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SQL: The Complete Reference
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limit either. For portability reasons, it s generally a good idea to avoid lists with only a single item, such as this one:
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CITY IN ('New York')
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and replace them with a simple comparison test:
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CITY = 'New York'
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The Pattern Matching Test (LIKE)
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You can use a simple comparison test to retrieve rows where the contents of a text column match some particular text. For example, this query retrieves a row of the CUSTOMERS table by name: Show the credit limit for Smithson Corp.
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SELECT COMPANY, CREDIT_LIMIT FROM CUSTOMERS WHERE COMPANY = 'Smithson Corp.'
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However, you might easily forget whether the company s name was Smith, Smithson, or Smithsonian. You can use SQL s pattern matching test to retrieve the data based on a partial match of the customer s name. The pattern matching test (LIKE), shown in Figure 6-10, checks to see whether the data value in a column matches a specified pattern. The pattern is a string that may include one or more wildcard characters. These characters are interpreted in a special way.
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Wildcard Characters
The percent sign (%) wildcard character matches any sequence of zero or more characters. Here s a modified version of the previous query that uses the percent sign for pattern matching:
SELECT COMPANY, CREDIT_LIMIT FROM CUSTOMERS WHERE COMPANY LIKE 'Smith% Corp.'
Figure 6-10.
Pattern matching test (LIKE) syntax diagram
6:
Simple Queries
The LIKE keyword tells SQL to compare the NAME column to the pattern Smith% Corp. Any of the following names would match the pattern:
Smith Corp. Smithson Corp. Smithsen Corp. Smithsonian Corp.
but these names would not:
SmithCorp Smithson Inc. RETRIEVING DATA
The underscore (_) wildcard character matches any single character. If you are sure that the company s name is either Smithson or Smithsen, for example, you can use this query:
SELECT COMPANY, CREDIT_LIMIT FROM CUSTOMERS WHERE COMPANY LIKE 'Smiths_n Corp.'
In this case, any of these names will match the pattern:
Smithson Corp. Smithsen Corp. Smithsun Corp.
but these names will not:
Smithsoon Corp. Smithsn Corp.
Wildcard characters can appear anywhere in the pattern string, and several wildcard characters can be within a single string. This query allows either the Smithson or Smithsen spelling and will also accept Corp., Inc., or any other ending on the company name:
SELECT COMPANY, CREDIT_LIMIT FROM CUSTOMERS WHERE COMPANY LIKE 'Smiths_n %'
You can locate strings that do not match a pattern by using the NOT LIKE form of the pattern matching test. The LIKE test must be applied to a column with a string data type. If the data value in the column is NULL, the LIKE test returns a NULL result. If you have used computers through a command-line interface (such as the UNIX shell), you ve probably seen string pattern matching before. Frequently, the asterisk (*) is used instead of SQL s percent sign (%), and the question mark ( ) is used instead of
SQL: The Complete Reference
SQL s underscore (_), but the pattern matching capabilities themselves are similar in most situations where a computer application offers the capability to match selected parts of a word or text.
Escape Characters *
One of the problems with string pattern matching is how to match the wildcard characters themselves as literal characters. To test for the presence of a percent sign character in a column of text data, for example, you can t simply include the percent sign in the pattern because SQL will treat it as a wildcard. With some popular SQL products, you cannot literally match the two wildcard characters. This usually doesn t pose serious problems, because the wildcard characters don t frequently appear in names, product numbers, and other text data of the sort that is usually stored in a database. The ANSI/ISO SQL standard does specify a way to literally match wildcard characters, using a special escape character. When the escape character appears in the pattern, the character immediately following it is treated as a literal character rather than as a wildcard character. (The latter character is said to be escaped.) The escaped character can be either of the two wildcard characters, or the escape character itself, which has now taken on a special meaning within the pattern. The escape character is specified as a one-character constant string in the ESCAPE clause of the search condition, as shown in Figure 6-10. Here is an example using a dollar sign ($) as the escape character: Find products whose product IDs start with the four letters "A%BC".
SELECT ORDER_NUM, PRODUCT FROM ORDERS WHERE PRODUCT LIKE 'A$%BC%' ESCAPE '$'
The first percent sign in the pattern, which follows an escape character, is treated as a literal percent sign; the second functions as a wildcard. The use of escape characters is very common in pattern matching applications, which is why the ANSI/ISO standard specified it. However, it was not a part of the early SQL implementations and has been slowly adopted. To insure portability, the ESCAPE clause should be avoided.
The Null Value Test (IS NULL)
NULL values create a three-valued logic for SQL search conditions. For any given row, the result of a search condition may be TRUE or FALSE, or it may be NULL because one of the columns used in evaluating the search condition contains a NULL value. Sometimes it s useful to check explicitly for NULL values in a search condition and handle them directly. SQL provides a special NULL value test (IS NULL), shown in Figure 6-11, to handle this task.
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