SQL definitions

Primary Key: A column in a table whose values uniquely identify the rows in the table. A primary key value cannot be NULL.
Unique Key: Unique Keys are used to uniquely identify each row in an Oracle table. There can be one and only one row for each unique key value.
Surrogate Key: A system generated key with no business value. Usually implemented with database generated sequences.
Foreign key (FK): A column or combination of columns that is used to establish and enforce a link between the data in two tables

Composite key consists of two or more columns, designated together as a table's primary key. Multiple-column primary keys can be defined only as table-level constraints:
Candidate key is a combination of attributes that can be uniquely used to identify a database record without any extraneous data. Each table may have one or more candidate keys. One of these candidate keys is selected as the table primary key.
Rownum is used to order the datas according to their ranks and it is used in inline views.
Level pseudocolumn is used in hierarchial retrival of database.....
An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created.