free barcode generator using vb.net SQL: The Complete Reference in Software

Generator ANSI/AIM Code 128 in Software SQL: The Complete Reference

SQL: The Complete Reference
USS Code 128 Reader In None
Using Barcode Control SDK for Software Control to generate, create, read, scan barcode image in Software applications.
Code 128 Code Set A Creator In None
Using Barcode encoder for Software Control to generate, create Code 128 image in Software applications.
Aliases and Synonyms (CREATE/DROP ALIAS)
Reading Code128 In None
Using Barcode scanner for Software Control to read, scan read, scan image in Software applications.
Generate Code-128 In C#.NET
Using Barcode maker for .NET Control to generate, create Code 128 image in VS .NET applications.
Production databases are often organized like the copy of the sample database shown in Figure 13-5, with all of their major tables collected together and owned by the database administrator. The database administrator gives other users permission to access the tables, using the SQL security scheme described in 15. Recall, however, that you must use qualified table names to refer to another user s tables. In practice, this means that every query against the major tables in Figure 13-5 must use qualified table names, which makes queries like the following one long and tedious to type: List the name, sales, office, and office sales for everyone.
Code 128A Encoder In VS .NET
Using Barcode drawer for ASP.NET Control to generate, create ANSI/AIM Code 128 image in ASP.NET applications.
Code 128 Creator In .NET
Using Barcode drawer for .NET framework Control to generate, create Code128 image in .NET applications.
SELECT NAME, OP_ADMIN.SALESREPS.SALES, OFFICE, OP_ADMIN.OFFICES.SALES FROM OP_ADMIN.SALESREPS, OP_ADMIN.OFFICES
Encode Code 128 Code Set C In VB.NET
Using Barcode printer for .NET framework Control to generate, create USS Code 128 image in .NET applications.
Making Barcode In None
Using Barcode printer for Software Control to generate, create bar code image in Software applications.
To address this problem, many SQL DBMS products provide an alias or synonym capability. A synonym is a name that you define that stands for the name of some other table. In DB2, you create an alias using the CREATE ALIAS statement. (Older versions of DB2 actually used a CREATE SYNONYM statement, and Oracle still uses this form of the statement, but it has the same effect as the CREATE ALIAS statement.) If you were
Making Barcode In None
Using Barcode generator for Software Control to generate, create barcode image in Software applications.
Paint EAN / UCC - 13 In None
Using Barcode generation for Software Control to generate, create EAN128 image in Software applications.
Figure 13-5.
Drawing UCC - 12 In None
Using Barcode creator for Software Control to generate, create UCC - 12 image in Software applications.
Paint ANSI/AIM Code 128 In None
Using Barcode maker for Software Control to generate, create Code 128C image in Software applications.
Typical organization of a production database
Draw International Standard Serial Number In None
Using Barcode printer for Software Control to generate, create International Standard Serial Number image in Software applications.
Bar Code Printer In Java
Using Barcode creation for Android Control to generate, create bar code image in Android applications.
13:
Generating Universal Product Code Version A In None
Using Barcode drawer for Font Control to generate, create Universal Product Code version A image in Font applications.
Scan ANSI/AIM Code 39 In C#
Using Barcode reader for .NET framework Control to read, scan read, scan image in .NET framework applications.
Creating a Database
Printing GTIN - 128 In Objective-C
Using Barcode drawer for iPad Control to generate, create EAN128 image in iPad applications.
USS Code 39 Printer In C#.NET
Using Barcode creator for .NET Control to generate, create Code39 image in .NET applications.
the user named George in Figure 13-5, for example, you might use this pair of CREATE ALIAS statements: Create synonyms for two tables owned by another user.
Decoding Code-128 In Visual Basic .NET
Using Barcode scanner for .NET Control to read, scan read, scan image in VS .NET applications.
Scan DataMatrix In Java
Using Barcode reader for Java Control to read, scan read, scan image in Java applications.
