Your database contains two tables named DomesticSalesOrders and InternationalSalesOrders. Both tables contain more than 100 million rows. Each table has a
Primary Key column named SalesOrderId. The data in the two tables is distinct from one another.
Business users want a report that includes aggregate information about the total number of global sales and total sales amounts.
You need to ensure that your query executes in the minimum possible time.
Which query should you use?

A.
SELECT COUNT(*) AS NumberOfSales, SUM(SalesAmount) AS TotalSalesAmount FROM (
SELECT SalesOrderId, SalesAmount
FROM DomesticSalesOrders
UNION ALL
SELECT SalesOrderId, SalesAmount
FROM InternationalSalesOrders
) AS p

B.
SELECT COUNT(*) AS NumberOfSales, SUM(SalesAmount) AS TotalSalesAmount FROM (
SELECT SalesOrderId, SalesAmount
FROM DomesticSalesOrders
UNION
SELECT SalesOrderId, SalesAmount
FROM InternationalSalesOrders
) AS p

C.
SELECT COUNT(*) AS NumberOfSales, SUM(SalesAmount) AS TotalSalesAmount FROM DomesticSalesOrders
UNION
SELECT COUNT(*) AS NumberOfSales, SUM(SalesAmount) AS TotalSalesAmount FROM InternationalSalesOrders

D.
SELECT COUNT(*) AS NumberOfSales, SUM(SalesAmount) AS TotalSalesAmount FROM DomesticSalesOrders
UNION ALL
SELECT COUNT(*) AS NumberOfSales, SUM(SalesAmount) AS TotalSalesAmount FROM InternationalSalesOrders

Explanation:
http://msdn.microsoft.com/en-us/library/ms180026.aspx Reference: http://blog.sqlauthority.com/2009/03/11/sql-server-difference-between-union-vsunion-all- optimalperformance-comparison/