free barcode generator in vb.net RETRIEVING DATA in Software

Paint Code 128C in Software RETRIEVING DATA

RETRIEVING DATA
Code128 Reader In None
Using Barcode Control SDK for Software Control to generate, create, read, scan barcode image in Software applications.
Code 128B Creator In None
Using Barcode printer for Software Control to generate, create Code 128 Code Set C image in Software applications.
SQL: The Complete Reference
Decode Code 128 Code Set A In None
Using Barcode recognizer for Software Control to read, scan read, scan image in Software applications.
Code 128 Generator In C#
Using Barcode creator for VS .NET Control to generate, create ANSI/AIM Code 128 image in Visual Studio .NET applications.
Multiple UNIONs*
Generate Code 128 Code Set A In VS .NET
Using Barcode creator for ASP.NET Control to generate, create Code-128 image in ASP.NET applications.
Draw USS Code 128 In .NET Framework
Using Barcode printer for VS .NET Control to generate, create Code 128 Code Set C image in VS .NET applications.
You can use the UNION operation repeatedly to combine three or more sets of query results, as shown in Figure 6-16. The union of Table B and Table C in the figure produces a single, combined table. This table is then combined with Table A in another UNION operation. The query in the figure is written this way:
Paint Code 128 In Visual Basic .NET
Using Barcode generation for .NET framework Control to generate, create Code 128 Code Set A image in .NET framework applications.
Printing Bar Code In None
Using Barcode printer for Software Control to generate, create barcode image in Software applications.
SELECT * FROM A UNION (SELECT FROM UNION SELECT FROM Bill Mary George Fred Sue Julia Harry
Encoding UPC Code In None
Using Barcode drawer for Software Control to generate, create UPC-A Supplement 5 image in Software applications.
Encoding Bar Code In None
Using Barcode maker for Software Control to generate, create bar code image in Software applications.
* B * C)
Data Matrix 2d Barcode Creator In None
Using Barcode printer for Software Control to generate, create Data Matrix 2d barcode image in Software applications.
Creating USS Code 128 In None
Using Barcode encoder for Software Control to generate, create Code 128 Code Set B image in Software applications.
Figure 6-16.
RM4SCC Generator In None
Using Barcode generation for Software Control to generate, create RoyalMail4SCC image in Software applications.
Barcode Printer In .NET
Using Barcode printer for ASP.NET Control to generate, create barcode image in ASP.NET applications.
Nested UNION operations
Printing ECC200 In None
Using Barcode encoder for Font Control to generate, create Data Matrix 2d barcode image in Font applications.
Bar Code Encoder In C#.NET
Using Barcode printer for .NET Control to generate, create barcode image in .NET framework applications.
6:
Drawing Code 128 Code Set B In Java
Using Barcode encoder for Android Control to generate, create Code 128A image in Android applications.
Code 128 Printer In .NET Framework
Using Barcode creator for Reporting Service Control to generate, create Code 128 Code Set A image in Reporting Service applications.
Simple Queries
GTIN - 13 Encoder In Java
Using Barcode encoder for Android Control to generate, create EAN / UCC - 13 image in Android applications.
UPC-A Scanner In Visual Basic .NET
Using Barcode recognizer for VS .NET Control to read, scan read, scan image in VS .NET applications.
The parentheses in the query indicate which UNION operation should be performed first. In fact, if all of the UNIONs in the statement eliminate duplicate rows, or if all of them retain duplicate rows, the order in which they are performed is unimportant. These three expressions are completely equivalent:
A UNION (B UNION C) (A UNION B) UNION C (A UNION C) UNION B RETRIEVING DATA
and produce seven rows of query results. Similarly, the following three expressions are completely equivalent and produce twelve rows of query results, because the duplicates are retained:
A UNION ALL (B UNION ALL C) (A UNION ALL B) UNION ALL C (A UNION ALL C) UNION ALL B
However, if the unions involve a mixture of UNION and UNION ALL, the order of evaluation matters. If this expression:
A UNION ALL B UNION C
is interpreted as:
A UNION ALL (B UNION C)
then it produces ten rows of query results (six from the inner UNION, plus four rows from Table A). However, if it is interpreted as:
(A UNION ALL B) UNION C
then it produces only four rows, because the outer UNION eliminates all duplicate rows. For this reason, it s always a good idea to use parentheses in UNIONs of three or more tables to specify the order of evaluation intended.
