asp.net mvc qr code generator Further Examples of Joins in .NET

Generator QR Code 2d barcode in .NET Further Examples of Joins

Further Examples of Joins
QR Creation In Visual Studio .NET
Using Barcode encoder for .NET Control to generate, create QR Code ISO/IEC18004 image in Visual Studio .NET applications.
www.OnBarcode.com
QR Code ISO/IEC18004 Decoder In Visual Studio .NET
Using Barcode scanner for Visual Studio .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
So far, I have demonstrated fundamental join types. There are other ways to categorize joins besides their fundamental type. In this section, I'll describe self joins, nonequijoins, queries with multiple joins, and semi joins.
Print Barcode In Visual Studio .NET
Using Barcode creation for .NET framework Control to generate, create bar code image in VS .NET applications.
www.OnBarcode.com
Reading Bar Code In .NET
Using Barcode scanner for .NET framework Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
Self Joins
Make QR Code ISO/IEC18004 In C#.NET
Using Barcode drawer for Visual Studio .NET Control to generate, create QR Code image in .NET applications.
www.OnBarcode.com
QR Code JIS X 0510 Maker In VS .NET
Using Barcode generator for ASP.NET Control to generate, create QR Code ISO/IEC18004 image in ASP.NET applications.
www.OnBarcode.com
A self join is simply a join between two instances of the same table. I've already shown examples of self joins without classifying them explicitly as such. Here's a simple example of a self join between two instances of the Employees table, one representing employees (E), and the other representing managers (M): USE Northwind; SELECT E.FirstName, E.LastName AS emp, M.FirstName, M.LastName AS mgr FROM dbo.Employees AS E LEFT OUTER JOIN dbo.Employees AS M ON E.ReportsTo = M.EmployeeID;
Making QR Code 2d Barcode In Visual Basic .NET
Using Barcode printer for Visual Studio .NET Control to generate, create Denso QR Bar Code image in .NET applications.
www.OnBarcode.com
Create Bar Code In Visual Studio .NET
Using Barcode creation for .NET Control to generate, create barcode image in .NET applications.
www.OnBarcode.com
The query produces the output shown in Table 5-4, where the employees' names are returned along with their managers' names.
Drawing Code 3/9 In .NET Framework
Using Barcode creation for VS .NET Control to generate, create Code-39 image in .NET applications.
www.OnBarcode.com
Create Code 128 Code Set B In .NET Framework
Using Barcode printer for .NET Control to generate, create Code 128 Code Set B image in .NET framework applications.
www.OnBarcode.com
Table 5-4. Employees and Their Managers
USS-128 Maker In .NET Framework
Using Barcode printer for Visual Studio .NET Control to generate, create UCC - 12 image in .NET applications.
www.OnBarcode.com
ANSI/AIM Codabar Generator In .NET Framework
Using Barcode generator for .NET framework Control to generate, create Uniform Symbology Specification Codabar image in VS .NET applications.
www.OnBarcode.com
emp Nancy Davolio
Making Bar Code In None
Using Barcode creation for Office Word Control to generate, create bar code image in Office Word applications.
www.OnBarcode.com
Paint Data Matrix ECC200 In .NET
Using Barcode maker for Reporting Service Control to generate, create Data Matrix image in Reporting Service applications.
www.OnBarcode.com
mgr Andrew Fuller
Paint Data Matrix In None
Using Barcode printer for Font Control to generate, create ECC200 image in Font applications.
www.OnBarcode.com
UCC-128 Generation In None
Using Barcode creator for Font Control to generate, create UCC.EAN - 128 image in Font applications.
www.OnBarcode.com
Andrew Fuller NULL Janet Leverling Margaret Peacock Steven Buchanan Michael Suyama Robert King Laura Callahan Anne Dodsworth Andrew Fuller Andrew Fuller Andrew Fuller Steven Buchanan Steven Buchanan Andrew Fuller Steven Buchanan
Recognize UCC.EAN - 128 In Visual C#.NET
Using Barcode scanner for .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
USS-128 Creation In Java
Using Barcode generator for Eclipse BIRT Control to generate, create EAN / UCC - 14 image in BIRT applications.
www.OnBarcode.com
I used a left outer join to include Andrewthe Vice President of Salesin the result. He has a NULL in the ReportsTo column because he has no manager.
