8 Managing Schema Objects

About Tables

The table is the basic unit of data storage in an Oracle database. It holds all user-accessible data. Each table is made up of columns and rows. In the employees table, for example, there are columns called last_name and employee_id. Each row in the table represents a different employee, and contains a value for last_name and employee_id.

When you create a table, you specify the table type, and define its columns and constraints. Constraints are rules that help preserve data integrity.

This section contains the following topics:

• About Table Types

• About Table Column Attributes

• About Table-Level Constraints

• Other Table Creation Considerations

Viewing Tables

You can use Database Control to list all the tables in a specified schema, and to view the definitions of individual tables.

To view tables:

1. Go to the Database Home page, logging in as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Schema to view the Schema subpage.

3. In the Database Objects section, click Tables.

   The Tables page appears.

4. In the Schema field, enter the name of a schema. Alternatively, click the flashlight icon adjacent to the Schema field to search for a schema.

   Examples of schema names include SYS and hr.

5. Leave the Object Name field blank to search for and display all tables in the schema. Alternatively, enter a table name or partial table name to limit the search.

   If you enter a search string in the Object Name field, then all tables that have names that start with the search string are displayed. If you precede the search string with an asterisk (*), then all tables that have the search string anywhere in the table name are displayed.

6. Click Go.

   The tables in the specified schema are displayed.

   
   Description of the illustration hr_tables.gif
   
   

7. To view the definition of a particular table, select the table and then click View. Alternatively, click the table name.

   The View Table page appears.

Viewing Table Data

Besides viewing table names and table definitions, you can view the data stored in the table, and the SQL statement used to display the data. You can also change the SQL statement to alter the results set.

To view table data:

1. Search for a table as described in "Viewing Tables". For example, search for the tables in the hr schema.

2. Select the table that contains the data.

   For example, select employees.

3. In the Actions list, select View Data, and then click Go.

   The View Data for Table page appears.

   
   Description of the illustration view_employees.gif
   
   

   The Query field displays the SQL query that was run to view the data for the table. The Result section shows the data in the table. You may have to use the horizontal scroll bar at the bottom of the page to view all columns.

4. (Optional) Click a column name to sort the data by that column.

5. (Optional) Click Refine Query to change the query and redisplay the data.

   The Refine Query for Table page appears. This page enables you to select the columns to display. It also enables you to specify a WHERE clause for the SQL SELECT statement to limit the results.

You can also write and submit your own SQL SELECT statement to see the contents of a table. You can run SQL statements by starting a SQL Worksheet session in Database Control. To do so, click SQL Worksheet in the Related Links section of the Database Home page.

A detailed description of the SELECT statement is in Oracle Database SQL Language Reference.

Example: Creating a Table

You can use Database Control to create a table. Before you create and populate a table, you can estimate its size to ensure that you have sufficient space to hold its data.

In the following example, you create a table called purchase_orders in the nick schema that you created in Chapter 7, "Administering User Accounts and Security". The table has the following columns:

To create the PURCHASE_ORDERS table in the NICK schema:

1. Go to the Database Home page, logging in as user nick or as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Schema to view the Schema subpage.

3. In the Database Objects section, click Tables.

   The Tables page appears.

4. Click Create.

   The Create Table: Table Organization page appears.

5. Select Standard, Heap Organized, and then click Continue.

   The Create Table page appears.

6. In the Name field, enter purchase_orders as the table name, and in the Schema field, enter nick.

   Because a default tablespace was specified when creating the user nick in the section "Example: Creating a User Account", accept the default tablespace setting for the table.

7. In the Columns section, enter column information for the purchase_orders table as specified in the table in the introduction to this topic. For example, for the first column in the purchase_orders table, enter the name PO_NUMBER and the data type NUMBER, and select the Not NULL check box.

   For all purchase_orders columns, you can leave Scale and Default Value blank.

   
   Description of the illustration purchase_orders2.gif
   
   

   Note:

   To create the table with partitions, click Partitions at the top of the page during this step.

8. (Optional) Obtain an estimate of the table size by completing the following steps:

   1. Click Estimate Table Size.

      The Estimate Table Size page appears.

   2. In the Projected Row Count field, enter 400000 (four hundred thousand), and then click Estimate Table Size.

      The estimated results are calculated and displayed.

