PostgreSQL views are virtual tables that represent the results of a query. They do not store data themselves but provide a named, reusable abstraction over one or more underlying tables or even other views.

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PostgreSQL –
VIEWS,SEQUENCE

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1. PostgreSQL – VIEWS: An Introduction
• Views are pseudo-tables.
• That is, they are not real tables; nevertheless appear as ordinary
tables to SELECT.
• A view can represent a subset of a real table, selecting
certain columns or certain rows from an ordinary table.
• A view can even represent joined tables.
• Because views are assigned separate permissions, you can use
them to restrict table access so that the users see only specific
rows or columns of a table.

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1. PostgreSQL – VIEWS: An Introduction …
• A view can contain all rows of a table or selected rows from one or more tables.
• A view can be created from one or many tables, which depends on the written PostgreSQL query to create a view.
• Views, which are kind of virtual tables, allow users to do the following −
• Structure data in a way that users or classes of users find natural or intuitive.
• Restrict access to the data such that a user can only see limited data instead of complete table.
• Summarize data from various tables, which can be used to generate reports.
• Since views are not ordinary tables, you may not be able to execute a DELETE, INSERT, or UPDATE
statement on a view.
• However, you can create a RULE to correct this problem of using DELETE, INSERT or UPDATE on a view.

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1.1 Creating Views
• The PostgreSQL views are created using the CREATE VIEW statement.
• The PostgreSQL views can be created from a single table, multiple tables, or another view.
• The basic CREATE VIEW syntax is as follows −
• You can include multiple tables in your SELECT statement in very similar way as you
use them in normal PostgreSQL SELECT query.
• If the optional TEMP or TEMPORARY keyword is present, the view will be created in the temporary
space.
• Temporary views are automatically dropped at the end of the current session.
CREATE [TEMP | TEMPORARY] VIEW view_name AS
SELECT column1, column2.....
FROM table_name
WHERE [condition];

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1.1 Creating Views :Example
• Consider, the COMPANY table is having the following records −

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1.1 Creating Views :Example …
• Now, following is an example to create a view from COMPANY table.
• This view would be used to have only few columns from COMPANY table −
• Now, you can query COMPANY_VIEW in a similar way as you query an actual table.
Following is the example −
• This would produce the following result −
CREATE VIEW COMPANY_VIEW AS
SELECT ID, NAME, AGE
FROM COMPANY;
SELECT * FROM COMPANY_VIEW;

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1.1 Creating Views :Example …
• This would produce the following result −

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1.2 Update VIEW
• You can modify the definition of a VIEW in PostgreSQL without
dropping it by using the CREATE OR REPLACE VIEW Statement.
• Syntax :The syntax for the CREATE OR REPLACE VIEW statement in
PostgreSQL is:
• Where, view_name The name of the view that you wish to update.
CREATE OR REPLACE VIEW view_name AS
SELECT columns
FROM table
WHERE conditions;

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1.2 Update VIEW …
• Example :Here is an example of how you would use the CREATE OR
REPLACE VIEW statement in PostgreSQL:
• This CREATE OR REPLACE VIEW example would update the definition of
the VIEW called current_inventory without dropping it.
CREATE or REPLACE VIEW current_inventory AS
SELECT product_name, quantity, category_name
FROM products
INNER JOIN categories
ON products.category_id = categories.category_id
WHERE quantity > 0;

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1.3 Dropping VIEWS
• To drop a view, simply use the DROP VIEW statement with the
view_name.
• The basic DROP VIEW syntax is as follows −
• The following command will delete COMPANY_VIEW view,
which we created in the last section −
DROP VIEW view_name;
DROP VIEW COMPANY_VIEW;

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1.4 Types of View
• There are two types of view,
• Simple View
• Complex View
Simple View Complex View
• Created from one table • Created from one or more table
• Does not contain functions • Contain functions
• Does not contain groups of data • Contains groups of data

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2. What is a sequence ?
• A sequence is simply a list of numbers, in which their orders
are important.
• For example, the {1,2,3} is a sequence while the {3,2,1} is an
entirely different sequence.
• In SQL Server, a sequence is a user-defined schema-bound
object that generates a sequence of numbers according to a
specified specification.
• A sequence of numeric values can be in ascending or descending
order at a defined interval and may cycle if requested.

