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Limiting Returned Results by Using the TABLESAMPLE Clause
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In some cases, you might want to evaluate only a small random subset of the returned values for a certain query. This can be especially relevant, for instance, when testing large queries. Instead of seeing the entire result set, you might want to analyze only a fraction of its rows. The TABLESAMPLE clause lets you specify a target number of rows or percentage of rows to be returned. The SQL Server query engine randomly determines the segment from which the rows will be taken. The following query returns approximately 10 percent of the addresses in the Address table:
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SELECT * FROM Person.Address TABLESAMPLE(10 PERCENT)
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TABLESAMPLE returns random rows
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The TABLESAMPLE clause works by returning rows from a random subset of data pages determined by the percentage specified. Because some data pages contain more rows than others, this means that the number of returned rows will almost never be exact. When using the TABLESAMPLE clause, do not write queries that expect an exact number of rows to be returned.
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Working with Transact-SQL
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Query and Pivot Employees Pay Rates
In the following practice exercises, you will write queries that retrieve employees pay rate information using aggregate functions and then pivot the data using the PIVOT operator.
Practice 1: Retrieve Employees Current Pay Rate Information
In this exercise, you will practice writing a query that uses aggregate functions to get employees current pay rate information from the AdventureWorks database. 1. Open SSMS and connect to your SQL Server. 2. Open a new query window and select AdventureWorks as the active database. 3. Type the following query and execute it:
SELECT EPH.EmployeeId, EPH.Rate, EPH.RateChangeDate FROM HumanResources.EmployeePayHistory EPH
4. This shows that the table EmployeePayHistory has one row for each employee s pay rate and the date it changed. 5. To find the current pay rate, you need to determine which change date is the maximum for each employee. 6. Type the following query and execute it:
SELECT EPH.EmployeeId, EPH.Rate, EPH.RateChangeDate FROM HumanResources.EmployeePayHistory EPH WHERE EPH.RateChangeDate = ( SELECT MAX(EPH1.RateChangeDate) FROM HumanResources.EmployeePayHistory EPH1 )
7. This query, however, returns rows for only a few of the employees; it uses a noncorrelated subquery, which gets the most recent RateChangeDate for the whole table. So only employees who had their rate changed on that day are returned. Instead, you need to use a correlated subquery. For each employee, the query needs to compare the most recent RateChangeDate.
Lesson 1: Querying Data
8. Type the following query and execute it:
SELECT EPH.EmployeeId, EPH.Rate, EPH.RateChangeDate FROM HumanResources.EmployeePayHistory EPH WHERE EPH.RateChangeDate = ( SELECT MAX(EPH1.RateChangeDate) FROM HumanResources.EmployeePayHistory EPH1 WHERE EPH1.EmployeeId = EPH.EmployeeId )
9. This query, which uses the correlated subquery, returns the most recent pay rate for every employee.
Practice 2: Pivot Employees Pay Rate History
In this exercise, you will practice writing a query that uses the PIVOT operator to create a report that shows each employee s pay rate changes in each year. 1. If necessary, open SSMS and connect to your SQL Server. 2. Open a new query window and select AdventureWorks as the active database. 3. Type the following query and execute it:
SELECT EmployeeId, YEAR(RateChangeDate) AS ChangeYear, Rate FROM HumanResources.EmployeePayHistory
4. This query returns the rate of each change made for each employee, along with the year in which the change was made. 5. Next, you need to store this information in a derived table, as the following query shows:
SELECT * FROM ( SELECT EmployeeId, YEAR(RateChangeDate) AS ChangeYear, Rate FROM HumanResources.EmployeePayHistory ) AS EmpRates
6. Execute the query and then analyze the years returned. Notice that the data ranges between 1996 and 2003.
5
Working with Transact-SQL
7. You can now pivot this derived table. One requirement of PIVOT is to use an aggregate function on the data being pivoted. Because that data is employee salary, the most obvious function is MAX, which would report the maximum change for each year. 8. Based on the date range in the data and the chosen aggregate function, the following PIVOT query can be written:
SELECT * FROM ( SELECT EmployeeId, YEAR(RateChangeDate) AS ChangeYear, Rate FROM HumanResources.EmployeePayHistory ) AS EmpRates PIVOT ( MAX(Rate) FOR ChangeYear IN ( [1996], [1997], [1998], [1999], [2000], [2001], [2002], [2003] ) ) AS Pvt
9. Executing this query returns a report with a column for each year, showing whether or not the employee received a pay rate change during that year. Years without changes show NULL for that employee.
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