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When you use the CREATE DATABASE statement, you must complete additional actions before you have an operational database. These actions include building views on the data dictionary tables and installing standard PL/SQL packages. You perform these actions by running the supplied scripts.

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If you have existing scripts for creating your database, then consider editing those scripts to take advantage of new Oracle Database features.

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The instructions in this section apply to single-instance installations only. See the Oracle Real Application Clusters (Oracle RAC) installation guide for your platform for instructions for creating an Oracle RAC database.

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Using the CREATE DATABASE SQL statement is a more manual approach to creating a database than using Oracle Database Configuration Assistant (DBCA). One advantage of using this statement over using DBCA is that you can create databases from within scripts.

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• About Creating a Database with the CREATE DATABASE Statement
  When you use the CREATE DATABASE statement, you must complete additional actions before you have an operational database. These actions include building views on the data dictionary tables and installing standard PL/SQL packages. You perform these actions by running the supplied scripts.

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• Step 1: Specify an Instance Identifier (SID)
  The ORACLE_SID environment variable is used to distinguish this instance from other Oracle Database instances that you may create later and run concurrently on the same host computer.
• Step 2: Ensure That the Required Environment Variables Are Set
  Depending on your platform, before you can start SQL*Plus (as required in a later step), you may have to set environment variables, or at least verify that they are set properly.
• Step 3: Choose a Database Administrator Authentication Method
  You must be authenticated and granted appropriate system privileges in order to create a database.
• Step 4: Create the Initialization Parameter File
  When an Oracle instance starts, it reads an initialization parameter file. This file can be a text file, which can be created and modified with a text editor, or a binary file, which is created and dynamically modified by the database. The binary file, which is preferred, is called a server parameter file. In this step, you create a text initialization parameter file. In a later step, you create a server parameter file from the text file.

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• Step 5: (Windows Only) Create an Instance
  On the Windows platform, before you can connect to an instance, you must manually create it if it does not already exist. The ORADIM command creates an Oracle Database instance by creating a new Windows service.
• Step 6: Connect to the Instance
  Start SQL*Plus and connect to your Oracle Database instance with the SYSDBA administrative privilege.
• Step 7: Create a Server Parameter File
  The server parameter file enables you to change initialization parameters with the ALTER SYSTEM command and persist the changes across a database shutdown and startup. You create the server parameter file from your edited text initialization file.
• Step 8: Start the Instance
  Start an instance without mounting a database.
• Step 9: Issue the CREATE DATABASE Statement
  To create the new database, use the CREATE DATABASE statement.
• Step 10: Create Additional Tablespaces
  To make the database functional, you must create additional tablespaces for your application data.
• Step 11: Run Scripts to Build Data Dictionary Views
  Run the scripts necessary to build data dictionary views, synonyms, and PL/SQL packages, and to support proper functioning of SQL*Plus.
• Step 12: (Optional) Run Scripts to Install Additional Options
  You may want to run other scripts. The scripts that you run are determined by the features and options you choose to use or install.
• Step 13: Run Datapatch
  Run Datapatch to update information stored in the database to the correct patch level.
• Step 14: Back Up the Database
  Take a full backup of the database to ensure that you have a complete set of files from which to recover if a media failure occurs.
• Step 15: (Optional) Enable Automatic Instance Startup
  You might want to configure the Oracle instance to start automatically when its host computer restarts.

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A null is the absence of a value in a column. Nulls indicate missing, unknown, or inapplicable data.

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Nulls are stored in the database if they fall between columns with data values. In these cases, they require 1 byte to store the length of the column (zero). Trailing nulls in a row require no storage because a new row header signals that the remaining columns in the previous row are null. For example, if the last three columns of a table are null, then no data is stored for these columns.

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See also

Oracle Database SQL Language Reference to learn more about null values

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A rowid is effectively a 10-byte physical address of a row.

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Every row in a heap-organized table has a rowid unique to this table that corresponds to the physical address of a row piece. For table clusters, rows in different tables that are in the same data block can have the same rowid.

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Oracle Database uses rowids internally for the construction of indexes. For example, each key in a B-tree index is associated with a rowid that points to the address of the associated row for fast access. Physical rowids provide the fastest possible access to a table row, enabling the database to retrieve a row in as little as a single I/O.

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See also

• “Rowid Format” to learn about the structure of a rowid
• “Overview of B-Tree Indexes” to learn about the types and structure of B-tree indexes

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The database stores rows in data blocks. Each row of a table containing data for less than 256 columns is contained in one or more row pieces.

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If possible, Oracle Database stores each row as one row piece. However, if all of the row data cannot be inserted into a single data block, or if an update to an existing row causes the row to outgrow its data block, then the database stores the row using multiple row pieces.

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Rows in a table cluster contain the same information as rows in nonclustered tables. Additionally, rows in a table cluster contain information that references the cluster key to which they belong.

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See also

“Data Block Format” to learn about the components of a data block

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Oracle Database uses a data segment in a tablespace to hold table data.

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A segment contains extents made up of data blocks. The data segment for a table (or cluster data segment, for a table cluster) is located in either the default tablespace of the table owner or in a tablespace named in the CREATE TABLE statement.

