Oracle Questions

Oracle NOT NULL is used on a column to ensure that the value for that column can never be NULL.
Example:
Below, the constraint is that the id should never be NULL. If it is, oracle throws an error.
create table employee ( id number NOT NULL, Name varchar(200) );

The CACHE option means how many sequences will be stored in memory for access by the application objects. The performance is faster. However in case of the database is down the data is memory is lost for the sequence.

The NO CACHE option means values are not stored in memory. So there might be some performance issue. 

A unique constraint on a column uniquely identifies the record by a combination of one or more fields. Few unique constraint fields can have a NULL value as long as the combination of values is unique.

Example:
create table employee ( id number NOT NULL, dob DATE, professor_id NOT NULL, Name varchar(200) Constraint id_unique UNIQUE(id,dob) );

A record data type represents a data type for that row in a database table. It lets u define your own records and not your own fields.

1) START- Used to run a SQL script.
2) ACCEPT- Accepts input from user.
3) GET- Gets the sql file from user to place in buffer.
4) LIST- Displays the last command executed.
5) RUN- Used to list and run the command in buffer
6) SHOW- Shows the environment settings. Example: SHOW PAGESIZE

New column can be added using the ALTER TABLE command as shown below:
Example:
ALTER TABLE employee ADD DateOfBirth date