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This chapter described the SQL Data Definition Language features that define and change the structure of a database: The CREATE TABLE statement creates a table and defines its columns. It can also be used to define primary key, foreign key (referential), and CHECK constraints as the table is created. The DROP TABLE statement removes a previously created table from the database. The ALTER TABLE statement can be used to add, change, or remove a column or a constraint on an existing table. The CREATE INDEX and DROP INDEX statements define indexes, which speed database queries but add overhead to database updates. Most DBMS brands support other CREATE, DROP, and ALTER statements used with DBMS-specific objects. The SQL standard specifies a database schema containing a collection of tables, and the database schema is manipulated with CREATE SCHEMA and DROP SCHEMA statements. Various DBMS brands use very different approaches to organizing the one or more databases that they manage, and these differences affect the way you design your databases and gain access to them.
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he tables of a database define the structure and organization of its data. However, SQL also lets you look at the stored data in other ways by defining alternative views of the data. A view is a SQL query that is permanently stored in the database and assigned a name. The results of the stored query are visible through the view, and SQL lets you access these query results as if they were, in fact, a real table in the database. Views are an important part of SQL for several reasons: Views let you tailor the appearance of a database so that different users see it from different perspectives. Views let you restrict access to data, allowing different users to see only certain rows and/or certain columns of a table. Views simplify database access by presenting the structure of the stored data in the way that is most natural for each user, including hiding complexities such as joins. This chapter describes how to create views and how to use views to simplify processing and enhance the security of a database.
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A view is a virtual table in the database whose contents are defined by a query, as shown in Figure 14-1. To the database user, the view appears just like a real table, with a set of named columns and rows of data. But unlike a real table, a view does not exist in the database as a stored set of data values. Instead, the rows and columns of data visible through the view are the query results produced by the query that defines the view. SQL creates the illusion of the view by giving the view a name like a table name and storing the definition of the view in the database. The view shown in Figure 14-1 is typical. It has been given the name REPDATA and is defined by this two-table query:
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SELECT NAME, CITY, REGION, QUOTA, SALESREPS.SALES FROM SALESREPS, OFFICES WHERE REP_OFFICE = OFFICE;
Part IV:
Database Structure
SALESREPS Table EMPL_NUM NAME 105 109 102 106 104 101 110 108 Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Tom Snyder Larry Fitch
AGE 37 31 48 52 33 45 41 62
QUOTA $350,000.00 $300,000.00 $350,000.00 $275,000.00 $200,000.00 $300,000.00 NULL $350,000.00
SALES $367,911.00 $392,725.00 $474,050.00 $299,912.00 $142,594.00 $305,673.00 $75,985.00 $361,865.00
REPDATA View 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
REGION Eastern Eastern Eastern Eastern Eastern Eastern Western Western Western
QUOTA $300,000.00 $275,000.00 $200,000.00 $275,000.00 $300,000.00 $350,000.00 $350,000.00 $350,000.00 $300,000.00
SALES $392,725.00 $299,912.00 $142,594.00 $286,775.00 $305,673.00 $367,911.00 $474,050.00 $361,865.00 $186,042.00
OFFICES Table OFFICE CITY 22 11 12 13 21 Denver New York Chicago Atlanta Los Angeles
REGION Western Eastern Eastern Eastern Western
MGR 108 106 104 NULL 108
FIGURE 14-1
A typical view with two source tables
The data in the view comes from the SALESREPS and OFFICES tables. These tables are called the source tables for the view because they are the source of the data that is visible through the view. This view contains one row of information for each salesperson, extended with the name of the city and region where the salesperson works. As shown in the figure, the view appears as a table, and its contents look just like the query results that you would obtain if you actually were to run the query. Once a view is defined, you can use it in a SELECT statement, just like a real table, as in this query:
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