SQL's Data Definition Language
(DDL) – View, Sequence, Index
DDL: View, Index, Sequence,
Synonym
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Objects are defined using the CREATE
statement
Some objects can be modified using the
ALTER statement
Some objects can be modified using the
ALTER statement
Some objects cannot be modified - they must
be dropped and then re-created
Objects are removed using the DROP
statement
View
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A view is a virtual table – no data is actually
maintained for a view
A view is defined based on existing tables by
using a SELECT statement:
CREATE VIEW viewname
AS SELECT …
When a view is referenced the view's defining
query is executed and data is retrieved from
tables referenced by the defining query
The defining query for a view is stored in the
data dictionary
View Example
CREATE VIEW management
AS SELECT Emp_no, ename
FROM employees
WHERE emp_no IN
(SELECT mgr
FROM employees
WHERE mgr IS NOT NULL);
 Using a view in a query causes the SELECT
statement that defines the view to be executed
and data values retrieved
eg SELECT ename
FROM management;
View Example 2
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Views are often used to simplify writing of
complex queries which involve multiple tables
or group functions
Eg: CREATE VIEW deptsummary
AS SELECT d.deptno, dept_name, sum(sal) Salaries
FROM departments d, employees e
WHERE d.dept_no = e.dept_no(+)
GROUP BY d.dept_no, dept_name;
SELECT * FROM deptsummary
WHERE Salaries > 5000;
View Example 3
Views are also used to permit access to only
certain rows and/or columns in a table to some
user(s) and not permit access to the remaining
columns and/or rows
Eg: CREATE VIEW emplist
AS SELECT emp_no, ename, fname, job
FROM employees
WHERE LOWER(job)!='president'
ORDER BY ename, fname;
SELECT * FROM emplist;
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Simple and Complex Views
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A simple view is based upon a query that
involves only a single table and uses no
grouping; any other view is referred to a
complex view
Data can be modified using INSERT, UPDATE
and DELETE statements applied to a simple
view but not to a complex view
eg: UPDATE emplist
SET job = 'Assistant'
WHERE job = 'Clerk';
Modifying Data using a View
Data can be modified using INSERT, UPDATE
and DELETE statements on a simple view but
this is not normally done
 A simple view defined using a WITH READ
ONLY clause will not permit data to be modified
eg CREATE VIEW emplist2
AS SELECT emp_no, ename, fname, job
FROM employees
WHERE LOWER(job)!='president'
ORDER BY ename, fname
WITH READ ONLY;
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Change View
Change definition of a view by using the OR
REPLACE clause in the CREATE VIEW
statement
eg CREATE OR REPLACE VIEW emplist
AS SELECT emp_no, ename, fname, job
FROM employees
ORDER BY ename, fname;
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Indexes
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Indexes store the value of a column(s) being
indexed as well as pointer(s) to the physical
location of the row(s) having this value
Most are implemented using a B*Tree
Indexes are used for 2 purposes: improving
performance of queries and enforcing
uniqueness of column values
CREATE INDEX indexname
ON tablename(col1name,col2name,…)
Indexing and Performance
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Indexes are often created to improve
performance of queries
However indexes slow performance of INSERT,
UPDATE and DELETE operations because the
index must be updated also
Be aware of trade-off between overhead
involved and usefulness of index
DBMS decides when an index should be used
to retrieve data – programmer cannot reference
an index in a query
Indexes
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Index can be used to enforce uniqueness on
column(s) in a table
PRIMARY KEY and UNIQUE constraints are
actually implemented by Oracle DBMS defining
a UNIQUE index
CREATE UNIQUE INDEX indexname
ON tablename(col1name,…)
eg CREATE UNIQUE INDEX SINIndex
ON STUDENT(SIN)
Composite Indexes
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If defining an index on a composite field, the
order of the columns is significant : list the most
commonly queried column first so that the index
can be used for this single column or for the
combined columns
eg CREATE INDEX SectionIndex
ON SECTION (subjcode,sectcode)
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SectionIndex can be used to search on subjcode
alone or subjcode and sectcode combined – but
is not used to search on sectcode alone
Common Reasons to Define an
Index on Large Tables
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Column(s) used frequently in joins – therefore
index defined on most foreign key fields
Column(s) used frequently in clauses (WHERE,
GROUP BY, ORDER BY)
Columns with a wide distribution of values so
index entries correspond to a limited number of
rows
Column with large occurrence of NULL values
since NULL values are not indexed
Sequence
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A sequence is an object which is used to
generate sequential values which are often
assigned to primary keys
A sequence is not physically linked with a table
or column
It is the responsibility of the developer to only
use a sequence with the primary key or column
that it was defined for
CREATE SEQUENCE seqname
INCREMENT BY n
START WITH n
Sequence(ctd)
Eg CREATE SEQUENCE emp_no_seq
START AT 1000
INCREMENT BY 1
 A sequence has two pseudo-columns:
- NEXTVAL : gets the next sequential value of
the sequence
- CURRVAL : gets the current value of the
sequence
Eg INSERT INTO employees
VALUES(emp_no_seq.nextval, …)
Eg SELECT emp_no_seq.currval FROM dual
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Synonym
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A synonym allows you to associate an alternate
name with an object such as a table or view
Eg CREATE SYNONYM emp
FOR employees;
SELECT * FROM emp;
Remove Object from Data
Dictionary
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Remove object from data dictionary
DROP VIEW viewname
DROP SEQUENCE sequencename
DROP INDEX indexname
DROP SYNONYM synonymname