barcode generator in vb.net code project The CREATE VIEW Statement in Software

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The CREATE VIEW Statement
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As mentioned in 1, views offer some great bene ts to database users by tailoring data to suit individual requirements and hiding complexity. Views have very little overhead when created correctly, and they store no data. In essence, a view is a stored SQL query that can be referenced in SQL DML and DQL statements as if it were a real table. Some like to think of views as virtual tables because they behave like tables (with some restrictions) but don t physically exist as tables. The general syntax for the CREATE VIEW statement is
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CREATE [OR REPLACE] VIEW view_name AS sql_query;
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The optional OR REPLACE keyword eliminates the need for you to drop an existing view before re-creating it. When OR REPLACE is speci ed, if the view already exists, it is replaced, and if it does not already exist, the new view is simply added to the database. The view name must conform to the same naming rules as tables and other database objects. As you will learn in 4, SQL queries name the objects from which they select data but not the object type. This means that view names must be unique among all tables and views in the database. Said another way, view names and table names must come from the same namespace, meaning the same domain of names. The SQL query included in the view de nition can be any valid SQL SELECT statement. You will learn about this essential statement in 4. Creating views is a natural progression you work with the SQL query, making changes and rerunning it until you get the results exactly the way you want them. Then, you simply add the CREATE VIEW statement in front of the query that you have worked out and run the statement to permanently store the query in the database as a view. It s a very productive (and enjoyable) way to work with databases.
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The ALTER TABLE Statement
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Once a table has been created, just about anything that was speci ed in the CREATE TABLE statement can be changed using the ALTER TABLE statement. In recent years, DBMS vendors have offered ever-increasing exibility in changing the de nition of tables in place (that is, without having to drop and recreate them). Much of this is out of necessity because of 7 24 uptime requirements (databases that can never be taken out of service) and rapid data growth rates, yielding tables so large that it s not practical to drop and re-create them. Another area where personal style and preference come into play is the use of the ALTER TABLE statement. Many database administrators prefer to keep their CREATE TABLE statements neat and simple, and thus they refrain from de ning constraints in the CREATE TABLE statement. Instead, they add ALTER TABLE statements after the CREATE TABLE statement to add all the constraints to the table (primary key, foreign key, unique, check, and so forth). The downside of this approach is that it requires a lot more typing. On the other hand, the CREATE TABLE statement is a lot easier to understand without constraints, and writing constraints independently makes it a lot easier to reuse the statements should you need to drop and re-create constraints.
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CHAPTER 3 De ning Database Objects Using SQL
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While there is a bit of variation across DBMS implementations, here is a list of the types of changes usually supported by the ALTER TABLE statement, along with the general syntax for each type: Adding a column to a table. The column de nition is exactly the same syntax as the one used in the CREATE TABLE statement.
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ALTER TABLE table_name ADD ( <column_definition> [,<column_definition> ...]; Example: ALTER TABLE CUSTOMER_ACCOUNT ADD (CUSTOMER_HOLD_DATE DATE NULL, HOLD_PLACED_BY VARCHAR(50));
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Changing the de nition of a column. Most DBMSs won t let you decrease column precision if there is data in the table, and very few will let you change the data type of an existing column. However, the ability to increase the precision of a column, add or change a column default, or change between NULL and NOT NULL is typically supported. Changing unnamed column constraints can be problematic, which is another good reason to name all your constraints.
ALTER TABLE table_name MODIFY [COLUMN] (column_definition) [,<column_definition> ...]; Example: ALTER TABLE CUSTOMER_ACCOUNT MODIFY (CUSTOMER_DEPOSIT_AMOUNT NUMERIC(7,2) DEFAULT 0 NOT NULL);
N OTE : Microsoft SQL Server, DB2, and PostgreSQL use the keyword ALTER COLUMN instead of MODIFY COLUMN. However, MySQL and Oracle use the MODIFY keyword as shown. Also note that most DBMSs will not permit you to change a column to NOT NULL if there are existing rows in the table that have null values in that column you have to change the null values to some other value rst. The UPDATE statement required to modify column data values is covered in 7. Adding a new constraint. The constraint de nition is identical to a table constraint de nition that could appear in a CREATE TABLE statement.
ALTER TABLE table_name ADD CONSTRAINT <constraint_definition>;
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