   3. Click OK to return to the Create Table page.

   The estimate of the table size can help you determine what values to use when specifying the storage parameters for the table.

9. Click Constraints to view the Constraints subpage, where you can designate a primary key for the table.

10. In the Constraints list, select PRIMARY, then click Add.

    The Add PRIMARY Constraint page appears.

    
    Description of the illustration primary_constraint.gif
    
    

11. In the Available Columns list, select PO_NUMBER, and then click Move.

    Note:

    You can also double-click PO_NUMBER.

    The po_number column moves to the Selected Columns list.

12. Click Continue to return to the Constraints subpage of the Create Table page.

13. Click OK.

    The Tables page returns, showing a confirmation message and listing the new table in the tables list. The purchase_orders table is now created with po_number as its primary key.

Modifying Table Attributes

You can use Database Control to add and delete table columns and to manage table constraints. This section contains the following topics:

• Example: Adding Table Columns

• Example: Deleting a Table Column

• Example: Adding a New Table Constraint

• Example: Modifying an Existing Table Constraint

• Example: Deleting a Table Constraint

Example: Loading Data into a Table

You can use Database Control to load data into a table. You can load data from a source file that is on your local computer—the one where your browser is running—or from a source file that is on the database host computer—the computer on which the Oracle instance is running. Because Database Control invokes the Oracle SQL*Loader utility to load the data, the format of the data in the source file can be of any format that is supported by SQL*Loader. In this example, you use a comma-delimited text file as the source file. In SQL*Loader terminology, the source file is referred to as the data file.

SQL*Loader also uses a control file to control the loading of data from the data file. The control file is a text file that contains statements written in the SQL*Loader command language. These statements specify where to find the data, how to parse and interpret the data, where to insert the data, and more. Database Control contains a Load Data wizard that takes you through the steps of preparing and running a data load job with SQL*Loader. (A wizard is an online, guided workflow.) The Load Data wizard can automatically create the SQL*Loader control file for you.

In this example, you load data into the PURCHASE_ORDERS table that you created in "Example: Creating a Table". For simplicity, this example loads only three rows.

To prepare for this example, you must create a text file named load.dat on the file system of the database host computer or on the file system of your local computer. The contents of the file should be as follows:

1, Office Equipment, 25-MAY-2006, 1201, 13-JUN-2006
2, Computer System, 18-JUN-2006, 1201, 27-JUN-2006
3, Travel Expense, 26-JUN-2006, 1340, 11-JUL-2006

To load data into the PURCHASE_ORDERS table:

1. Go to the Database Home page, logging in as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Data Movement.

   The Data Movement subpage appears.

3. In the Move Row Data section, click Load Data from User Files.

   The Load Data: Generate or Use Existing Control File page appears.

4. Select Automatically Generate Control File, and enter host computer credentials (user name and password) for the database host computer.

5. Click Continue.

   The first page of the Load Data wizard appears. Its title is Load Data: Data Files.

6. Follow the steps in the wizard, clicking Next to proceed to each new step.

   For information about using any of the wizard pages, click Help on that page. At the conclusion of the wizard, you submit a job that runs SQL*Loader. A job status page is then displayed. If necessary, refresh the status page until you see a succeeded (or failed) status.

7. If the job succeeded, then confirm that the data was successfully loaded by doing one of the following:

   • View the table data.

     See "Viewing Table Data".

   • Examine the SQL*Loader log file, which is written to the host computer directory that you designated for the SQL*Loader data file.

   Note:

   If the job succeeds, then it means only that Database Control was able to run the SQL*Loader utility. It does not necessarily mean that SQL*Loader ran without errors. For this reason, you must confirm that the data loaded successfully.

8. If the job failed, then examine the SQL*Loader log file, correct any errors, and try again.

Deleting a Table

If you no longer need a table, then you can delete it using Database Control. When you delete a table, the database deletes the data and dependent objects of the table (such as indexes), and removes the table from the data dictionary.

When you delete a table from a locally managed tablespace that is not the SYSTEM tablespace, the database does not immediately reclaim the space associated with the table. Instead, it places the table and any dependent objects in the recycle bin. You can then restore the table, its data, and its dependent objects from the recycle bin if necessary. You can view the contents of the recycle bin by clicking Recycle Bin on the Tables page. Note that users can see only tables that they own in the recycle bin. See Oracle Database Administrator's Guide for more information about the recycle bin, including how to view, purge, and recover tables for which you are not the owner.

