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ITEC 334 Fall 2010
Online (Business) Application Development
Database Review
Professor J. Alberto Espinosa
Agenda
• Quick review of database design
concepts
• Quick review of SQL SELECT
command for queries
• Learn SQL DELETE, INSERT and
UPDATE commands
• Learn how to embed an SQL
statement in a script statement
p.2
DBMS with Web Access (DBMS server + Web server):
Very common when there are large numbers of users and would be impractical to deploy and install a
DBSM client  access to the database is done through a browser (e.g., on-line purchases)
e.g., Microsoft’s Internet Information Server
(IIS) with Active Server Pages (ASP) +
Microsoft’s SQL Server
Request (ex. get a price quote, place an order)
Response (ex. query results with HTML-formatted
product price or order confirmation notice)
p.3
Database Design Review
p.4
Entity Integrity
• Is ensuring that every record in each table in the database can
be addressed (i.e., found) – this means that there each
record has to have a unique identifier that is not duplicate
or null (i.e., not blank)
• Examples: every student has an AU ID; every purchase order
has a unique number; every customer has an ID
Primary key (PK)  helps enforce Entity Integrity:
• Field(s) that uniquely identifies a record in a table
(e.g., AU user ID)
• Entity integrity = PK is not duplicate & not blank
• PK can be:
– A single field (e.g., UserID), or
– Or more than one field (e.g., OrderNo, LineItem)
p.5
Referential Integrity
• Is ensuring that the data that is entered in one table
is consistent with data in other tables
• Examples: purchase orders can only be placed by
valid customers; accounting transactions can only be
posted to valid company accounts
Foreign key (FK)  helps enforce referential Integrity:
• A field in a table that is a PK in another table
• That is, a field that “must” exist in another table
• This is how referential integrity is maintained
p.6
Data Model Example (Entity Relationship Diagram--ERD):
Course Registration System
Cardinality
1 to
Many
Enrolls
Entities
Relationships
p.7
Cardinality
• Is an important database concept that helps understand
how two entities (i.e., tables) relate to each other
• Cardinality: describes how many records of one entity
(i.e., table) can be associated with records from another
table
• Examples:
1 student can only park 1 car
 1 to 1 relationship
1 professor can teach many courses
1 to many relationship
1 client can place many orders
 1 to many relationship
p.8
Cardinality: 1 to 1 (MS Access notation)
p.9
Cardinality: 1 to many
(MS Access notation)
p.10
Example:
Course Registration System
p.11
Example:
Package Delivery Tracking System
p.12
Example:
Airline Reservation System
p.13
Update Rules
What can be updated/modified in the database and when?
1. It is OK to update values in any non-PK fields, provided that
referential integrity and business rules are respected
2. It is OK to update values in the PK in one table if it is not linked to
a FK in another table, provided that entity integrity, referential
integrity and business rules are respected
3. If a PK is linked to a FK in another table, we need to ensure that
referential integrity is maintained. Depending on what makes sense
for business and the data itself, you can either:
• Disallow updates of values in the PK, or
• Allow updates, but cascade the change
– i.e., change the corresponding values the related FK
p.14
Delete Rules
What can be deleted in the database and when?
1. It is OK to delete records in a table [only] if its PK is not
linked to a FK in another table
2. If its PK is linked to a FK in another table, we need to
ensure that referential integrity is maintained when we
delete the record. Depending on what makes sense for
business and the data itself, either:
• Disallow deletion of records in the table that
has the PK
• Allow deletions of records but cascade
deletions – i.e., delete the corresponding
records in the related table that contain the FK
p.15
Important decisions to make when
designing a web database
•
•
Where do you implement entity integrity, referential integrity,
relationships, update rules, delete rules, etc.?
Two choices:
1. In the database design itself – but then if your program is allowing bad
data to be submitted to the database you will get cryptic error
messages.
2. In the script itself – but then if your program doesn’t enforce these
good database design principles you will end up with bad data
•
•
Decisions, decisions, decisions – the correct answer depends on
your programming orientation and experience
I personally favor 1 because it is best to prevent bad data to be
entered into the database.
p.16
SQL Queries Review
p.17
Useful SQL Commands for Scripting
• You will need to know how to write SQL commands to
(1) display; (2) delete; (3) enter; and (4) modify data
• SELECT: Display existing records
• DELETE: Delete records
• INSERT: Enter new records
• UPDATE: Modify existing records
p.18
SQL SELECT Command: Displaying Data
SELECT column1, column2, etc. – columns you want to display
FROM table1, table2, etc. – tables that contain the data
WHERE condition1 – which records (i.e., rows) to retrieve
AND condition2 etc – you can combine more than one
condition using AND or OR
ORDER BY field1, field2, etc. – to sort the query results
SELECT can be followed by:
DISTINCT (eliminates duplicate rows from result)
TOP x
(lists only the top x rows of result, e.g. TOP 5)
*
(lists all columns in the table)
p.19
“Simple” SELECT Queries:
When all the data is available in a single table
and there is no need to aggregate data
Example: list the product ID, product name, product
type and price from the Products table, for items that
are price at $300 or more:
SELECT ProdID, ProdName, Type, Price
FROM Products
WHERE Price>=300;
Click here to download this database [local copy]
p.20
SQL Queries With Aggregate Functions
• These queries yield a single number result
(i.e., a table with 1 column and 1 row)
• The only thing you can include in the SELECT line are the fields
you are aggregating
• Aggregate functions you can use:
Avg, Sum, Min, Max, Count
• These functions aggregate vertically a column of (usually numeric)
values (e.g., salaries, payment amounts, etc.)
