Les10[1]Creating and Managing Tables

Copyright ‫س‬ Oracle Corporation, 1999. All rights reserved.
10
Creating and Managing
Tables

10-2 Copyright ‫س‬ Oracle Corporation, 1999. All rights reserved.
Objectives
After completing this lesson, you should
be able to do the following:
• Describe the main database objects
• Create tables
• Describe the datatypes that can be used
when specifying column definition
• Alter table definitions
• Drop, rename, and truncate tables

Database Objects
Object Description
Table Basic unit of storage; composed of rows
and columns
View Logically represents subsets of data from
one or more tables
Sequence Generates primary key values
Index Improves the performance of some queries
Synonym Gives alternative names to objects

Naming Conventions
• Must begin with a letter
• Can be 1–30 characters long
• Must contain only A–Z, a–z, 0–9, _, $,
and #
• Must not duplicate the name of another
object owned by the same user
• Must not be an Oracle Server reserved
word

The CREATE TABLE Statement
• You must have :
– CREATE TABLE privilege
– A storage area
• You specify:
– Table name
– Column name, column datatype, and
column size
CREATE [GLOBAL TEMPORARY] TABLE [schema.]table
(column datatype [DEFAULT expr][, ...]);

Referencing Another User’s
Tables
• Tables belonging to other users are not
in the user’s schema.
• You should use the owner’s name as a
prefix to the table.

The DEFAULT Option
• Specify a default value for a column during
an insert.
… hiredate DATE DEFAULT SYSDATE, …
• Legal values are literal value, expression,
or SQL function.
• Illegal values are another column’s name or
pseudocolumn.
• The default datatype must match the
column datatype.

Creating Tables
SQL> CREATE TABLE dept
2 (deptno NUMBER(2),
3 dname VARCHAR2(14),
4 loc VARCHAR2(13));
Table created.
• Create the table.
• Confirm table creation.
SQL> DESCRIBE dept
Name Null? Type
--------------------------- -------- ---------
DEPTNO NUMBER(2)
DNAME VARCHAR2(14)
LOC VARCHAR2(13)

Tables in the Oracle Database
• User Tables
– Collection of tables created and
maintained by the user
– Contain user information
• Data Dictionary
– Collection of tables created and
maintained by the Oracle server
– Contain database information

Querying the Data Dictionary
• View distinct object types owned by the
user.
• View tables, views, synonyms, and
sequences owned by the user.
SQL> SELECT *
2 FROM user_tables;
SQL> SELECT DISTINCT object_type
2 FROM user_objects;
SQL> SELECT *
2 FROM user_catalog;
• Describe tables owned by the user.

Datatypes
Datatype Description
VARCHAR2(size) Variable-length character data
CHAR(size) Fixed-length character data
NUMBER(p,s) Variable-length numeric data
DATE Date and time values
LONG Variable-length character data
up to 2 gigabytes
CLOB Single-byte character data up to 4
gigabytes
RAW and LONG RAW Raw binary data
BLOB Binary data up to 4 gigabytes
BFILE Binary data stored in an external
file; up to 4 gigabytes

Creating a Table
by Using a Subquery
• Create a table and insert rows by
combining the CREATE TABLE statement
and AS subquery option.
• Match the number of specified columns
to the number of subquery columns.
• Define columns with column names and
default values.
CREATE TABLE table
[(column, column...)]
AS subquery;

SQL> CREATE TABLE dept30
2 AS
3 SELECT empno, ename, sal*12 ANNSAL, hiredate
4 FROM emp
5 WHERE deptno = 30;
Table created.
Creating a Table
by Using a Subquery
Name Null? Type
---------------------------- -------- -----
EMPNO NOT NULL NUMBER(4)
ENAME VARCHAR2(10)
ANNSAL NUMBER
HIREDATE DATE
SQL> DESCRIBE dept30