To delete a table:

1. Search for the table to delete, as explained in "Viewing Tables".

2. Select the table, and then click Delete With Options.

   The Delete With Options page appears.

3. Select Delete the table definition, all its data, and dependent objects (DROP).

4. Select Delete all referential integrity constraints (CASCADE CONTRAINTS).

5. Click Yes.

   The Tables page returns and displays a confirmation message.

About Indexes

Indexes are optional schema objects that are associated with tables. You create indexes on tables to improve query performance. Just as the index in a guide helps you to quickly locate specific information, an Oracle Database index provides quick access to table data.

You can create as many indexes on a table as you need. You create each index on one or more columns of a table. For example, in a purchase orders table, if you create an index on the vendor number column, then you can then sequentially access the rows of the table in vendor number order, without having to actually sort the rows. Additionally, you can directly access all purchase orders issued to a particular vendor without having to scan the entire table.

After an index is created, it is automatically maintained and used by the database. Changes to the data or structure of a table, such as adding new rows, updating rows, or deleting rows, are automatically incorporated into all relevant indexes. This is transparent to the user.

Some indexes are created implicitly through constraints that are placed on a table. For example, the database automatically creates an index on the columns of a primary key constraint or unique key constraint.

The following topics provide more background information about indexes:

• Indexes and Performance

• Index Attributes

Viewing Indexes

You use the Indexes page of Database Control to view the indexes in your database.

To view indexes:

1. Go to the Database Home page, logging in as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Schema to view the Schema subpage.

3. In the Database Objects section, click Indexes.

   The Indexes page appears.

4. In the Search By list, do one of the following:

   • Select Index Name to search for indexes by name.

     Every index has a system-assigned or user-assigned name.

   • Select Table Name to search for indexes that belong to a particular table.

5. In the Schema field, enter the name of a schema. Alternatively, click the flashlight icon adjacent to the Schema field to search for a schema.

6. Do one of the following:

   • If you are searching by index name, then leave the Object Name field blank to search for and display all indexes in the schema. Alternatively, enter an index name or partial index name as a search string.

     If you enter a search string in the Object Name field, then all indexes that have names that start with the search string are displayed. If you precede the search string with an asterisk (*), then all indexes that have the search string anywhere in the index name are displayed.

   • If you are searching by table name, then enter a table name or partial table name in the Object Name field.

     If you enter a partial table name as a search string, then indexes are displayed for all tables that have names that start with the search string. If you precede the search string with an asterisk (*), then indexes are displayed for all tables that have the search string anywhere in the table name.

7. Click Go.

   The indexes in the specified schema are displayed.

8. To view the definition of a particular index, select the index and then click View. Alternatively, double-click the index name.

   The View Index page appears. This page includes basic information about the index, including its status, the table and column or columns on which it is built, the space used by the index, the options used in its definition, and index statistics.

Example: Creating an Index

When you create an index, you specify one or more table columns to be indexed and the type of index to create.

In this example, you create a standard B-tree index on the SUPPLIER_ID column in the SH.PRODUCTS table. (The SH schema is part of the sample schemas.)

To create a supplier index on the SH.PRODUCTS table:

1. View the tables in the SH schema, by following the instructions in the section "Viewing Tables".

2. Select the PRODUCTS table.

3. In the Actions list, select Create Index, and then click Go.

   The Create Index page appears.

   
   Description of the illustration create_index.gif
   
   

4. Enter the following information:

   • In the Name field, enter PRODUCTS_SUPPLIER_IDX.

   • For the Tablespace field, accept the default value.

   • For Index Type, select Standard - B-tree.

   • In the Table Columns list, select the SUPPLIER_ID column by entering 1 in the Order column.

     If your index were to consist of multiple columns (a concatenated index), then you would enter 2 in the next column to include, and so on. These numbers indicate the order in which the columns are to be concatenated, from left to right, or from most significant in the sort order to least significant.

   • For Sorting Order, accept the default selection of ASC (ascending).

5. Click OK to create the index.

   The Indexes page returns and displays a confirmation message. The new index is listed in the table of indexes.

