Other database objects

(Sequence and view)

What Is a Sequence?

A sequence:• Automatically generates sequential numbers• Is a sharable object• Is typically used to create a primary key value• Sequence numbers are stored and generated independently of tables. Therefore, the same sequence can be used for multiple tables

The CREATE SEQUENCE Statement Syntax

• Define a sequence to generate sequential numbers automatically. Syntax:CREATE SEQUENCE sequenceINCREMENT BY n

START WITH n MAXVALUE n

MINVALUE n [CYCLE | NOCYCLE] [CACHE n | NOCACHE];

The CREATE SEQUENCE Statement Syntax

• INCREMENT BY n specifies the interval between sequence numbers where n is an integer (If this clause is omitted, the sequence increments by 1.)

• START WITH n specifies the first sequence number to be generated (If this clause use is omitted, the sequence starts with 1.)

• MAXVALUE n specifies the maximum value the sequence can generate

• MINVALUE n specifies the minimum value the sequence can generate

• CYCLE | NOCYCLE specifies whether the sequence continues to generate values after reaching its maximum value (NOCYCLE is the default option.)

• CACHE n | NOCACHE specifies how many values the Oracle Server preallocates and keep in memory (By default, the Oracle Server caches 20 values.)

Creating a Sequence

• Create a sequence named DEPT_DEPTID_SEQ to be used for the primary key of the DEPARTMENTS table.• Do not use the CYCLE option.CREATE SEQUENCE dept_deptid_seqINCREMENT BY 10START WITH 120MAXVALUE 9999NOCACHENOCYCLE;

NEXTVAL and CURRVAL• NEXTVAL returns the next available sequence

value.• It returns a unique value every time it is

referenced, even for different users. • CURRVAL obtains the current sequence value.

NEXTVAL must be issued for that sequence before CURRVAL contains a value.

Using a Sequence

• Insert a new department named “Support” in location ID 2500.INSERT INTO departments VALUES (dept_deptid_seq.NEXTVAL, 'Support', 2500);

• View the current value for the DEPT_DEPTID_SEQ sequence.SELECT dept_deptid_seq.CURRVALFROM dual;

The DUAL Table• The DUAL table is owned by the user SYS and

can be accessed by all users. It contains one column, DUMMY, and one row with the value X. The DUAL table is useful when you want to return a value once only: for instance, the value of a constant, or expression that is not derived from a table with user data. The DUAL table is generally used for SELECT clause because both

SELECT and FROM clauses are mandatory, and several calculations do not need to select from actual tables.

• SELECT 1 + 1 FROM DUAL;• -> 2

Modifying a Sequence

• Change the increment value, maximum value

• ALTER SEQUENCE dept_deptid_seqINCREMENT BY 20MAXVALUE 999999NOCACHENOCYCLE;

Guidelines for Modifying a Sequence

• You must be the owner or have the ALTER privilege for the sequence.

• Only future sequence numbers are affected.

• The START WITH option cannot be changed using ALTER SEQUENCE. The sequence must be dropped and re-created in order to restart the sequence at a different number.

Guidelines for Modifying a Sequence

• Some validation is performed. For example, a new MAXVALUE that is less than the current sequence number cannot be imposed.ALTER SEQUENCE dept_deptid_seqINCREMENT BY 20MAXVALUE 90NOCACHENOCYCLE;ALTER SEQUENCE dept_deptid_seq*ERROR at line 1:ORA-04009: MAXVALUE cannot be made to be less than the currentvalue

Removing a Sequence

• Remove a sequence from the data dictionary by using the DROP SEQUENCE statement.

• Once removed, the sequence can no longer be referenced.DROP SEQUENCE dept_deptid_seq;

• You must be the owner of the sequence or have the DROP ANY SEQUENCE privilege to remove it.

VIEWS

• A database view is a logical or virtual table based on a query.

• It is useful to think of a view as a stored query.• Views are created through use of a CREATE

VIEW command that incorporates use of the SELECT statement.

• Views are queried just like tables.

