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Distinction between subelement and attribute ● In the context of documents, attributes are part of markup, while subelement contents are part of the basic document contents ● In the context of data representation, the difference is unclear and may be confusing  Same information can be represented in two ways –

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To find all books that have the word “database” as a keyword, select title from books where ‘database’ in (unnest(keyword­set )) ■ We can access individual elements of an array by using indices ● E.g.: If we know that a particular book has three authors, we could write: select author­array[1], author­array[2], author­array[3] from books whe...

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A Web server can easily serve as a front end to a variety of information services. ■ The document name in a URL may identify an executable program, that, when run, generates a HTML document. ● When a HTTP server receives a request for such a document, it executes the program, and sends back the HTML document that is generated. ● The Web clie...

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Atomicity is actually a property of how the elements of the domain are used. ● Example: Strings would normally be considered indivisible ● Suppose that students are given roll numbers which are strings of the form CS0012 or EE1127 ● If the first two characters are extracted to find the department, the domain of roll numbers is not atomic. ● ...

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The combination of primary keys of the participating entity sets forms a super key of a relationship set. ● (customer_id, account_number) is the super key of depositor ● NOTE: this means a pair of entity sets can have at most one relationship in a particular relationship set.  Example: if we wish to track all access_dates to each account by ...

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Allows the expression of constraints on domain variables that are either inconvenient or impossible to express within the skeleton tables. ■ Complex conditions can be used in condition boxes ■ Example: Find the loan numbers of all loans made to Smith, to Jones, or to both jointly

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The SQL standard defines embeddings of SQL in a variety of programming languages such as C, Java, and Cobol. ■ A language to which SQL queries are embedded is referred to as a host language, and the SQL structures permitted in the host language comprise embedded SQL. ■ The basic form of these languages follows that of the System R embedding o...

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Find all customers who have an account at all branches located in Brooklyn. select distinct S.customer_name from depositor as S where not exists ( (select branch_name from branch where branch_city = 'Brooklyn') except (select R.branch_name from depositor as T, account as R where T.account_number = R.account_number and S.customer_name = ...

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Aggregation function takes a collection of values and returns a single value as a result. avg: average value min: minimum value max: maximum value sum: sum of values count: number of values ■ Aggregate operation in relational algebra E is any relational­algebra expression ● G 1, G2 , Gn is a list of attributes on which to group (can be em...

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Alternative ways of evaluating a given query ● Equivalent expressions ● Different algorithms for each operation Cost difference between a good and a bad way of evaluating a query can be enormous Need to estimate the cost of operations ● Depends critically on statistical information about relations which the database must maintain ● Need to ...

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