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2. What is a sequence ? …
• Sequence is a feature supported by some database systems to
produce unique values on demand.
• Some DBMS like MySQL supports AUTO_INCREMENT in place of
Sequence.
• AUTO_INCREMENT is applied on columns, it automatically
increments the column value by 1 each time a new record is
inserted into the table.
• Sequence is also some what similar to AUTO_INCREMENT but it has
some additional features too.

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2.1 How Do I Use A Sequence In A Table?
• Sequences are most commonly used via the serial pseudotype.
• A serial is a special data type that encodes the following information:
it indicates that the values for the column will be generated by consulting the
sequence
therefore, it creates a new sequence object, and sets the default value for the column
to be the next value produced by the sequence
since a sequence always produces non-NULL values, it adds a NOT NULL constraint to
the column
since the sequence that is produced is created "behind the scenes", PostgreSQL
assumes that the sequence is only used to generate values for the table containing the
serial column.
 Therefore, if this column is dropped, the sequence will be automatically removed.

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2.2 Sequence : Syntax
CREATE [ TEMPORARY | TEMP ]
SEQUENCE name
[ INCREMENT [ BY ] increment ]
[ MINVALUE minvalue | NO MINVALUE ]
[ MAXVALUE maxvalue | NO MAXVALUE ]
[ START [ WITH ] start ]
[ CACHE cache ]
[ [ NO ] CYCLE ]
[ OWNED BY { table_name.column_name | NONE } ]

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2.2 Sequence : Syntax …
Option Description
TEMPORARY or
TEMP
• If specified, the sequence object is created only for this session and is automatically dropped on
session exit.
name • The name of the sequence to be created.
increment
• The optional clause INCREMENT BY increment specifies which value is added to the current
sequence value to create a new value.
• A positive value will make an ascending sequence, a negative one a descending sequence.
• The default value is 1.
minvalue
NO MINVALUE
• The optional clause MINVALUE minvalue determines the minimum value a sequence can generate.
• If NO MINVALUE is specified, then defaults will be used.
• The defaults are 1 and -263-1 for ascending and descending sequences, respectively.
maxvalue
NO MAXVALUE
• The optional clause MAXVALUE maxvalue determines the maximum value for the sequence.
• If NO MAXVALUE is specified, then default values will be used.
• The defaults are 263-1 and -1 for ascending and descending sequences, respectively.

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2.2 Sequence : Syntax …
Option Description
start • The optional clause START WITH start allows the sequence to begin anywhere.
• The default starting value is minvalue for ascending sequences and maxvalue for descending ones.
cache • The optional clause CACHE cache specifies how many sequence numbers are to be preallocated and stored in memory
for faster access.
• The minimum value is 1which is also the default.
CYCLE
NO CYCLE
• The CYCLE option allows the sequence to wrap around when the maxvalue or minvalue has been reached by an
ascending or descending sequence respectively.
• If the limit is reached, the next number generated will be the minvalue or maxvalue, respectively.
• For NO CYCLE, any calls to nextval after the sequence has reached its maximum value will return an error.
• NO CYCLE is the default.
OWNED BY
table_name.colum
n_name
OWNED BY NONE
• The OWNED BY option causes the sequence to be associated with a specific table column, such that if that column (or
its whole table) is dropped, the sequence will be automatically dropped as well.
• The specified table must have the same owner and be in the same schema as the sequence.
• OWNED BY NONE, the default, specifies that there is no such association.

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2.3 Drop a Sequence:
• Use DROP SEQUENCE to remove a sequence.

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2.4 Examples:
• Create an ascending sequence called idno, starting at 50:
• Code:
postgres=# CREATE SEQUENCE idno START 50;
CREATE SEQUENCE
postgres=#

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2.4 Examples …
• Select the next number from this sequence:
• Sample Output:
• Select the next number from this sequence :
• Sample Output:
• Note: Here nextval() function is used to get the next value instead of the
standard's NEXT VALUE FOR expression.

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2.4 Examples …
• Let use this sequence in an INSERT command:
• Now use the above sequence in an INSERT command :
• Sample table 'test':
• Sample Output:

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2.4 Examples …
• Now insert some records:
• Sample Output:

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2.4 Examples …
• Inside the sequence:
• Sample Output:

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2.5 Alter the Sequence:
• To alter the sequence so that IDs start a different number,
you can't just do an update, you have to use the alter
sequence command.
• When you're truncating a table, you can truncate and restart
IDs from 1 in one command:
alter sequence idno restart with 1000;
truncate test restart identity;