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Queries
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Whenever possible, avoid using SELECT * queries, which return all columns from the specified tables. Instead, always specify a column list, which will ensure that you don t bring back any more columns than you re intending to, even as additional columns are added to underlying tables.
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Learning query basics
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For more information about writing queries, see the Query Fundamentals topic in SQL Server 2005 Books Online, which is installed as part of SQL Server 2005. Updates for SQL Server 2005 Books Online are available for download at www.microsoft.com/technet/prodtechnol/sql/2005/ downloads/books.mspx.
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How to Create Subqueries
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Subqueries are queries that are nested in other queries and relate in some way to the data in the query in which they are nested. The query in which a subquery participates is called the outer query. As you work with Transact-SQL, you will find that you often have many ways to write a query to get the same output, and each method will have different performance characteristics. For example, in many cases, you can use subqueries instead of joins to tune difficult queries. You can use subqueries in a variety of different ways and in any of the clauses of a SELECT statement. There are several types of subqueries available to database developers.
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Lesson 1: Querying Data
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The most straightforward subquery form is a noncorrelated subquery. Noncorrelated means that the subquery does not use any columns from the tables in the outer query. For instance, the following query selects all the employees from the Employee table if the employee s ID is in the EmployeeAddress table:
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SELECT * FROM HumanResources.Employee AS E WHERE E.EmployeeId IN ( SELECT AddressId FROM HumanResources.EmployeeAddress )
The outer query in this case selects from the Employee table, whereas the subquery selects from the EmployeeAddress table. You can also write this query using the correlated form of a subquery. Correlated means that the subquery uses one or more columns from the outer query. The following query is logically equivalent to the preceding noncorrelated version:
SELECT * FROM HumanResources.Employee AS E WHERE EXISTS ( SELECT * FROM HumanResources.EmployeeAddress EA WHERE E.EmployeeId = EA.EmployeeId )
In this case, the subquery correlates the outer query s EmployeeId value to the subquery s EmployeeId value. The EXISTS predicate returns true if at least one row is returned by the subquery. Although they are logically equivalent, the two queries might perform differently depending on your data or indexes. If you re not sure whether to use a correlated or noncorrelated subquery when tuning a query, test both options and compare their performances. You can also use subqueries in the SELECT list. The following query returns every employee s ID from the Employee table and uses a correlated subquery to return the employee s address ID:
SELECT EmployeeId, ( SELECT EA.AddressId FROM HumanResources.EmployeeAddress EA WHERE EA.EmployeeId = E.EmployeeId ) AS AddressId FROM HumanResources.Employee AS E
5
Working with Transact-SQL
Note that in this case, if the employee did not have an address in the EmployeeAddress table, the AddressId column would return NULL for that employee. In many cases such as this, you can use correlated subqueries and outer joins interchangeably to return the same data.
Quick Check
What is the difference between a correlated and noncorrelated subquery A correlated subquery references columns from the outer query; a noncorrelated subquery does not.
Quick Check Answer
Creating Queries That Use Complex Criteria
You often must write queries to express intricate business logic. The key to effectively doing this is to use a Transact-SQL feature called a case expression, which lets you build conditional logic into a query. Like subqueries, you can use case expressions in virtually all parts of a query, including the SELECT list and the WHERE clause. As an example of when to use a case expression, consider a business requirement that salaried employees receive a certain number of vacation hours and sick-leave hours per year, and nonsalaried employees receive only sick-leave hours. The following query uses this business rule to return the total number of hours of paid time off for each employee in the Employee table:
SELECT EmployeeId, CASE SalariedFlag WHEN 1 THEN VacationHours + SickLeaveHours ELSE SickLeaveHours END AS PaidTimeOff FROM HumanResources.Employee
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