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See also

“User Segments” to learn about the types of segments and how they are created

Table Organization

By default, a table is organized as a heap, which means that the database places rows where they fit best rather than in a user-specified order. Thus, a heap-organized table is an unordered collection of rows.

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Note

Index-organized tables use a different principle of organization.

As users add rows, the database places the rows in the first available free space in the data segment. Rows are not guaranteed to be retrieved in the order in which they were inserted.

The hr.departments table is a heap-organized table. It has columns for department ID, name, manager ID, and location ID. As rows are inserted, the database stores them wherever they fit. A data block in the table segment might contain the unordered rows shown in the following example:

Copy50,Shipping,121,1500
120,Treasury,,1700
70,Public Relations,204,2700
30,Purchasing,114,1700
130,Corporate Tax,,1700
10,Administration,200,1700
110,Accounting,205,1700

The column order is the same for all rows in a table. The database usually stores columns in the order in which they were listed in the CREATE TABLE statement, but this order is not guaranteed. For example, if a table has a column of type LONG, then Oracle Database always stores this column last in the row. Also, if you add a new column to a table, then the new column becomes the last column stored.

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A table can contain a virtual column, which unlike normal columns does not consume space on disk. The database derives the values in a virtual column on demand by computing a set of user-specified expressions or functions. You can index virtual columns, collect statistics on them, and create integrity constraints. Thus, virtual columns are much like nonvirtual columns.

An integrity constraint is a named rule that restrict the values for one or more columns in a table.

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Data integrity rules prevent invalid data entry into tables. Also, constraints can prevent the deletion of a table when certain dependencies exist.

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If a constraint is enabled, then the database checks data as it is entered or updated. Oracle Database prevents data that does not conform to the constraint from being entered. If a constraint is disabled, then Oracle Database allows data that does not conform to the constraint to enter the database.

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In Example 2-1, the CREATE TABLE statement specifies NOT NULL constraints for the last_name, email, hire_date, and job_id columns. The constraint clauses identify the columns and the conditions of the constraint. These constraints ensure that the specified columns contain no null values. For example, an attempt to insert a new employee without a job ID generates an error.

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You can create a constraint when or after you create a table. You can temporarily disable constraints if needed. The database stores constraints in the data dictionary.

A format model is a character literal that describes the format of datetime or numeric data stored in a character string. A format model does not change the internal representation of the value in the database.

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When you convert a character string into a date or number, a format model determines how the database interprets the string. In SQL, you can use a format model as an argument of the TO_CHAR and TO_DATE functions to format a value to be returned from the database or to format a value to be stored in the database.

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The following statement selects the salaries of the employees in Department 80 and uses the TO_CHAR function to convert these salaries into character values with the format specified by the number format model '$99,990.99':

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CopySQL> SELECT last_name employee, TO_CHAR(salary, '$99,990.99') AS "SALARY"
  2  FROM   employees
  3  WHERE  department_id = 80 AND last_name = 'Russell';
 
EMPLOYEE                  SALARY
------------------------- -----------
Russell                    $14,000.00

The following example updates a hire date using the TO_DATE function with the format mask 'YYYY MM DD' to convert the string '1998 05 20' to a DATE value:

CopySQL> UPDATE employees
  2  SET hire_date = TO_DATE('1998 05 20','YYYY MM DD')
  3  WHERE last_name = 'Hunold';

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Every table in an Oracle database has a pseudocolumn named ROWID.

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A pseudocolumn behaves like a table column, but is not actually stored in the table. You can select from pseudocolumns, but you cannot insert, update, or delete their values. A pseudocolumn is also similar to a SQL function without arguments. Functions without arguments typically return the same value for every row in the result set, whereas pseudocolumns typically return a different value for each row.

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Values of the ROWID pseudocolumn are strings representing the address of each row. These strings have the data type ROWID. This pseudocolumn is not evident when listing the structure of a table by executing SELECT or DESCRIBE, nor does the pseudocolumn consume space. However, the rowid of each row can be retrieved with a SQL query using the reserved word ROWID as a column name.

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The following example queries the ROWID pseudocolumn to show the rowid of the row in the employees table for employee 100:

CopySQL> SELECT ROWID FROM employees WHERE employee_id = 100;

ROWID
------------------
AAAPecAAFAAAABSAAA

See also

• “Rowid Format”
• Oracle Database Development Guide to learn how to identify rows by address
• Oracle Database SQL Language Reference to learn about rowid types

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Oracle Database uses rowids internally for the construction of indexes.

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A B-tree index, which is the most common type, contains an ordered list of keys divided into ranges. Each key is associated with a rowid that points to the associated row’s address for fast access.

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End users and application developers can also use rowids for several important functions:

• Rowids are the fastest means of accessing particular rows.
• Rowids provide the ability to see how a table is organized.
• Rowids are unique identifiers for rows in a given table.
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You can also create tables with columns defined using the ROWID data type. For example, you can define an exception table with a column of data type ROWID to store the rowids of rows that violate integrity constraints. Columns defined using the ROWID data type behave like other table columns: values can be updated, and so on.

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