CREATE ALIAS REPS FOR OP_ADMIN.SALESREPS CREATE ALIAS OFFICES FOR OP_ADMIN.OFFICES
Once you have defined a synonym or alias, you can use it just like a table name in SQL queries. The previous query thus becomes:
SELECT NAME, REPS.SALES, OFFICE, OFFICES.SALES FROM REPS, OFFICES
The use of aliases doesn t change the meaning of the query, and you must still have permission to access the other users tables. Nonetheless, synonyms simplify the SQL statements you use and make it appear as if the tables were your own. If you decide later that you no longer want to use the synonyms, they can be removed with the DROP ALIAS statement: Drop the synonyms created earlier.
DATABASE STRUCTURE DROP ALIAS REPS DROP ALIAS OFFICES
Synonyms or aliases are supported by DB2, Oracle, and Informix. They are not specified by the ANSI/ISO SQL standard.
Indexes (CREATE/DROP INDEX)
One of the physical storage structures that is provided by most SQL-based database management systems is an index, which is a structure that provides rapid access to the rows of a table based on the values of one or more columns. Figure 13-6 shows the PRODUCTS table and two indexes that have been created for it. One of the indexes
SQL: The Complete Reference
Figure 13-6.
Two indexes on the PRODUCTS table
provides access based on the DESCRIPTION column. The other provides access based on the primary key of the table, which is a combination of the MFR_ID and PRODUCT_ ID columns. The DBMS uses the index as you might use the index of a book. The index stores data values and pointers to the rows where those data values occur. In the index the data values are arranged in ascending or descending order, so that the DBMS can quickly search the index to find a particular value. It can then follow the pointer to locate the row containing the value. The presence or absence of an index is completely transparent to the SQL user who accesses a table. For example, consider this SELECT statement: Find the quantity and price for size 4 widgets.
13:
Creating a Database
SELECT QTY_ON_HAND, PRICE FROM PRODUCTS WHERE DESCRIPTION = 'Size 4 Widget'
The statement doesn t say whether there is an index on the DESCRIPTION column, and the DBMS will carry out the query in either case. If there were no index for the DESCRIPTION column, the DBMS would be forced to process the query by sequentially scanning the PRODUCTS table, row by row, examining the DESCRIPTION column in each row. To make sure it had found all of the rows that satisfied the search condition, it would have to examine every row in the table. For a large table with thousands or millions of rows, the scan of the table could take minutes or hours. With an index for the DESCRIPTION column, the DBMS can locate the requested data with much less effort. It searches the index to find the requested value ( Size 4 widget ) and then follows the pointer to find the requested row(s) of the table. The index search is very rapid because the index is sorted and its rows are very small. Moving from the index to the row(s) is also very rapid because the index tells the DBMS where on the disk the row(s) are located. As this example shows, the advantage of having an index is that it greatly speeds the execution of SQL statements with search conditions that refer to the indexed column(s). One disadvantage of having an index is that it consumes additional disk space. Another disadvantage is that the index must be updated every time a row is added to the table and every time the indexed column is updated in an existing row. This imposes additional overhead on INSERT and UPDATE statements for the table. In general, it s a good idea to create an index for columns that are used frequently in search conditions. Indexing is also more appropriate when queries against a table are more frequent than inserts and updates. Most DBMS products always establish an index for the primary key of a table, because they anticipate that access to the table will most frequently be via the primary key. Most DBMS products also automatically establish an index for any column (or column combination) defined with a uniqueness constraint. The DBMS must check the value of such a column in any new row to be inserted, or in any update to an existing row, to make certain that the value does not duplicate a value already contained in the table. Without an index on the column(s), the DBMS would have to sequentially search through every row of the table to check the constraint. With an index, the DBMS can simply use the index to find a row (if it exists) with the value in question, which is a much faster operation than a sequential search. In the sample database, these columns are good candidates for additional indexes: I The COMPANY column in the CUSTOMERS table should be indexed if customer data is often retrieved by company name. I The NAME column in the SALESREPS table should be indexed if data about salespeople is often retrieved by salesperson name.
Copyright © OnBarcode.com . All rights reserved.