ECC200 Generator In Objective-C
Using Barcode generation for iPad Control to generate, create Data Matrix image in iPad applications.
www.OnBarcode.com
EAN / UCC - 14 Generation In Java
Using Barcode creation for Java Control to generate, create UCC.EAN - 128 image in Java applications.
www.OnBarcode.com
Note
When joining two instances of the same table, you must alias at least one of the tables. This provides a unique name or alias to each instance so that there is no ambiguity in the result column names and in the column names in the intermediate virtual tables.
Nonequijoins
Equijoins are joins with a join condition based on an equality operator. Nonequijoins have operators other than equality in their join condition. As an example, suppose that you need to generate all pairs of two different employees from an Employees table. Assume that currently the table contains employee IDs A, B, and C. A cross join would generate the following nine pairs: A, A A, B A, C B, A
B, B B, C C, A C, B C, C Obviously, a "self" pair (x, x) that has the same employee ID twice is not a pair of two different employees. Also, for each pair (x, y), you will find its "mirror" pair (y, x) in the result. You need to return only one of the two. To take care of both issues, you can specify a join condition that filters pairs where the key from the left table is smaller than the key from the right table. Pairs where the same employee appears twice will be removed. Also, one of the mirror pairs (x, y) and (y, x) will be removed because only one will have a left key smaller than the right key. The following query returns the required result, without mirror pairs and without self pairs: SELECT E1.EmployeeID, E1.LastName, E1.FirstName, E2.EmployeeID, E2.LastName, E2.FirstName FROM dbo.Employees AS E1 JOIN dbo.Employees AS E2 ON E1.EmployeeID < E2.EmployeeID;
You can also calculate row numbers using a nonequijoin. As an example, the following query calculates row numbers for orders from the Orders table, based on increasing OrderID: SELECT O1.OrderID, O1.CustomerID, O1.EmployeeID, COUNT(*) AS rn FROM dbo.Orders AS O1 JOIN dbo.Orders AS O2 ON O2.OrderID <= O1.OrderID GROUP BY O1.OrderID, O1.CustomerID, O1.EmployeeID;
You can find similarities between this solution and the preSQL Server 2005 set-based solution I showed in the previous chapter using subqueries. The join condition here contains the same logical expression I used in a subquery before. After applying the first two phases in query logical processing (Cartesian product and ON filter), each order from O1 is matched with all orders from O2 that have a smaller or equal OrderID. This means that a row from O1 with a target row number n will be matched with n rows from O2. Each row from O1 will be duplicated in the result of the join n times. If this is confusing, bear with me as I try to demonstrate this logic with an example. Say you have orders with the following IDs (in order): x, y, and z. The result of the join would be the following: x, x y, x y, y z, x z, y z, z
The join created duplicates out of each row from O1as many as the target row number. The next step is to collapse each group of rows back to one row, returning the count of rows as the row number: x, 1 y, 2 z, 3 Note that you must include in the GROUP BY clause all attributes from O1 that you want to return. Remember that in an aggregate query, an attribute that you want to return in the SELECT list must appear in the GROUP BY clause. This query suffers from the same N2 performance issues I described with the subquery solution. This query also demonstrates an "expand-collapse" technique, where the join achieves the expansion of the number of rows by generating duplicates, and the grouping achieves the collapsing of the rows allowing you to calculate aggregates. I find the subquery technique more appealing because it's so much more intuitive. I find the "expand-collapse" technique to be artificial and nonintuitive. Remember that in both solutions to generating row numbers you used an aggregate functiona count of rows. Very similar logic can be used to calculate other aggregates either with a subquery or with a join (expand-collapse technique). I will elaborate on this technique in 6 in the "Running Aggregations" section. I'll also describe there scenarios in which I'd still consider using the "expandcollapse" technique even though I find it less intuitive than the subquery technique.
Copyright © OnBarcode.com . All rights reserved.