Creating Oracle Tables ( Lesson 2 )

Transcription

1 Creating Oracle Tables ( Lesson 2 ) 2.1 Demo Application During this course we will be using Application Express and Oracle 10g Express Edition to create an application. The application will be used to reinforce concepts and display functionality. In this particular case the application is a Job Recruitment System (JRS). The Job Recruitment System will have the following features: An attractive front page for login Self Registration Capabilities A main menu (customized by user type) A user maintenance screen for admins A job vacancies screen for posting job vacancies An Apply (or Application) screen for responding to job advertisements. Additionally on this screen users may upload their resume to the website. A message/discussion board for user interaction These features will be demonstrated in class. 2.2 Recap of Tables So what is a table? A table is an object on the database which stores data. Data is structured on the table where a row of data represents an instance of related data and columns represent attributes of the data. Graphically one can imagine a table as being like a spreadsheet, however databases and database tables are more powerful than spreadsheets. The application depends on database tables to store information, e.g. the users table will store information about users logging into the application. One instance of a user (Charlie) is that the user is a job seeker looking to fill a job vacancy another instance of a user (Dave) could be a job recruiter looking for suitable candidates. Each user would be represented on the database table as a row of data and each attribute such as first name, last name etc. would be represented by columns in the database as seen below. Finally each attribute or column would have a particular datatype associated with it, basic datatypes used are as follows: Number(precision, scale) A numeric datatype, where precision represents how many characters the number can consist of (before and after the decimal point) and scale represents the number of characters after the decimal point. Example, a column with a datatype of number(10,2) would have 8 characters before the decimal point and two characters after it. Varchar2(size) Varchar2 represents variable length character column, the size specification determines the maximum number of characters that can be stored in the field. Example a column such as an address may have a datatype of varchar2(255), this means that at maximum length the field holds 255 characters however if there is less, it does not blank pad the item to make it 255.

2 Char(size) Char is just like varchar2, however with char if the number of characters are less than the size specified then the rest of the space is blank filled. This is useful for small fixed with type columns; larger columns with char type would only waste space. Date This is the date and time datatype, the date and time is actually stored as an internally recognizable format, a default database format is used for presenting the date portion of the information (usually of the form DD-MON-YYYY), however for alternate formats, a function called to_char is used to present the date in a particular format, e.g. to_char(date_column, DD- MM-YYYY ) can possibly return alternately to_char(date_column, DD MONTH,YYYY ) can possibly return 25 NOVEMBER, Table Constraints Table Constraints are rules on the tables. Some of the basic rules are as follows: Not Null the item must not remain empty Unique the item must be unique on the table Primary Key the item identifies the row of data on the table (being both not null and unique) Foreign Key The item must exist as a primary key on this or another table or else be null. Check the item has a basic condition on it, e.g. < 50 (less than 50) In general, constraints are effective in maintaining data integrity i.e. the data is meaningful and does not eventually grow in to meaningless collection of raw data. To illustrate think of an administrator being allowed to create a user but not entering the user s first and last name, how meaningful would that be? Primary Key Constraint More specifically Primary keys are useful to identify the row of data. These usually take the form of a number e.g. User Id in the users table is a number however there is no limitation as whether a primary key can be a number or not, nor whether it can be one column or a combination of columns. However using a number is always a good idea. To illustrate, had we used first name and last name as the primary key then there would be several Steve Singh, Susan Mohammed and John Henry (to name a few). This is not a new idea, think about your driver s permit, passport and government Id card, each has a number which is what uniquely identifies you. As you can imagine there can only be one primary key constraint on a table however nothing prevents you from having multiple unique keys on a table. Foreign Key Constraint Tables often don t exist in isolation but are often used in combination with other tables in an application. Foreign keys establish a formal relationship between tables. As an example two tables on our application is the thread table ( which contains the discussion topic ) and the message table ( which contains users comments on any particular topic ). If there is no relationship established between these tables how would we know which messages go with which topic? The solution is a foreign key, the Thread ID is the primary key for the thread table, and the thread id must also exist on the message table. The thread id on the message table must be a number which exist on the thread table or else do not have a value. However a message without an associated thread subject is meaningless and so we must employ another constraint here, the not null constraint.

