free barcode generator in vb.net RETRIEVING DATA in Software

Print Code 128 Code Set C in Software RETRIEVING DATA

RETRIEVING DATA
Reading Code 128C In None
Using Barcode Control SDK for Software Control to generate, create, read, scan barcode image in Software applications.
Code 128 Code Set C Creation In None
Using Barcode generation for Software Control to generate, create Code 128C image in Software applications.
Outer Joins *
USS Code 128 Decoder In None
Using Barcode reader for Software Control to read, scan read, scan image in Software applications.
Code 128C Creation In Visual C#.NET
Using Barcode drawer for Visual Studio .NET Control to generate, create Code 128A image in .NET applications.
The SQL join operation combines information from two tables by forming pairs of related rows from the two tables. The row pairs that make up the joined table are those where the matching columns in each of the two tables have the same value. If one of the rows of a table is unmatched in this process, the join can produce unexpected results, as illustrated by these queries: List the salespeople and the offices where they work.
Printing Code 128 Code Set A In Visual Studio .NET
Using Barcode drawer for ASP.NET Control to generate, create Code 128 image in ASP.NET applications.
Make Code 128B In .NET Framework
Using Barcode printer for .NET Control to generate, create Code 128B image in .NET framework applications.
SELECT NAME, REP_OFFICE FROM SALESREPS NAME REP_OFFICE -------------- ----------Bill Adams 13 Mary Jones 11 Sue Smith 21
Paint ANSI/AIM Code 128 In VB.NET
Using Barcode printer for .NET Control to generate, create Code 128A image in .NET framework applications.
UPC-A Generation In None
Using Barcode generation for Software Control to generate, create UPCA image in Software applications.
SQL: The Complete Reference
Make Bar Code In None
Using Barcode drawer for Software Control to generate, create barcode image in Software applications.
Printing ANSI/AIM Code 128 In None
Using Barcode creator for Software Control to generate, create Code 128C image in Software applications.
Sam Clark Bob Smith Dan Roberts Tom Snyder Larry Fitch Paul Cruz Nancy Angelli
Painting EAN 128 In None
Using Barcode maker for Software Control to generate, create EAN 128 image in Software applications.
Data Matrix Generator In None
Using Barcode creation for Software Control to generate, create Data Matrix image in Software applications.
11 12 12 NULL 21 12 22
British Royal Mail 4-State Customer Code Encoder In None
Using Barcode generation for Software Control to generate, create British Royal Mail 4-State Customer Code image in Software applications.
EAN-13 Supplement 5 Reader In None
Using Barcode decoder for Software Control to read, scan read, scan image in Software applications.
List the salespeople and the cities where they work.
Print Code 128 Code Set C In Java
Using Barcode maker for Java Control to generate, create Code 128B image in Java applications.
Code 128B Drawer In Objective-C
Using Barcode creation for iPhone Control to generate, create Code 128 image in iPhone applications.
SELECT NAME, CITY FROM SALESREPS, OFFICES WHERE REP_OFFICE = OFFICE NAME -------------Mary Jones Sam Clark Bob Smith Paul Cruz Dan Roberts Bill Adams Sue Smith Larry Fitch Nancy Angelli CITY -----------New York New York Chicago Chicago Chicago Atlanta Los Angeles Los Angeles Denver
Barcode Decoder In VS .NET
Using Barcode recognizer for .NET framework Control to read, scan read, scan image in Visual Studio .NET applications.
Painting UPC-A Supplement 5 In VS .NET
Using Barcode maker for .NET framework Control to generate, create Universal Product Code version A image in .NET applications.
Based on the English-language descriptions of these two queries, you would probably expect them to produce the same number of rows. But the first query includes a row for each of the ten salespeople, while the second query produces only nine. Why Because Tom Snyder is currently unassigned and has a NULL value in the REP_OFFICE column (which is the matching column for the join). This NULL value doesn t match any of the office numbers in the OFFICES table, so Tom s row in the SALESREPS table is unmatched. As a result, it vanishes in the join. The standard SQL join thus has the potential to lose information if the tables being joined contain unmatched rows. Based on the English-language version of the request, you would probably expect the second query to produce results like these: List the salespeople and the cities where they work.
Reading Barcode In Java
Using Barcode recognizer for Java Control to read, scan read, scan image in Java applications.
Generating Data Matrix ECC200 In None
Using Barcode maker for Excel Control to generate, create Data Matrix 2d barcode image in Microsoft Excel applications.