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VIEWS

CREATE VIEW employee_parking (parking_space, last_name,

first_name, ssn) ASSELECT emp_parking_space, emp_last_name, emp_first_name, emp_ssn

FROM employeeORDER BY emp_parking_space;View Created.

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VIEWS

SELECT *FROM employee_parking; PARKING_SPACE LAST_NAME FIRST_NAME SSN------------- ---------- ----------- -------- 1 Bordoloi Bijoy 999666666 3 Joyner Suzanne 999555555 32 Zhu Waiman 999444444more rows are displayed…

 • Notice that the only columns in the query are

those defined as part of the view.

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VIEWS

• Additionally, we have renamed the columns in the view so that they are slightly different than the column names in the underlying employee table.

• Further, the rows are sorted by parking_space column even though there is no ORDER BY in the SELECT command used to access the view.

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CREATING A VIEW

• CREATE VIEW Syntax

CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW <view name> [(column alias name….)] AS <query> [WITH [CHECK OPTION] [READ ONLY] [CONSTRAINT]];

• The OR REPLACE option is used to create a view that already exists. This option is useful for modifying an existing view without having to drop or grant the privileges that system users have acquired with respect to the view .

• If you attempt to create a view that already exists without using the OR REPLACE option, Oracle will return the ORA-00955: name is already used by an existing object error message and the CREATE VIEW command will fail.

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CREATING A VIEW

• The FORCE option allows a view to be created even if a base table that the view references does not already exist.

• This option is used to create a view prior to the actual creation of the base tables and accompanying data. Before such a view can be queried, the base tables must be created and data must be loaded into the tables. This option can also be used if a system user does not currently have the privilege to create a view.

• The NOFORCE option is the opposite of FORCE and allows a system user to create a view if they have the required permissions to create a view, and if the tables from which the view is created already exist. This is the default option.

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CREATING A VIEW

• The WITH READ ONLY option allows creation of a view that is read-only. You cannot use the DELETE, INSERT, or UPDATE commands to modify data for the view.

• The WITH CHECK OPTION clause allows rows that can be selected through the view to be updated. It also enables the specification of constraints on values.

• The CONSTRAINT clause is used in conjunction with the WITH CHECK OPTION clause to enable a database administrator to assign a unique name to the CHECK OPTION. If the DBA omits the CONSTRAINT clause, Oracle will automatically assign the constraint a system-generated name that will not be very meaningful.

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ExampleCREATE VIEW empview7 AS SELECT emp_ssn, emp_first_name, emp_last_nameFROM employeeWHERE emp_dpt_number=7;View created.

•A simple query of the empview7 shows the following data.

SELECT *FROM empview7;EMP_SSN EMP_FIRST_NAME EMP_LAST_NAME--------- ------------------------- -------------------------999444444 Waiman Zhu999111111 Douglas Bock999333333 Dinesh Joshi999888888 Sherri Prescott

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Example• It is also possible to create a view that has exactly

the same structure as an existing database table. • The view named dept_view shown next has exactly

the same structure as department table.

CREATE VIEW dept_view AS SELECT * FROM department;View created.

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Example• We can recreate the view by using the OR REPLACE clause to

create a view that is read-only by specifying a WITH READ ONLY clause.

• The new version of dept_view will restrict data manipulation language operations on the view to the use of the SELECT command.

CREATE OR REPLACE VIEW dept_view AS SELECT * FROM department WITH READ ONLY CONSTRAINT

vw_dept_view_read_only;View created.

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FUNCTIONS AND VIEWS – A JOIN VIEW

• In addition to specifying columns from existing tables, you can use single row functions consisting of number, character, date, and group functions as well as expressions to create additional columns in views.

• This can be extremely useful because the system user will have access to data without having to understand how to use the underlying functions.

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ExampleCREATE OR REPLACE VIEW dept_salary (name, min_salary, max_salary, avg_salary) AS SELECT d.dpt_name, MIN(e.emp_salary), MAX(e.emp_salary), AVG(e.emp_salary)FROM employee e, department dWHERE e.emp_dpt_number=d.dpt_noGROUP BY d.dpt_name;

View created.

