create barcode image c# SIMPLE QUERIES ON ONE TABLE in Font

Maker PDF-417 2d barcode in Font SIMPLE QUERIES ON ONE TABLE

CHAPTER 2 SIMPLE QUERIES ON ONE TABLE
PDF417 Generator In None
Using Barcode creator for Font Control to generate, create PDF417 image in Font applications.
www.OnBarcode.com
Encode Data Matrix 2d Barcode In None
Using Barcode drawer for Font Control to generate, create ECC200 image in Font applications.
www.OnBarcode.com
a) With duplicates
Make Barcode In None
Using Barcode generator for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Generate Code 39 Extended In None
Using Barcode generator for Font Control to generate, create Code 3 of 9 image in Font applications.
www.OnBarcode.com
b) Without duplicates
Make EAN / UCC - 14 In None
Using Barcode generation for Font Control to generate, create USS-128 image in Font applications.
www.OnBarcode.com
Barcode Generation In None
Using Barcode generation for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Figure 2-8. Projecting the MemberType column from the Member table
EAN-13 Supplement 5 Drawer In None
Using Barcode creation for Font Control to generate, create EAN13 image in Font applications.
www.OnBarcode.com
ANSI/AIM I-2/5 Maker In None
Using Barcode encoder for Font Control to generate, create 2 of 5 Interleaved image in Font applications.
www.OnBarcode.com
What does SQL do If we say SELECT MemberType FROM Member, we will get the output in Figure 2-8a with all the duplicates included. If we do not want the duplicates, then we can use the keyword DISTINCT, as in Listing 2-21.
Draw PDF 417 In .NET Framework
Using Barcode generator for Reporting Service Control to generate, create PDF417 image in Reporting Service applications.
www.OnBarcode.com
Decoding PDF 417 In Visual Basic .NET
Using Barcode scanner for Visual Studio .NET Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
Listing 2-21. Retrieving a List of Unique Membership Types SELECT DISTINCT m.MemberType FROM Member m
Barcode Scanner In VB.NET
Using Barcode decoder for .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
QR Code JIS X 0510 Generation In Java
Using Barcode creator for Eclipse BIRT Control to generate, create QR Code ISO/IEC18004 image in Eclipse BIRT applications.
www.OnBarcode.com
Whether you keep the duplicates depends very much on the information you require, so you need to give it some careful thought. If you were expecting the set of rows in Figure 2-8b and got Figure 2-8a, you would most likely notice. With the two sets of rows in Figure 2-7, it is much more difficult to spot that you have perhaps made a mistake. Get into the habit of thinking about duplicates for all your queries.
Code 39 Extended Generator In Java
Using Barcode drawer for Java Control to generate, create Code-39 image in Java applications.
www.OnBarcode.com
EAN / UCC - 14 Encoder In None
Using Barcode encoder for Excel Control to generate, create UCC - 12 image in Excel applications.
www.OnBarcode.com
CH A PT ER 2 S IMP LE QUER IES O N O NE T AB LE
Code 128A Drawer In None
Using Barcode drawer for Software Control to generate, create Code 128 Code Set A image in Software applications.
www.OnBarcode.com
Decode Barcode In Java
Using Barcode Control SDK for Eclipse BIRT Control to generate, create, read, scan barcode image in Eclipse BIRT applications.
www.OnBarcode.com
Ordering Output
Generating GS1-128 In .NET
Using Barcode drawer for .NET framework Control to generate, create GS1 128 image in VS .NET applications.
www.OnBarcode.com
Draw QR In Java
Using Barcode maker for Java Control to generate, create Quick Response Code image in Java applications.
www.OnBarcode.com
Every now and then I refer to a set of rows rather than a table or a virtual table. Our formal definition of a database is a set of tables, and for a table the definition is a set of rows. 3 The word set has two implications. One is that there are no duplicates (and we have discussed that a lot!). The other implication is that there is no particular order to the rows in our set. In theory, we don t have a first row or a last row or a next row. If we run a query to retrieve all the rows, or just some of the rows, from a table, then we have no guarantee in what order they will be returned. However, sometimes we might like to display the result in a particular order. We can do this with the key phrase ORDER BY. Listing 2-22 shows how to retrieve member information ordered alphabetically by LastName.
Print Barcode In Java
Using Barcode creation for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
Barcode Maker In Visual Studio .NET
Using Barcode generator for .NET Control to generate, create Barcode image in .NET applications.
www.OnBarcode.com
Listing 2-22. Retrieving the Members in Order of LastName SELECT * FROM Member m ORDER BY m.LastName
We can also order by two or more values. For example, if we want to order members with the same LastName by the value of their FirstName, we can include those two attributes (in that order) in the ORDER BY clause. The ORDER BY clause is the final clause in an SQL query. Listing 2-23 shows how to list all the senior members ordered by LastName and where the last names are the same by FirstName.
Listing 2-23. Retrieving the Senior Members Ordered by LastName and Then FirstName SELECT * FROM Member m WHERE m.MemberType = 'Senior' ORDER BY m.LastName, m.FirstName
The type of a field determines how the values will be ordered. Text fields will be ordered alphabetically, number fields will be ordered numerically, and date and time fields will be ordered chronologically.
Performing Simple Counts
As well as retrieving a subset of rows and columns from a table, we can also use SQL queries to retrieve some statistics. There are SQL functions that allow us to count records, total or average values, find maximum and minimum values, and so on. In this section, we will look at some simple queries for counting records. (We will return to this topic in 8.)
3. To be really formal, a database is actually a set of relations, and a relation is a set of tuples.
CHAPTER 2 SIMPLE QUERIES ON ONE TABLE
Listing 2-24 shows a simple query to return the number of records in the Member table. COUNT is a function in SQL, and * means count each record.
Listing 2-24. Retrieving the Number of Records in a Table SELECT COUNT(*) FROM Member
We can also count a subset of rows by adding a WHERE clause to specify those rows we want to include. Listing 2-25 shows how to count the number of senior members.
Listing 2-25. Retrieving the Number of Senior Members SELECT COUNT(*) FROM Member m WHERE m.MemberType = 'Senior'
Because we have just been talking about Nulls and duplicate values, it is worth briefly mentioning here how these will affect our counts. Rather than use * as a parameter to the COUNT function so that it counts all the rows, we can put an attribute such as Handicap in parentheses, as shown in Listing 2-26. This will count the number of records that have a value for the Handicap attribute.
Listing 2-26. Retrieving the Number of Records with a Non-Null Value for Handicap SELECT COUNT(Handicap) FROM Member
We can also specify that we want to count the number of unique values for an attribute, that is, how many different handicaps there are. Listing 2-27 shows how to do this.
Listing 2-27. Retrieving the Number of Different Values for Handicap SELECT COUNT(DISTINCT Handicap) FROM Member
It is worth reiterating that different database software will support different parts of the SQL standard syntax. For example, Microsoft Access does not support the statement in Listing 2-27. There is usually a way to work around this and find an equivalent query, and we will look at that and other issues related to aggregates and summaries in 8.
Copyright © OnBarcode.com . All rights reserved.