SELECT NAME, CITY FROM SALESREPS, OFFICES WHERE REP_OFFICE *= OFFICE
7:
Multitable Queries (Joins)
NAME -------------Tom Snyder Mary Jones Sam Clark Bob Smith Paul Cruz Dan Roberts Bill Adams Sue Smith Larry Fitch Nancy Angelli
CITY -----------NULL New York New York Chicago Chicago Chicago Atlanta Los Angeles Los Angeles Denver
RETRIEVING DATA
These query results are generated by using a different type of join operation, called an outer join (indicated by the *= notation in the WHERE clause). The outer join is an extension of the standard join described earlier in this chapter, which is sometimes called an inner join. The SQL1 standard specifies only the inner join; it does not include the outer join. The earlier IBM SQL products also support only the inner join. However, the outer join is a well-understood and useful part of the relational database model, and it has been implemented in many non-IBM SQL products, including the flagship database products from Microsoft, Sybase, Oracle, and IBM s Informix. The outer join is also the most natural way to express a certain type of query request, as shown in the remainder of this section. To understand the outer join well, it s useful to move away from the sample database and consider the two simple tables in Figure 7-12. The GIRLS table lists five girls and the cities where they live; the BOYS table lists five boys and the cities where they live. To find the girl/boy pairs who live in the same city, you could use this query, which forms the inner join of the two tables: List the girls and boys who live in the same city.
SELECT * FROM GIRLS, BOYS WHERE GIRLS.CITY = BOYS.CITY GIRLS.NAME ----------Mary Mary Susan Betty GIRLS.CITY ----------Boston Boston Chicago Chicago BOYS.NAME ---------John Henry Sam Sam BOYS.CITY ---------Boston Boston Chicago Chicago
SQL: The Complete Reference
Figure 7-12.
Anatomy of an outer join
The inner join produces four rows of query results. Notice that two of the girls (Anne and Nancy) and two of the boys (James and George) are not represented in the query results. These rows cannot be paired with any row from the other table, and so they are missing from the inner join results. Two of the unmatched rows (Anne and James) have valid values in their CITY columns, but they don t match any cities in the opposite table. The other two unmatched rows (Nancy and George) have NULL values in their CITY columns, and by the rules of SQL NULL handling, the NULL value doesn t match any other value (even another NULL value). Suppose you wanted to list the girl/boy pairs who share the same cities and include the unmatched girls and boys in the list. The outer join of the GIRLS and BOYS tables produces exactly this result. The following list shows the procedure for constructing the outer join, and the outer join is shown graphically in Figure 7-12. 1. Begin with the inner join of the two tables, using matching columns in the normal way. 2. For each row of the first table that is not matched by any row in the second table, add one row to the query results, using the values of the columns in the first table, and assuming a NULL value for all columns of the second table.
7:
Multitable Queries (Joins)
3. For each row of the second table that is not matched by any row in the first table, add one row to the query results, using the values of the columns in the second table, and assuming a NULL value for all columns of the first table. 4. The resulting table is the outer join of the two tables. Here is the SQL statement that produces the outer join: List girls and boys in the same city, including any unmatched girls or boys.
SELECT * FROM GIRLS, BOYS WHERE GIRLS.CITY *=* BOYS.CITY GIRLS.NAME ----------Mary Mary Susan Betty Anne Nancy NULL NULL GIRLS.CITY ----------Boston Boston Chicago Chicago Denver NULL NULL NULL BOYS.NAME ---------John Henry Sam Sam NULL NULL James George BOYS.CITY ---------Boston Boston Chicago Chicago NULL NULL Dallas NULL RETRIEVING DATA
The outer join of the two tables contains eight rows. Four of the rows are identical to those of the inner join between the two tables. Two other rows, for Anne and Nancy, come from the unmatched rows of the GIRLS table. These rows have been NULLextended by matching them to an imaginary row of all NULLs in the BOYS table, and adding them to the query results. The final two rows, for James and George, come from the unmatched rows of the BOYS table. These rows have also been NULL-extended by matching them to an imaginary row of all NULLs in the GIRLS table and then adding them to the query results. As this example shows, the outer join is an information-preserving join. Every row of the BOYS table is represented in the query results (some more than once). Similarly, every row of the GIRLS table is represented in the query results (again, some more than once).
Left and Right Outer Joins *
Technically, the outer join produced by the previous query is called the full outer join of the two tables. Both tables are treated symmetrically in the full outer join. Two other well-defined outer joins do not treat the two tables symmetrically. The left outer join between two tables is produced by following Step 1 and Step 2 in the previous numbered list but omitting Step 3. The left outer join thus includes
Copyright © OnBarcode.com . All rights reserved.