SELECT * FROM dept_salary;NAME MIN_SALARY MAX_SALARY AVG_SALARY------------------------- ------------------ ------------------ ------------------Admin and Records 25000 43000 31000Headquarters 55000 55000 55000Production 25000 43000 34000

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VIEW STABILITY

• A view does not actually store any data. The data needed to support queries of a view are retrieved from the underlying database tables and displayed to a result table whenever a view is queried. The result table is only stored temporarily.

• If a table that underlies a view is dropped, then the view is no longer valid. Attempting to query an invalid view will produce an ORA-04063: view "VIEW_NAME" has errors error message.

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INSERTING , UPDATING, AND DELETING TABLE ROWS THROUGH VIEWS

• You can insert a row if the view in use is one that is updateable (not read-only).

• A view is updateable if the INSERT command does not violate any constraints on the underlying tables.

• This rule concerning constraint violations also applies to UPDATE and DELETE commands.

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ExampleCREATE OR REPLACE VIEW dept_view ASSELECT dpt_no, dpt_name FROM department;

INSERT INTO dept_view VALUES (18, 'Department 18');INSERT INTO dept_view VALUES (19, 'Department 20');

SELECT *FROM dept_view;DPT_NO DPT_NAME------------ -------------------- 7 Production 3 Admin and Records 1 Headquarters 18 Department 18 19 Department 20

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Example

UPDATE dept_view SET dpt_name = 'Department 19‘ WHERE dpt_no = 19;1 row updated.

SELECT *FROM departmentWHERE dpt_no >= 5; DPT_NO DPT_NAME DPT_MGRSS DPT_MGR_S----------- -------------------- ------------------ ----------------- 7 Production 999444444 22-MAY-98 18 Department 18 19 Department 19more rows are displayed…

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More Examples

DELETE dept_view WHERE dpt_no = 18 OR dpt_no = 19;2 rows deleted.

SELECT *FROM department;DPT_NO DPT_NAME DPT_MGRSS DPT_MGR_S------------ ------------------------ ------------------ ---------------- 7 Production 999444444 22-MAY-98 3 Admin and Records 999555555 01-JAN-01 1 Headquarters 999666666 19-JUN-81

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CREATING A VIEW WITH ERRORS

• If there are no syntax errors in a CREATE VIEW statement, Oracle will create a view even if the view-defining query refers to a non-existent table or an invalid column of an existing table.

• The view will also be created even if the system user does not have privileges to access the tables which a view references.

• The new view will be unusable and is categorized as “created with errors.”

• In order to create such a view, the system user must use the FORCE option of the CREATE VIEW command.

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CREATING A VIEW WITH ERRORS

• In the CREATE VIEW command shown below, the table named divisions does not exist and the view is created with errors. Oracle returns an appropriate warning message.

CREATE FORCE VIEW div_view AS SELECT * FROM divisions;Warning: View created with

compilation errors.

• If we now create a table named divisions, a query of the invalid div_view view will execute, and the view is automatically recompiled and becomes valid.

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DROPPING VIEW• A DBA or view owner can drop a view with the

DROP VIEW command. The following command drops a view named dept_view.

DROP VIEW dept_view;View dropped.

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A Summary of VIEW Facts • A view does not store data, but a view does

display data through a SELECT query as if the data were stored in the view.

• A view can simplify data presentation as well as provide a kind of data security by limiting access to data based on a "need to know."

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A Summary of VIEW Facts • A view can display data from more than one table. • Views can be used to update the underlying

tables. Views can also be limited to read-only access.

• Views can change the appearance of data. For example, a view can be used to rename columns from tables without affecting the base table.

• A view that has columns from more than one table cannot be modified by an INSERT, DELETE, or UPDATE command if a grouping function, GROUP BY clause is part of the view definition.

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A Summary of VIEW Facts • A view cannot reference the nextval and currval

pseudocolumns created through the use of sequences.

• A row cannot be inserted in a view in which the base table has a column with the NOT NULL or other constraint that cannot be satisfied by the new row data.

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