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:
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-alloptimalperformance-comparison/
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Form MSDN: Performance and query time
Note that UNION take longer time, since the duplicates must be removed after the two selects has ended.
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The question says “The data in the two tables is distinct from one another.” Hence, there’s no need to remove duplicates. However, the UNION ALL clause is more effective, as the SQL Engine implicitly eliminates the SORT operation required to return a DISTINCT result set.
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Option D aggregates the result set individually for both Domestic Sales and International Sales, and then merges the result into one contiguous result set; the UNION ALL eliminates duplicates (even though these may not be necessary).
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