c# generate barcode Diagnosing Problems in Font

Printer PDF-417 2d barcode in Font Diagnosing Problems

Diagnosing Problems
Draw PDF 417 In None
Using Barcode generator for Font Control to generate, create PDF417 image in Font applications.
www.OnBarcode.com
Create USS Code 39 In None
Using Barcode generator for Font Control to generate, create ANSI/AIM Code 39 image in Font applications.
www.OnBarcode.com
In the previous sections, we looked at some problems you could possibly find with database design and inconsistent data. Most of the time, however, if the result of your query is not looking quite right, it is probably because you have the wrong SQL statement. Your statement may be retrieving rows that are different from what you were expecting. We looked at checking the query output in 10. I cannot overemphasize the importance of checking your query results, as sometimes two queries may be subtly different, and it can be tricky to spot if you have mistakenly asked the wrong question. In the previous chapter, I suggested a way to approach queries that lets you build the query up slowly so you can check that each step is returning appropriate rows. However, if you are presented with a full-blown, complex query that is not delivering as expected, you need to pare it down until you find where the problem lies. If you have noticed a problem, then you have a good place to start. You have either noticed an expected row is missing or that an inappropriate row has been retrieved. Concentrate on finding where in the query that problem is. The following sections offer some suggestions.
Barcode Generation In None
Using Barcode maker for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Draw EAN13 In None
Using Barcode printer for Font Control to generate, create EAN 13 image in Font applications.
www.OnBarcode.com
C HA PTER 11 C OMMON PROBLEMS
Creating Barcode In None
Using Barcode creator for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
PDF-417 2d Barcode Printer In None
Using Barcode generation for Font Control to generate, create PDF 417 image in Font applications.
www.OnBarcode.com
Check Parts of Nested Queries Independently
USS-128 Printer In None
Using Barcode creation for Font Control to generate, create EAN / UCC - 13 image in Font applications.
www.OnBarcode.com
Draw Code 9/3 In None
Using Barcode printer for Font Control to generate, create USS-93 image in Font applications.
www.OnBarcode.com
Where you have one query nested inside another, the first thing to check is that the nested part is behaving itself. Take a look at Listing 11-17.
Printing PDF 417 In Java
Using Barcode drawer for Java Control to generate, create PDF-417 2d barcode image in Java applications.
www.OnBarcode.com
PDF417 Printer In .NET Framework
Using Barcode generator for Reporting Service Control to generate, create PDF417 image in Reporting Service applications.
www.OnBarcode.com
Listing 11-17. SQL Statement to Retrieve Juniors with Handicaps Lower Than the Average SELECT * FROM Member m WHERE m.MemberType = 'Junior' AND Handicap < (SELECT AVG(Handicap) FROM Member)
Encoding Matrix In Java
Using Barcode drawer for Java Control to generate, create 2D Barcode image in Java applications.
www.OnBarcode.com
EAN13 Printer In None
Using Barcode generation for Microsoft Excel Control to generate, create GTIN - 13 image in Office Excel applications.
www.OnBarcode.com
If you are having trouble with a query like this, cut and paste the inner query and run it independently. Check to see if it is returning the correct result. If this is OK, you can try doing the outer query on its own. To do this, just put some value in place of the inner query such as Handicap < 10 and see if that returns the correct results. If you can narrow down the problem to one part of the query, then use some of the ideas in the following sections. This approach doesn t work if the inner and outer parts of the query are related (see 4), but some of the following techniques might help with that situation.
Draw USS Code 39 In Java
Using Barcode generator for Android Control to generate, create Code-39 image in Android applications.
www.OnBarcode.com
UPC - 13 Drawer In None
Using Barcode drawer for Software Control to generate, create GS1 - 13 image in Software applications.
www.OnBarcode.com
Understand How the Tables Are Being Combined
Creating DataMatrix In Java
Using Barcode encoder for BIRT Control to generate, create ECC200 image in Eclipse BIRT applications.
www.OnBarcode.com
Reading Barcode In Visual Studio .NET
Using Barcode Control SDK for ASP.NET Control to generate, create, read, scan barcode image in ASP.NET applications.
www.OnBarcode.com
Many queries involve combining tables with some relational operation (join, union, and so on). Make sure you understand how the tables are being combined and if that is appropriate. Consider a query such as the one in Listing 11-18.
Barcode Reader In VB.NET
Using Barcode Control SDK for .NET framework Control to generate, create, read, scan barcode image in .NET framework applications.
www.OnBarcode.com
GTIN - 13 Scanner In VB.NET
Using Barcode reader for .NET Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
Listing 11-18. How Are the Tables Being Combined SELECT m.LastName, m.FirstName FROM Member m, Entry e, Tournament t WHERE m.MemberID = e.MemberID AND e.TourID = t.TourID AND t.TourType = 'Open' AND e.Year = 2006
Quick Response Code Generation In Objective-C
Using Barcode generator for iPhone Control to generate, create QR-Code image in iPhone applications.
www.OnBarcode.com
Draw EAN13 In Java
Using Barcode drawer for Java Control to generate, create EAN 13 image in Java applications.
www.OnBarcode.com
Three tables are involved in this query. It can take a moment to figure out that they are being joined. Make sure that is appropriate for the question being asked. 10 has examples of key words in questions and the appropriate ways to combine tables.
Remove Extra WHERE Clauses
After combining tables, usually only some of the resulting rows are required. In Listing 11-18, part of the WHERE clause is needed for the join operations. However, after the join, only the rows satisfying t.TourType = 'Open' AND e.Year = 2006 are retained. If you have rows
CHAPTER 11 C OM MON PROBLE MS
missing from your result, it is often useful to remove the part of the WHERE clause that is selecting a final subset of the rows. If the rows are still missing, then you know that (for this example) the problem is occurring in the join.
Retain All the Columns
I m a big fan of always saying SELECT * in the early stages of developing queries that involve joins. Consider the query in Listing 11-18. If we suspect a problem with the joins, then by leaving all the columns visible, we can see if the join conditions are behaving as expected. Once we are happy with the rows being retrieved, we can retain just the columns required. However, if you are combining tables with set operations, this approach will be counterproductive, as projecting the right columns is critical (see the Do You Have Correct Columns in Set Operations section later in this chapter).
Copyright © OnBarcode.com . All rights reserved.