Example: Deleting an Index

If you no longer need an index, then you can delete it using Database Control.

In this example, you delete the PRODUCTS_SUPPLIER_IDX index that you created previously on the SH.PRODUCTS table in "Example: Creating an Index".

To delete the supplier index on the SH.PRODUCTS table:

1. Go to the Database Home page, logging in as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Schema to view the Schema subpage.

3. In the Database Objects section, click Indexes.

   The Indexes page appears.

4. In the Search By list, select Table Name.

5. In the Schema field, enter SH.

6. In the Object Name field, enter PROD.

   You can enter only the first few letters of the table name.

7. Click Go.

   All indexes on the PRODUCTS table are displayed.

8. Select the PRODUCTS_SUPPLIER_IDX index, and then click Delete.

   A confirmation page appears.

9. Click Yes to delete the index.

   The Indexes page returns and displays a confirmation message.

About Views

Views are customized presentations of data in one or more tables or other views. You can think of them as stored queries. Views do not actually contain data, but instead derive their data from the tables upon which they are based. These tables are referred to as the base tables of the view.

Similar to tables, views can be queried, updated, inserted into, and deleted from, with some restrictions. All operations performed on a view actually affect the base tables of the view. Views can provide an additional level of security by restricting access to a predetermined set of rows and columns of a table. They can also hide data complexity and store complex queries.

Many important views are in the SYS schema. There are two types: static data dictionary views and dynamic performance views. Complete descriptions of the views in the SYS schema are in Oracle Database Reference.

Static Data Dictionary Views

The data dictionary views are called static views because they change infrequently, only when a change is made to the data dictionary. Examples of data dictionary changes include creating a new table or granting a privilege to a user.

Many data dictionary tables have three corresponding views:

• A DBA_ view displays all relevant information in the entire database. DBA_ views are intended only for administrators.

  An example of a DBA_ view is DBA_TABLESPACES, which contains one row for each tablespace in the database.

• An ALL_ view displays all the information accessible to the current user, including information from the schema of the current user, and information from objects in other schemas, if the current user has access to those objects through privileges or roles.

  An example of an ALL_ view is ALL_TABLES, which contains one row for every table for which the user has object privileges.

• A USER_ view displays all the information from the schema of the current user. No special privileges are required to query these views.

  An example of a USER_ view is USER_TABLES, which contains one row for every table owned by the user.

The columns in the DBA_, ALL_, and USER_ views are usually nearly identical.

Dynamic Performance Views

Dynamic performance views monitor ongoing database activity. They are available only to administrators. The names of dynamic performance views start with the characters V$. For this reason, these views are often referred to as V$ views.

An example of a V$ view is V$SGA, which returns the current sizes of various System Global Area (SGA) memory components.

Displaying Views

You can use Oracle Enterprise Manager Database Control (Database Control) to list the views in a specified schema. You can also display the view definitions.

To display views:

1. Go to the Database Home page, logging in as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Schema to view the Schema subpage.

3. In the Database Objects section, click Views.

   The Views page appears.

4. In the Schema field, enter the name of a schema. Alternatively, click the flashlight icon adjacent to the Schema field to search for a schema.

   Examples of schema names include SYS and hr.

5. Leave the Object Name field blank to search for and display all views in the schema. Alternatively, enter a view name or partial view name to limit the search.

   If you enter a search string in the Object Name field, then all views that have names that start with the search string are displayed. If you precede the search string with an asterisk (*), then all views that have the search string anywhere in the view name are displayed.

6. Click Go.

   The views in the specified schema are displayed.

7. To view the definition of a particular view, select the view and then click View. Alternatively, double-click the view name.

   The View page appears.

Example: Creating a View

In this example, you create a view named king_view, which uses the hr.employees table as its base table. (The hr schema is part of the sample schemas.) This view filters the table data so that only employees who report directly to the manager King, whose employee ID is 100, are returned in queries. In an application scenario, this view adds an additional level of security to the hr.employees table while providing a suitable presentation of relevant information for manager King.

To create the KING_VIEW view on the HR.EMPLOYEES table:

1. Go to the Database Home page, logging in as user hr or as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Schema to view the Schema subpage.

3. In the Database Objects section, click Views.

   The Views page appears.

