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
http://blog.sqlauthority.com/2009/03/11/sql-server-difference-between-union-vs-unionall-optimalperformance-comparison/
A
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A.
SELECT COUNT(*) AS NumberOfSales, SUM(SalesAmount) AS TotalSalesAmount
FROM (
SELECT SalesOrderId, SalesAmount
FROM DomesticSalesOrders
UNION ALL
SELECT SalesOrderId, SalesAmount
FROM InternationalSalesOrders
) AS p
UNION ALL – A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.
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