SQL: The Complete Reference
Summary
This chapter is the first of four chapters about SQL queries. It described the following query features: I The SELECT statement is used to express a SQL query. Every SELECT statement produces a table of query results containing one or more columns and zero or more rows. I The FROM clause specifies the table(s) containing the data to be retrieved by a query. I The SELECT clause specifies the column(s) of data to be included in the query results, which can be columns of data from the database, or calculated columns. I The WHERE clause selects the rows to be included in the query results by applying a search condition to rows of the database. I A search condition can select rows by comparing values, by checking a value against a range or set of values, by matching a string pattern, and by checking for NULL values. I Simple search conditions can be combined with AND, OR, and NOT to form more complex search conditions. I The ORDER BY clause specifies that the query results should be sorted in ascending or descending order, based on the values of one or more columns. I The UNION operation can be used within a SELECT statement to combine two or more sets of query results into a single set.
7
Multitable Queries (Joins)
Copyright 2002 by The McGraw-Hill Companies, Inc. Click Here for Terms of Use.
SQL: The Complete Reference
any useful queries request data from two or more tables in a database. For example, these requests for data in the sample database draw data from two, three, or four tables:
I List the salespeople and the offices where they work (SALESREPS and OFFICES tables). I List each order placed last week, showing the order amount, the name of the customer who placed it, and the name of the product ordered (ORDERS, CUSTOMERS, and SALESREPS tables). I Show all orders taken by salespeople in the Eastern region, showing the product description and salesperson (ORDERS, SALESREPS, OFFICES, and PRODUCTS tables). SQL allows you to retrieve data that answers these requests through multitable queries that join data from two or more tables. These queries and the SQL join facility are described in this chapter.
A Two-Table Query Example
The best way to understand the facilities that SQL provides for multitable queries is to start with a simple request that combines data from two different tables: List all orders, showing the order number and amount, and the name and credit limit of the customer who placed it. The four specific data items requested are clearly stored in two different tables, as shown in Figure 7-1. I The ORDERS table contains the order number and amount of each order, but doesn t have customer names or credit limits. I The CUSTOMERS table contains the customer names and balances, but it lacks any information about orders. There is a link between these two tables, however. In each row of the ORDERS table, the CUST column contains the customer number of the customer who placed the order, which matches the value in the CUST_NUM column in one of the rows in the CUSTOMERS table. Clearly, the SELECT statement that handles the request must somehow use this link between the tables to generate its query results. Before examining the SELECT statement for the query, it s instructive to think about how you would manually handle the request, using paper and pencil. Figure 7-2 shows what you would probably do:
7:
Multitable Queries (Joins)
1. Start by writing down the four column names for the query results. Then move to the ORDERS table, and start with the first order. 2. Look across the row to find the order number (112961) and the order amount ($31,500.00) and copy both values to the first row of query results. 3. Look across the row to find the number of the customer who placed the order (2117), and move to the CUSTOMERS table to find customer number 2117 by searching the CUST_NUM column. 4. Move across the row of the CUSTOMERS table to find the customer s name ( J.P. Sinclair ) and credit limit ($35,000.00), and copy them to the query results table. 5. You ve generated a row of query results! Move back to the ORDERS table, and go to the next row. Repeat the process, starting with Step 2, until you run out of orders. Of course this isn t the only way to generate the query results, but regardless of how you do it, two things will be true: I Each row of query results draws its data from a specific pair of rows, one from the ORDERS table and one from the CUSTOMERS table. I The pair of rows are found by matching the contents of corresponding columns from the tables.
Copyright © OnBarcode.com . All rights reserved.