4. Click Create.

   The Create View page appears.

5. Enter the following information:

   • In the Name field, enter king_view.

   • In the Schema field, enter hr.

   • In the Query Text field, enter the following SQL statement:

     SELECT * FROM hr.employees
     WHERE manager_id = 100
     

6. Click OK.

   The Views page returns and displays a confirmation message. The new view appears in the list of views.

To test the new KING_VIEW view:

1. On the Views page, select king_view and then select View Data from the Actions list.

2. Click Go.

   The View Data for View page appears. The data selected by the view appears in the Result section.

3. (Optional) You can also test the view by submitting the following SQL statement in SQL*Plus or SQL Developer:

   SELECT * FROM king_view
   

Example: Deleting a View

If you no longer need a view, then you can delete it using Database Control.

In this example, you delete the hr.king_view view that you created previously in "Example: Creating a View".

To delete the HR.KING_VIEW view:

1. Go to the Database Home page, logging in as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Schema to view the Schema subpage.

3. In the Database Objects section, click Views.

   The Views page appears.

4. In the Schema field, enter hr.

5. In the Object Name field, enter king.

   You can enter just the first few letters of the view name.

6. Click Go.

   KING_VIEW is displayed in the list of views.

7. Select KING_VIEW, and then click Delete.

   A Confirmation page appears.

8. Click Yes to delete the view.

   The Views page returns and displays a confirmation message.

About Program Code Stored in the Database

Oracle Database offers the ability to store program code in the database. Developers write program code in PL/SQL or Java, and store the code in schema objects. You, as the DBA, can use Oracle Enterprise Manager Database Control (Database Control) to manage program code objects such as:

• PL/SQL packages, procedures, functions, and triggers

• Java source code (Java sources) and compiled Java classes

The actions that you can perform include creating, compiling, creating synonyms for, granting privileges on, and showing dependencies for these code objects. You access administration pages for these objects by clicking links in the Programs section of the Schema subpage.

Note that creating and managing program code objects is primarily the responsibility of application developers. However, as a DBA you might have to assist in managing these objects. Your most frequent task for program code objects might be to revalidate (compile) them, because they can become invalidated if the schema objects on which they depend change or are deleted.

Validating (Compiling) Invalid Schema Objects

As a database administrator (DBA), you may be asked to revalidate schema objects that have become invalid. Schema objects (such as triggers, procedures, or views) might be invalidated when changes are made to objects on which they depend. For example, if a PL/SQL procedure contains a query on a table and you modify table columns that are referenced in the query, then the PL/SQL procedure becomes invalid. You revalidate schema objects by compiling them.

Database Control notifies you when schema objects become invalid by displaying an alert in the Alerts section of the Database Home page.

There are two ways to display schema objects that require validation: by following an alert on the Database Home page, or by viewing the appropriate object page (Views page, Procedures page, and so on) and searching for the objects.

To validate schema objects starting from an alert:

1. Go to the Database Home page, logging in as user SYSTEM.

   See "Accessing the Database Home Page".

2. In the Alerts section, search for alerts with the following message:

   n object(s) are invalid in the schema_name schema.
   

   An example of such a message is the following:

   4 object(s) are invalid in the HR schema.
   

   There is a separate message for each schema that contains invalid objects.

   
   Description of the illustration invalid_object_alerts.gif
   
   

3. In the Message column, click an invalid object message.

   The Owner's Invalid Object Count page appears.

4. Under Related Links, click Invalid Objects Details.

   The Invalid Object Details page appears, showing a list of invalid objects.

   
   Description of the illustration invalid_object_details.gif
   
   

5. Select one or more objects, and then click Recompile.

   A confirmation message appears, and the newly validated objects are removed from the list.

To validate a schema object starting from an object page:

1. Go to the Database Home page, logging in as user SYSTEM.

   See "Accessing the Database Home Page".

2. At the top of the page, click Schema to view the Schema subpage.

3. In the Database Objects section, click the link for the object type to validate.

   For example, to validate a view, click Views.

4. On the object page (for example, the Views page), enter a schema name and, optionally, an object name or partial object name, and then click Go.

   The schema objects are displayed.

5. Select the schema object to validate.

6. In the Actions list, select Compile, and then click Go.

   A confirmation message appears.