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Defining a WHERE Clause
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As shown in the previous example, the WHERE clause is used in most cases to limit the number of rows returned in the result set. Including well-written WHERE clauses typically increases query performance by limiting the amount of data that needs to be sent back to the client application.
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UnDeRStanDinG QUeRY PROceSSinG anD PeRFORMance
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For more in-depth information about how SQL Server processes queries and about advanced querying techniques, see Inside Microsoft SQL Server 2008: T-SQL Querying (Microsoft Press, 2009), by Itzik Ben-Gan, Lubor Kollar, Steve Kass, and Dejan Sarka.
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The WHERE clause can include a variety of search conditions that can include Boolean operators and predicates such as LIKE , BETWEEN, EXISTS, IS NULL, IS NOT NULL, and CONTAINS. As mentioned earlier, the IS (IS NOT) NULL clause returns rows based on the existence of NULL values in the named column. The CONTAINS clause is available only when you create a full text index on the column being compared. For more information about full text searches, see 8, Extending Microsoft SQL Server Functionality with the Spatial, Full-Text Search, and Service Broker. Boolean operators, which include AND, OR, and NOT, can be used to define more than one criterion in a WHERE clause. The following sample code returns only products that have a color attribute of Silver and also have a list price greater than $200:
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SELECT * FROM Production.Product WHERE Color = 'Silver' AND ListPrice > 200
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The following sample assumes that the Product table includes newly added products that do not yet have a list price assigned to them. The following sample code includes Silver products with a price over $200, as well as the new Silver products with a list price of $0 (the default list price for new products added). If the parentheses were not added, the query would return Silver products with a price over $200 and all colors of products with a list price of $0 because of the order in which the conditions are evaluated, which is based on the Boolean order of operations:
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SELECT * FROM Production.Product WHERE Color = 'Silver' AND (ListPrice > 200 OR ListPrice = 0)
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iMPortant
ORDeR OF OPeRatiOnS
When the database engine parses and compiles a query, conditions that include Boolean operators are evaluated in the following order: NOT, AND, OR. This order of operations is important to understand because misunderstanding this can cause the query to return an unintended result set. You can use parentheses to control the order of operations.
Lesson 1: Querying Data
real World
Ann Weber
long time ago, I was teaching a class on Microsoft System Management Server (SMS). One of my students was trying to query the SQL database created by SMS
that contained all the information about client computers on the network. He was
trying to build a report that included computers with certain hardware specifications that did not have Microsoft Windows 98 (I told you it was a long time ago) installed on them yet. Because he was using a combination of AND, NOT, and OR and did not understand the order of operations for the Boolean operators, he was getting incorrect results and was blaming SMS because the installations were failing for various reasons (Windows 98 already installed, improper hardware configurations, and so on). I asked him to bring his query to class with him the next day. We added some parentheses to his query and he took it back to work to try it. The installations succeeded because the new query returned the correct set of workstations from the database. To avoid problems like this, use the following rule: When in doubt, use parentheses.
Using the AND operator typically results in a smaller result set, thus improving performance. The NOT operator typically hurts performance because the query optimizer cannot use indexes for the WHERE clause when a NOT operator is specified. For indexes to be utilized when an OR operator is specified, all columns referenced by the OR condition must be included in an index or none of the indexes are used. The LIKE clause allows you to match a character string found in a column to a specified pattern in the WHERE clause. The LIKE clause uses the following wildcard characters:
Percent (%) Replaces any number of characters (including 0 characters) in the string. For example, %at would match at, cat, hat, and that. Underscore(_)
Replaces exactly one character in the string. For example, _at would match cat and hat, but it would not match that or at.
Square brackets ([ ]) Replaces any one character within a set or a range of characters. A set is frequently displayed as a straight list of characters, for example, [abcd]; but the characters can be separated by commas to add clarity, for example, [a,b,c,d]. A range is separated by a dash, for example, [a d]. Each of these three options includes all rows where the specified character is an a, b, c, or d.
Any character not within a set or range of characters. For example, [^a-d] would be equal to [e z].
caret (^)
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