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:
Verified answer as correct.
Reference: http://msdn.microsoft.com/en-us/library/ms180026.aspx
Reference: http://blog.sqlauthority.com/2009/03/11/sql-server-difference-between-union-vs-union-all-optimal-performance-comparison/