A table is the data structure that holds data in a relational database. A table is composed of rows and columns.

A table in Oracle Ver. 7.3 can have maximum 255 Columns and in Oracle Ver. 8 and above a table can have maximum 1000 columns. Number of rows in a table is unlimited in all the versions.

A table can represent a single entity that you want to track within your system. This type of a table could represent a list of the employees within your organization, or the orders placed for your company's products.

A table can also represent a relationship between two entities. This type of a table could portray the association between employees and their job skills, or the relationship of products to orders. Within the tables, foreign keys are used to represent relationships.

Although some well designed tables could represent both an entity and describe the relationship between that entity and another entity, most tables should represent either an entity or a relationship.

The following sessions explain how to create, alter, and drop tables. Some simple guidelines to follow when managing tables in your database are included.

Designing Tables

Consider the following guidelines when designing your tables:

• Use descriptive names for tables, columns, indexes, and clusters.
• Table Names, Columns Names can contain maximum of 30 characters and they should start with an alphabet.
• Be consistent in abbreviations and in the use of singular and plural forms of table names and columns.
• Select the appropriate datatype for each column.
• Arrange columns that can contain NULL Values in the last, to save storage space.

Before creating a table, you should also determine whether to use integrity constraints. Integrity constraints can be defined on the columns of a table to enforce the business rules of your database automatically.

Before creating a Table you also have to decide what type of data each column can contain. This is known as datatype.  Lets Discuss what datatypes are available in Oracle.

Datatypes

A datatype associates a fixed set of properties with the values that can be used in a column of a table or in an argument of a procedure or function. These properties cause Oracle to treat values of one datatype differently from values of another datatype. For example, Oracle can add values of NUMBER datatype, but not values of RAW datatype.

Oracle supplies the following built-in datatypes:

Character datatypes

• CHAR
• NCHAR
• VARCHAR2 and VARCHAR
• NVARCHAR2
• CLOB
• NCLOB
• LONG

Numeric datatypes

• NUMBER

Time and date datatypes:

• DATE
• INTERVAL DAY TO SECOND
• INTERVAL YEAR TO MONTH
• TIMESTAMP
• TIMESTAMP WITH TIME ZONE
• TIMESTAMP WITH LOCAL TIME ZONE

Binary datatypes

• BLOB
• BFILE
• RAW
• LONG RAW

Another datatype, ROWID, is used for values in the ROWID pseudocolumn, which represents the unique address of each row in a table.

The following table summarizes the information about each Oracle built-in datatype.

Representing Character Data

Use the character datatypes to store alphanumeric data:

CHAR and NCHAR datatypes store fixed-length character strings.

VARCHAR2 and NVARCHAR2 datatypes store variable-length character strings. (The VARCHAR datatype is synonymous with the VARCHAR2 datatype.)

NCHAR and NVARCHAR2 datatypes store Unicode character data only.

CLOB and NCLOB datatypes store single-byte and multibyte character strings of up to four gigabytes.

The LONG datatype stores variable-length character strings containing up to two gigabytes, but with many restrictions.

This datatype is provided for backward compatibility with existing applications; in general, new applications should use CLOB and NCLOB datatypes to store large amounts of character data, and BLOB and BFILE to store large amounts of binary data.

When deciding which datatype to use for a column that will store alphanumeric data in a table, consider the following points of distinction:

To store data more efficiently, use the VARCHAR2 datatype. The CHAR datatype blank-pads and stores trailing blanks up to a fixed column length for all column values, while the VARCHAR2 datatype does not add any extra blanks.

For example if you define  empname as char(20) then if you store names like “Sami” then name will occupy 20 bytes( 4 bytes for characters “Sami” and 16 blank spaces)

And if you define  empname as varchar2(20) then if you store names like “Sami” then oracle will take 4 bytes only.

Use the CHAR datatype when you require ANSI compatibility in comparison semantics (when trailing blanks are not important in string comparisons). Use the VARCHAR2 when trailing blanks are important in string comparisons.

The CHAR and VARCHAR2 datatypes are and will always be fully supported. At this time, the VARCHAR datatype automatically corresponds to the VARCHAR2 datatype and is reserved for future use.

Representing Numeric Data

Use the NUMBER datatype to store real numbers in a fixed-point or floating-point format. Numbers using this datatype are guaranteed to be portable among different Oracle platforms, and offer up to 38 decimal digits of precision. You can store positive and negative numbers of magnitude 1 x 10-130 through 9.99 x10125, as well as zero, in a NUMBER column.

You can specify that a column contains a floating-point number, for example:

distance NUMBER

Or, you can specify a precision (total number of digits) and scale (number of digits to right of decimal point):

price NUMBER (8, 2)

Although not required, specifying precision and scale helps to identify bad input values. If a precision is not specified, the column stores values as given. The following table shows examples of how data different scale factors affect storage.

Representing Date and Time Data

Use the DATE datatype to store point-in-time values (dates and times) in a table. The DATE datatype stores the century, year, month, day, hours, minutes, and seconds.

Use the TIMESTAMP datatype to store precise values, down to fractional seconds. For example, an application that must decide which of two events occurred first might use TIMESTAMP. An application that needs to specify the time for a job to execute might use DATE.

Date Format

For input and output of dates, the standard Oracle default date format is DD-MON-RR. For example:

'13-NOV-1992'

To change this default date format on an instance-wide basis, use the NLS_DATE_FORMAT parameter. To change the format during a session, use the ALTER SESSION statement. To enter dates that are not in the current default date format, use the TO_DATE function with a format mask. For example:

TO_DATE ('November 13, 1992', 'MONTH DD, YYYY')

Be careful using a date format like DD-MON-YY. The YY indicates the year in the current century. For example, 31-DEC-92 is December 31, 2092, not 1992 as you might expect. If you want to indicate years in any century other than the current one, use a different format mask, such as the default RR.

Time Format

Time is stored in 24-hour format, HH24:MI:SS. By default, the time in a date field is 12:00:00 A.M. (midnight) if no time portion is entered. In a time-only entry, the date portion defaults to the first day of the current month. To enter the time portion of a date, use the TO_DATE function with a format mask indicating the time portion, as in:

INSERT INTO Birthdays_tab (bname, bday) VALUES
    ('ANNIE',TO_DATE('13-NOV-92 10:56 A.M.','DD-MON-YY HH:MI A.M.'));

Creating Tables in Oracle

Once you have designed the table and decided about datatypes use the following SQL command to create a table.

For example, the following statement creates a table named Emp.

CREATE TABLE Emp (
   Empno      NUMBER(5),
   Ename      VARCHAR2(15),
   Hiredate   DATE,
   Sal        NUMBER(7,2)
          );

To insert rows in the table you can use SQL  INSERT command.

For example the following statement creates a row in the above table.

SQL> insert into emp values (101,’Sami’,3400);

To insert rows continuously in SQL Plus you can give the following command.

SQL> insert into emp values (&empno,’&name’,&sal);

These &Empno, &name and &sal  are known as substitution variables. That is SQLPlus will prompt you for these values and then rewrites the statement with supplied values.

To see the rows you have inserted give the following command.

SQL> Select * from emp; 

To see the structure of the table i.e. column names and their datatypes and widths. Give the following command.

SQL> desc  emp

To see how many tables are in your schema give the following command.

SQL> select * from cat;

or

SQL> select * from tab;