDER BY DATEPART(yyyy,OrderDate);

GROUP BY vs. DISTINCT Performance

• While DISTINCT better explains the intent, and GROUP BY is only required when aggregations are present, they are interchangeable in many cases.
• In complex queries, DISTINCT can end up doing more work.
  • Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates.
  • GROUP BY can filter out the duplicate rows before performing any of that work.

HAVING Clause

• The WHERE clause filters out rows prior to them being aggregated.
• The HAVING clause allows filters out the aggregated rows based on a criteria.

SELECT DATEPART(yyyy,OrderDate) AS N'Year',
	SUM(TotalDue) AS N'Total Order Amount'
FROM Sales.SalesOrderHeader
GROUP BY DATEPART(yyyy,OrderDate)
HAVING DATEPART(yyyy,OrderDate) >= 2003
ORDER BY DATEPART(yyyy,OrderDate);

[ROLLUP()](<https://learn.microsoft.com/en-us/sql/t-sql/queries/select-group-by-transact-sql?group-by-rollup>)

• It generates the simple GROUP BY aggregate rows, plus subtotal or super-aggregate rows, and also a grand total row.
• The number of groupings that is returned equals the number of expressions in the <composite element list> plus one.

SELECT
	[Region] = [T].[Group],
	[Territory] = [T].[Name],
	[Year] = DATEPART(yyyy, [S].[OrderDate]),
	[Month] = DATEPART(mm, [S].[OrderDate]),
	[TotalRevenue] = SUM([S].[TotalDue])
FROM Sales.SalesOrderHeader AS [S]
INNER JOIN Sales.SalesTerritory AS [T]
	ON [S].TerritoryID = [T].TerritoryID
GROUP BY ROLLUP
(
	[T].[Group],
	[T].[Name],
	DATEPART(yyyy, [S].[OrderDate]), 
	DATEPART(mm, [S].[OrderDate])
)

[CUBE()](<https://learn.microsoft.com/en-us/sql/t-sql/queries/select-group-by-transact-sql?#group-by-cube-->)

• The CUBE does the same function but, instead of providing a hierarchy of totals in ordered super-aggregate rows, it provides all the ‘super-aggregate’ permutations (‘symmetric super-aggregate’ rows), also known as cross-tabulation rows.