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Indexes and more Table
Creation
Indexes
Increase database performance
must be explicitly defined
once defined, are transparent to the
user
once created, the system maintains it
more than one can exist on a given
table
Creating an Index
Syntax
CREATE [UNIQUE] INDEX index_name
ON table_name (column_name)
Example
create index auind
on authors (au_id)
Composite Index
•Used when columns have a logical
relationship and would be searched as a
unit
•Example
create index au_name_ind
on authors (au_lname, au_fname)
•order not important, but performance is
better when primary search col is first
2 Kinds of Indexes
Unique Index
Clustered Index
Unique Index
No 2 rows are permitted to have the same value
system checks data upon creation and data
addition
rejects duplicates and returns an error
should only be created on a column that
requires uniqueness eg. ssn, acct code
can be created as a composite or single column
helps in maintaining data integrity
boosts search performance
Clustered Index
•System sorts rows on an ongoing basis so that
the physical order is the same as the indexed
order
•only 1 can exist per table
•should only be created for a column that is most
often retrieved in order
•greatly increases performance when searching
for contiguous key values… especially a range
•slows down data updates due to the sorting
involved
Things to Consider
Indexes greatly increase query response
time
every index requires system resources to
store and maintain
indexes can actually slow down the
performance of UPDATES, INSERTS, and
DELETES due to index maintenance
So… don’t over index
What Should We
Index?
Any column frequently used in retrieval
primary key columns
columns that are often queried in a sorted
order
columns that are used in joins
columns that are often searched for ranges
We Should NOT Index…
Columns rarely used in queries
columns with 2 or 3 possible values
eg. Male or Female
small tables
SQL-92 Create Table
Constraints
PRIMARY KEY

rejects duplicates and nulls
UNIQUE

rejects duplicates, allows nulls
DEFAULT

inserts the default value when no value is
entered
CHECK

validates data format
FOREIGN KEY and REFERENCES

ties foreign key to the primary key it references
Put it on paper!
Column
Datatype
Null?
Key
title_id
title
char(6)
varchar(80)
not null
not null
primary, unique
unique
type
pub_id
price
advance
char(12)
char(4)
money
money
Default
Reference
2 letter then 2 dig
unclass
null
null
Check
business,
mod_cook,
trad_cook
publishers,
pub_id
Then write your SQL
create table title
(title_id char(6) not null
constraint tididx primary key
constraint tidcheck check
(title_id like ‘[A-Z] [A-Z] [0-9] [0-9]…’),
title varchar(80) not null,
type char(12)
default ‘unclassified’ null
constraint typechk check
(type in(‘business’, ‘mod_cook’,
‘trad_cook’)),
pub_id char(4) null
reference publishers (pub_id)
)
Changing a Table
Syntax
 ALTER table table_name
add column_name datatype null|not null
Removing Objects
Database


DROP DATABASE db_name
deletes ALL tables and data within it!!
Table


DROP TABLE table_name
deletes table and its contents
Index


DROP INDEX table_name.index_name
deletes named index on named table
Joins
What columns do I need?
What tables have these columns?
Are all the tables related in some way?
If not, are there other tables that can relate them?
How are they all related?
Link them together by setting their common fields
equal in the WHERE clause.
Restrict the WHERE clause to the record(s) of
interest.
2 ways of looking at a
Join
Looking at all the tables, linking them
together and treating them like one big
table.
Setting the main search criteria and
then linking the common fields to the
data that is of interest.