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The General Form of a Query
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ascending group let where descending in on
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All queries share a general form, which is based on a set of contextual keywords, shown here:
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Of these, the following begin query clauses:
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As mentioned, a query must begin with the keyword from and end with either a select or group clause The select clause determines what type of value is enumerated by the query The group clause returns the data by groups, with each group able to be enumerated individually As the preceding examples have shown, the where clause specifies criteria that an item must meet in order for it to be returned The remaining clauses help you fine-tune a query The following sections examine each query clause
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Filter Values with where
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As explained, where is used to filter the data returned by a query The preceding examples have shown only its simplest form, in which a single condition is used A key point to understand is that you can use where to filter data based on more than one condition One way to do this is through the use of multiple where clauses For example, consider the following program that displays only those values in the array that are both positive and less than 10
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// Use multiple where clauses using System; using SystemLinq; class TwoWheres { static void Main() { int[] nums = { 1, -2, 3, -3, 0, -8, 12, 19, 6, 9, 10 };
// Create a query that obtains positive values less than 10: var posNums = from n in nums where n > 0 Use two where clauses where n < 10 select n; ConsoleWriteLine("The positive values less than 10:"); // Execute the query and display the results foreach(int i in posNums) ConsoleWriteLine(i); } }
The output is shown here:
The positive values less than 10: 1 3 6 9
As you can see, only positive values less than 10 are retrieved
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Although it is not wrong to use two where clauses as just shown, the same effect can be achieved in a more compact manner by using a single where in which both tests are combined into a single expression Here is the query rewritten to use this approach:
var posNums = from n in nums where n > 0 && n < 10 select n;
In general, a where condition can use any valid C# expression that evaluates to a Boolean result For example, the following program defines an array of strings Several of the strings are Internet addresses The query netAddrs retrieves only those strings that have more than four characters and that end with net Thus, it finds those strings that contain Internet addresses that use the net domain name
// Demonstrate another where clause using System; using SystemLinq; class WhereDemo2 { static void Main() { string[] strs = { "com", "net", "hsNameAcom", "hsNameBnet", "test", "network", "hsNameCnet", "hsNameDcom" }; // Create a query that obtains Internet addresses that // end with net var netAddrs = from addr in strs where addrLength > 4 && addrEndsWith("net") select addr; // Execute the query and display the results foreach(var str in netAddrs) ConsoleWriteLine(str); } }
A more complicated where expression
The output is shown here:
hsNameBnet hsNameCnet
Notice that the program makes use of another of string s method called EndsWith( ) It returns true if the invoking string ends with the character sequence specified as an argument
Sort Results with orderby
Often you will want the results of a query to be sorted For example, you might want to obtain a list of past-due accounts, in order of the remaining balance, from greatest to least Or, you
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might want to obtain a customer list, alphabetized by name Whatever the purpose, LINQ gives you an easy way to produce sorted results: the orderby clause The general form of orderby is shown here: orderby sort-on how The item on which to sort is specified by sort-on This can be as inclusive as the entire element stored in the data source or as restricted as a portion of a single field within the element The value of how determines if the sort is ascending or descending, and it must be either ascending or descending The default direction is ascending, so you won t normally specify ascending Here is an example that uses orderby to retrieve the values in an int array in ascending order:
// Demonstrate orderby using System; using SystemLinq; class OrderbyDemo { static void Main() { int[] nums = { 10, -19, 4, 7, 2, -5, 0 };
// Create a query that obtains the values in sorted order var sNums = from n in nums Sort the result orderby n select n; ConsoleWrite("Values in ascending order: "); // Execute the query and display the results foreach(int i in sNums) ConsoleWrite(i + " "); ConsoleWriteLine(); } }
The output is shown here:
Values in ascending order: -19 -5 0 2 4 7 10
To change the order to descending, simply specify the descending option, as shown here:
var sNums = from n in nums orderby n descending select n;
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