free barcode generator using vb.net Part III: Updating Data in Software

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Part III: Updating Data
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10: 11: 12: Database Updates Data Integrity Transaction Processing
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10: Database Updates
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Overview
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SQL is a complete data manipulation language that is used not only for database queries, but also to modify and update data in the database. Compared to the complexity of the SELECT statement, which supports SQL queries, the SQL statements that modify database contents are extremely simple. However, database updates pose some challenges for a DBMS beyond those presented by database queries. The DBMS must protect the integrity of stored data during changes, ensuring that only valid data is introduced into the database, and that the database remains self-consistent, even in the event of system failures. The DBMS must also coordinate simultaneous updates by multiple users, ensuring that the users and their changes do not interfere with one another. This chapter describes the three SQL statements that are used to modify the contents of a database: INSERT, which adds new rows of data to a table, DELETE, which removes rows of data from a table, and
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UPDATE, which modifies existing data in the database. In 11, SQL facilities for maintaining data integrity are described. 12 covers SQL support for multi-user concurrency.
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Adding Data to the Database
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A new row of data is typically added to a relational database when a new entity represented by the row "appears in the outside world." For example, in the sample database: When you hire a new salesperson, a new row must be added to the SALESREPS table to store the salesperson's data. When a salesperson signs a new customer, a new row must be added to the CUSTOMERS table, representing the new customer. When a customer places an order, a new row must be added to the ORDERS table to contain the order data. In each case, the new row is added to maintain the database as an accurate model of the real world. The smallest unit of data that can be added to a relational database is a single row. In general, a SQL-based DBMS provides three ways to add new rows of data to a database: A single-row INSERT statement adds a single new row of data to a table. It is commonly used in daily applications for example, data entry programs. A multi-row INSERT statement extracts rows of data from another part of the database and adds them to a table. It is commonly used in end-of-month or end-of-year processing when "old" rows of a table are moved to an inactive table. A bulk load utility adds data to a table from a file that is outside of the database. It is commonly used to initially load the database or to incorporate data downloaded from another computer system or collected from many sites.
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The Single-Row INSERT Statement
The single-row INSERT statement, shown in Figure 10-1, adds a new row to a table. The INTO clause specifies the table that receives the new row (the target table), and the VALUES clause specifies the data values that the new row will contain. The column list indicates which data value goes into which column of the new row
Figure 10-1: Single-row INSERT statement syntax diagram
Suppose you just hired a new salesperson, Henry Jacobsen, with the following personal data:
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Name: Age: Employee Number: Title: Office: Hire Date: Quota: Year-to-Date Sales:
Henry Jacobsen 36 111 Sales Manager Atlanta (office number 13) July 25, 1990 Not yet assigned $0.00
Here is the INSERT statement that adds Mr. Jacobsen to the sample database: Add Henry Jacobsen as a new salesperson. INSERT INTO SALESREPS (NAME, AGE, EMPL_NUM, SALES, TITLE, HIRE_DATE, REP_OFFICE) VALUES ('Henry Jacobsen', 36, 111, 0.00, 'Sales Mgr', '25-JUL-90', 13) 1 row inserted. Figure 10-2 graphically illustrates how SQL carries out this INSERT statement. Conceptually, the INSERT statement builds a single row of data that matches the column structure of the table, fills it with the data from the VALUES clause, and then adds the new row to the table. The rows of a table are unordered, so there is no notion of inserting the row "at the top" or "at the bottom" or "between two rows" of the table. After the INSERT statement, the new row is simply a part of the table. A subsequent query against the SALESREPS table will include the new row, but it may appear anywhere among the rows of query results.
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