The ALTER TABLE Statement
Use the ALTER TABLE statement to:
• Add a new column
• Modify an existing column
• Define a default value for the new column
ALTER TABLE table
ADD (column datatype [DEFAULT expr]
[, column datatype]...);
ALTER TABLE table
MODIFY (column datatype [DEFAULT expr]
[, column datatype]...);

Adding a Column
DEPT30
EMPNO ENAME ANNSAL HIREDATE
------ ---------- --------
7698 BLAKE 34200 01-MAY-81
7654 MARTIN 15000 28-SEP-81
7499 ALLEN 19200 20-FEB-81
7844 TURNER 18000 08-SEP-81
...
“…add a
new
column
into
DEPT30
table…”
DEPT30
EMPNO ENAME ANNSAL HIREDATE
------ ---------- --------
7698 BLAKE 34200 01-MAY-81
7654 MARTIN 15000 28-SEP-81
7499 ALLEN 19200 20-FEB-81
7844 TURNER 18000 08-SEP-81
...
JOB
JOB
New column

Adding a Column
• You use the ADD clause to add columns.
EMPNO ENAME ANNSAL HIREDATE JOB
--------- ---------- --------- --------- ----
7698 BLAKE 34200 01-MAY-81
7654 MARTIN 15000 28-SEP-81
7499 ALLEN 19200 20-FEB-81
7844 TURNER 18000 08-SEP-81
...
6 rows selected.
SQL> ALTER TABLE dept30
2 ADD (job VARCHAR2(9));
Table altered.
• The new column becomes the last column.

Modifying a Column
• You can change a column’s datatype,
size, and default value.
• A change to the default value affects
only subsequent insertions to the table.
2 MODIFY (ename VARCHAR2(15));
Table altered.

Dropping a Column
You use the DROP COLUMN clause drop
columns you no longer need from the
table.
2 DROP COLUMN job ;
Table altered.

SET UNUSED Option
• You use the SET UNUSED option to
mark one or more columns as unused.
• You use the DROP UNUSED COLUMNS
option to remove the columns that are
marked as UNUSED.
ALTER TABLE table
(column);
ALTER TABLE table
COLUMN column;
OR
ALTER TABLE table
DROP UNUSED COLUMNS;
SET UNUSED
SET UNUSED

Dropping a Table
• All data and structure in the table is
deleted.
• Any pending transactions are
committed.
• All indexes are dropped.
• You cannot roll back this statement.
SQL> DROP TABLE dept30;
Table dropped.

Changing the Name of an Object
• To change the name of a table, view,
sequence, or synonym, you execute the
RENAME statement.
• You must be the owner of the object.
SQL> RENAME dept TO department;
Table renamed.

Truncating a Table
• The TRUNCATE TABLE statement:
– Removes all rows from a table
– Releases the storage space used by
that table
• You cannot roll back row removal when
using TRUNCATE.
• Alternatively, you can remove rows by
using the DELETE statement.
SQL> TRUNCATE TABLE department;
Table truncated.

Adding Comments to a Table
• You can add comments to a table or
column by using the COMMENT
statement.
• Comments can be viewed through the
data dictionary views.
– ALL_COL_COMMENTS
– USER_COL_COMMENTS
– ALL_TAB_COMMENTS
– USER_TAB_COMMENTS
SQL> COMMENT ON TABLE emp
2 IS 'Employee Information';
Comment created.

Summary
Statement Description
CREATE TABLE Creates a table
ALTER TABLE Modifies table structures
DROP TABLE Removes the rows and table structure
RENAME Changes the name of a table, view,
sequence, or synonym
TRUNCATE Removes all rows from a table and
releases the storage space
COMMENT Adds comments to a table or view

Practice Overview
• Creating new tables
• Creating a new table by using the
CREATE TABLE AS syntax
• Modifying column definitions
• Verifying that the tables exist
• Adding comments to tables
• Dropping tables
• Altering tables

Les10[1]Creating and Managing Tables

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Les10[1]Creating and Managing Tables