SELECT Avg(Price) AS AvgPrice
FROM Products
WHERE Price>=120
AND Type=“Percussion”;
Note: the AS clause is optional; it does not
change the query results; it only changes the
column label in the results
SELECT Max(Price) AS MaxPrice, Avg(Price) AS AvgPrice
FROM Products
WHERE Type=“Guitars”;
SELECT Count(*) as TotOrders
FROM Orders
WHERE OrderStatus = “Top Priority”
Note: you can use more
than one aggregate
function in one SELECT
command
Note: the Count function counts how many rows
meet the Where criteria, so it you can use any
column you wish to count and you will get the
same results – the easiest thing is to use
Count(*)
p.21
SQL Queries With
Aggregate Functions and Grouping
• The ONLY things you can include in the SELECT line
are:
(1) the fields you are aggregating [e.g., Avg(Price)]
(2) and the fields you are using to group [e.g, Type]
SELECT Type, Avg(Price) AS AvgPrice, Max(Price) AS MaxPrice
FROM Products
Note: the WHERE
clause is evaluated
WHERE Price>=1000
BEFORE the grouping
GROUP BY Type
SELECT Type, Avg(Price) AS AvgPrice, Max(Price) as MaxPrice
FROM Products
Note: the HAVING
clause is evaluated
GROUP BY Type
AFTER the grouping
HAVING Avg(Price)>1000
p.22
“Complex” SELECT (“Join”) Queries
Practical rule: you should query the minimum number of tables possible to get the data you
need. If you need to get the data from more than one table, you MUST JOIN the tables:
Tables: Orders (OrderNo, ClientID, OrderDate, OrderStatus)
LineItems (OrderNo, LineItem, ProdID, Qty)
Table Join (2 ways):
SELECT Orders.OrderNo, ClientID, LineItem, ProdID, Qty
FROM Orders, LineItems
WHERE Orders.OrderNo = LineItems.OrderNo;
Table Product
Join Condition
(This query is wrong – NEVER OMIT THE JOIN CONDITION!!) :
SELECT Orders.OrderNo, ClientID, LineItem, ProdID, Qty
FROM Orders, LineItems;
Complex SELECT Queries with Joins: TIPS
COMPLEX queries that JOIN 2 tables are identical to SIMPLE queries, except
for 2 additional rules you MUST ALWAYS apply:
1.
The two tables need to be JOINED through the common field that links them
e.g., WHERE Orders.OrderNo = LineItems.OrderNo
2.
ANY time you refer to a COMMOND FIELD that exists in both tables, you
must use a TABLE PREFIX to eliminate the ambiguity
e.g., SELECT Orders.OrderNo; WHERE Orders.OrderNo = 990001
For complex queries that JOIN 3 or more tables apply rule 1 for EACH link, and
always apply rule 2 – e.g.,
SELECT Clients.ClientID, ClientName, Orders.OrderNo,
OrderStatus, LineItem, ProdID, Qty
FROM Clients, Orders, LineItems
WHERE Clients.ClientID = Orders.ClientID
AND Orders.OrderNo = LineItems.OrderNo
SQL DELETE Command: Deleting Data
DELETE * FROM table1, table2, etc. – use wildcard *
WHERE condition1 – which records (i.e., rows) to delete
AND condition2 etc – you can combine more than one
condition using AND or OR
e.g.: DELETE * FROM Products WHERE ProdID=“bongos”
Note: the DELETE command is IDENTICAL to the SELECT
command – the only difference is that SELECT displays the
selected records, whereas DELETE deletes them.
Strong suggestion: write a SELECT command first, visually inspect
the records display to ensure that they are the ones you want to
delete, then replace the SELECT keyword with DELETE.
p.25
SQL Insert Command: Entering New Data
Syntax:
INSERT INTO table_name
(column1, column2, etc., columnx, etc.)
VALUES (value1, value2, etc., valuex, etc.)
Ex.: Insert (add) a complete record (values in all fields):
INSERT INTO Friends
VALUES (“ae”, “Espinosa”, “Alberto”, 12/12/2002,
“885-1958”, “Looks tired, needs a vacation”)
Ex.: Insert (add) partial record (values in some fields only –
strongly recommended alternative):
INSERT INTO Friends
(FriendID, LastName, FirstName)
VALUES (“ae”, “Espinosa”, “Alberto”)
p.26
SQL Update Command: Modifying Existing Data
Ex.: Update (modify) record with new values:
UPDATE table_name
SET column1 = value1, columnx= valuex, etc.
WHERE condition AND condition2 etc.
Ex.: Update (modify) record with new values:
UPDATE Friends
SET LastName=“Espinosa”, FirstName=“Alberto”
WHERE FriendID = “alberto”
Friends is the table name, goes after UPDATE
SET indicate the columns where to make the changes
and the values you want to change to
WHERE indicates in which rows the change will take place; this WHERE
keyword works exactly like the WHERE keyword in the SELECT
command
p.27