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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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II Relational Databases
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The McGraw Hill Companies, 2001
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even if an attribute name can be derived from the base relations as in the preceding example, we may want to change the attribute name in the result Hence, SQL provides a way of renaming the attributes of a result relation For example, if we want the attribute name loan-number to be replaced with the name loan-id, we can rewrite the preceding query as select customer-name, borrowerloan-number as loan-id, amount from borrower, loan where borrowerloan-number = loanloan-number
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The as clause is particularly useful in de ning the notion of tuple variables, as is done in the tuple relational calculus A tuple variable in SQL must be associated with a particular relation Tuple variables are de ned in the from clause by way of the as clause To illustrate, we rewrite the query For all customers who have a loan from the bank, nd their names, loan numbers, and loan amount as select customer-name, Tloan-number, Samount from borrower as T, loan as S where Tloan-number = Sloan-number Note that we de ne a tuple variable in the from clause by placing it after the name of the relation with which it is associated, with the keyword as in between (the keyword as is optional) When we write expressions of the form relation-nameattribute-name, the relation name is, in effect, an implicitly de ned tuple variable Tuple variables are most useful for comparing two tuples in the same relation Recall that, in such cases, we could use the rename operation in the relational algebra Suppose that we want the query Find the names of all branches that have assets greater than at least one branch located in Brooklyn We can write the SQL expression select distinct Tbranch-name from branch as T, branch as S where Tassets > Sassets and Sbranch-city = Brooklyn Observe that we could not use the notation branchasset, since it would not be clear which reference to branch is intended SQL permits us to use the notation (v1 , v2 , , vn ) to denote a tuple of arity n containing values v1 , v2 , , vn The comparison operators can be used on tuples, and the ordering is de ned lexicographically For example, (a1 , a2 ) <= (b1 , b2 ) is true if a1 < b1 , or (a1 = b1 ) (a2 <= b2 ); similarly, the two tuples are equal if all their attributes are equal
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SQL speci es strings by enclosing them in single quotes, for example, Perryridge , as we saw earlier A single quote character that is part of a string can be speci ed by
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
II Relational Databases
4 SQL
The McGraw Hill Companies, 2001
4
using two single quote characters; for example the string It s right can be speci ed by It s right The most commonly used operation on strings is pattern matching using the operator like We describe patterns by using two special characters: Percent (%): The % character matches any substring Underscore ( ): The character matches any character
Patterns are case sensitive; that is, uppercase characters do not match lowercase characters, or vice versa To illustrate pattern matching, we consider the following examples: Perry% matches any string beginning with Perry %idge% matches any string containing idge as a substring, for example, Perryridge , Rock Ridge , Mianus Bridge , and Ridgeway matches any string of exactly three characters % matches any string of at least three characters
SQL expresses patterns by using the like comparison operator Consider the query Find the names of all customers whose street address includes the substring Main This query can be written as
select customer-name from customer where customer-street like %Main% For patterns to include the special pattern characters (that is, % and ), SQL allows the speci cation of an escape character The escape character is used immediately before a special pattern character to indicate that the special pattern character is to be treated like a normal character We de ne the escape character for a like comparison using the escape keyword To illustrate, consider the following patterns, which use a backslash (\) as the escape character: like ab\%cd% escape \ matches all strings beginning with ab%cd like ab\\cd% escape \ matches all strings beginning with ab\cd
SQL allows us to search for mismatches instead of matches by using the not like comparison operator SQL also permits a variety of functions on character strings, such as concatenating (using ), extracting substrings, nding the length of strings, converting between uppercase and lowercase, and so on SQL:1999 also offers a similar to operation, which provides more powerful pattern matching than the like operation; the syntax for specifying patterns is similar to that used in Unix regular expressions
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