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/
Answer is A.
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A
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Answer is A.
In this scenario, the data is distinct so UNION ALL will be faster.
Using UNION clause will try to identify the distinct records which impacts the query performance.
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A
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Why A and not D?
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Yes… Why A? Someone who can give an answer?
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It’s A, test it out.
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Best Regards!
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UNION removes duplicate records (where all columns in the results are the same), UNION ALL does not.
There is a performance hit when using UNION vs UNION ALL, since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports).
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No, you wrong!
I made a test right now…
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