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XQuery path expressions can provide the XML equivalent of the simple SQL SELECT statement with a WHERE clause. Assume that a collection of XML documents contains the XML equivalent of the contents of the sample database, with the top-level documents named with the names of the tables in the sample database and the individual row structures named with the singular equivalents of those names (e.g., the OFFICES document contains individual OFFICE elements to represent the rows of the OFFICES table, etc.). Here are some query requests and their corresponding path expressions: Identify the of ces managed by employee number 108.
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/offices/office[mgr=108]
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Find all of ces with sales over target.
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/offices/office[sales > target]
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Find all orders for manufacturer ACI with amounts over $30,000.
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/orders/order[mfr = 'ACI' and amount > 30000.00]
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Because the sample database is a shallow row/column structure, the XML hierarchy is only three levels deep. To illustrate the query possibilities in more hierarchical XML documents, consider once again the book document in Figure 25-1. Here are some query requests and their corresponding path expressions:
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These expressions don t give you the same control over query results as the SELECT list provides in SQL queries. They also don t provide the equivalent of SQL cursors for row-byrow processing. XQuery provides both of these capabilities through For/Let/Where/ Returns expressions (FLWR expressions, pronounced flower ). An example is the best way to illustrate the capability. Once again, assume a set of XML documents structured to resemble the sample database, as in the previous examples. This query implements a twotable join and generates three specific columns of query results: List the salesperson s name, order date, and amount of all orders under $5000, sorted by amount.
<smallOrders> { for $o in document("orders.xml")//orders[amount < 5000.00], $r in document("salesreps.xml")//salesreps[empl_num=$o/rep] return <smallOrder> { $r/name, $o/order_date, $o/amount } </smallOrder> sortby(amount) } </smallOrders>
At the outer level, the contents of the smallOrders element are specified by the XQuery expression enclosed in the outer braces. The for expression uses two variables to iterate through two documents, corresponding to the ORDERS and SALESREPS tables. These two variables effectively implement a join between the two tables (documents). The predicates (search arguments) at the end of each of the two lines following the for keyword correspond to the SQL WHERE clause. The predicate in the first line restricts the query to orders with amounts under $5000. The predicate in the second row implements the join, using the $o variable to link rows in the SALESREPS table (document) with rows in the ORDERS table (document). The return part of the for expression specifies which elements should be returned as the results of the expression evaluation. It corresponds to the select list in a SQL query. The returned value will be an XML sequence of smallOrder elements, and each element comes from one corresponding element in the source tables (documents). Once again, the iteration variables are used to qualify the specific path to the element whose contents are to be returned. Finally, the sortby part of the expression functions in the same way as the corresponding ORDER BY clause of a SQL query. A few additional query-processing capabilities are not illustrated in this example. You can use a let expression within the for iteration to capture additional variable values within the for loop that you may need in predicates or other expressions. An if then else expression supports conditional execution. Aggregate functions support grouped
PART VI
Part VI:
SQL Today and Tomorrow
XQuery queries, corresponding to the SQL summary queries described in 8. With these capabilities, the flexibility of XQuery is comparable to that of SQL query expressions. However, as you can see from the example, the style of the query expression is quite different, reflecting both the expression orientation and the very strong navigational orientation of XQuery and XML documents.
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