3 2.4 Demo Application Tables The demo application JRS consist of five main tables: Not Null? Column Type Size Other Constraints Column Name USERS USERID YES Number Primary Key USERNAME YES Varchar2 30 Unique Key FIRSTNAME Varchar2 45 LASTNAME Varchar2 45 COMPANY Varchar2 45 PASSWORD YES Varchar2 30 USERTYPE YES Varchar2 15 ADDRESS1 Varchar2 45 ADDRESS2 Varchar2 45 CITY Varchar2 45 COUNTRY Varchar2 45 YES Varchar2 255 WEBSITE Varchar2 255 VACANCY VACANCYID YES Number Primay Key POSITION YES Varchar2 45 JOBDESCRIPTION YES Varchar2 255 USERID YES Number References Users(Userid) DATEPOSTED YES Date EXPIRYDATE YES Date INDUSTRY YES Varchar2 45 APPLICATION APPLICATIONID YES Number Primary Key VACANCYID YES Number References Vacancy(Vacancyid) USERID YES Number References Users(Userid) DATEAPPLIED YES Date RESUME Number THREAD THREADID YES Number Primary Key TOPIC YES Varchar2 255 DATECREATED YES Date USERID YES Number References Users(Userid) MESSAGE MESSAGEID YES Number Primary Key MESSAGEDETAIL YES Varchar THREADID YES Number References Thread(Threadid) USERID YES Number References Users(Userid)

4 DATEPOSTED YES Date 2.4 Application Express Table Creation Wizard On application express the object browser is used to create tables. Create the Users table 1. Navigate to object browser and click on the create button to the far right. 2. Create options should appear including Click on the Table link.

5 3. The table creation wizard should appear, enter the table name and populate the columns as given from the definitions in the previous section. Click Next. 4. Choose a primary key, choose Populated from a new sequence and select the primary key as the USERID then click next 5. There are no foreign keys defined for the users table so click next on foreign keys screen 6. On the Constraints screen, create a unique constraint for username. To do this select the unique radio button then click on the blank field below the radio button. It should automatically change to the field selection screen as seen below. Select username on the left hand side then click on right arrow to move it to the right hand side. Rename the constraint name to something meaningful e.g. USERS_USERNAME_UK. Click the add button at the top of the section. Finally, click on the finish button at the top.

6 7. Click on the create button to create the table. Create Vacancy table 1. Repeat steps 1 to 3 above 2. Choose a primary key, choose Populated from a new sequence and select the primary key as the VACANCYID then click next 3. Create foreign key for userid. Type in a meaningful name e.g. VACANCY_USERID_FK. Select USERID field on the left hand side and click on the right arrow to move it to the right hand side.

7 Click the up arrow to select the USERS table then click the down arrow to get the second column selection window. Select USERID field on the left hand side and click on the right arrow to move it to the right hand side. Click on the ADD button. If you made a mistake you can click on the red X and start over again. Click on the next button when finished. 4. There are no additional constraints so click Finish on the constraints screen. 5. Finally click create to create the table. Use these steps as the basis for creating the remaining tables. 2.5 Additionally Created Supporting Objects When you create a table with the option of populated by a new sequence, oracle creates additional objects to support the function. Basically, this option allows rows of data to be entered and the primary key to be guaranteed unique. Two objects are required for this: Sequence: an object which generates numbers in increments. Trigger: code executed on a specific event

8 The sequence generates the new number and then the trigger executes prior to inserting a row of data into the table. The trigger inserts the number generated from the sequence into the primary key. create or replace trigger "BI_VACANCY" before insert on "VACANCY" for each row begin select "VACANCY_SEQ".nextval into :NEW.VACANCYID from dual; end;