Group of tables physically stored together because they share common columns and are often used together is called Cluster.
Lookup tables are like constraints which hold a value. The content in the lookup tables doesn’t change often, hence referred as lookup (reference
Example : Countries table
Countries are not added on regular bases hence one can consider the table as a lookup table. As and when needed in some other table the countries information can be fetched base on PK - Country-Code column. Eg - Emp_Per_Info.
Different types of joins are
-Cartesian Product or Cross Join
-Equi Join or Natural Join
-Non Equi Join
-Left Outer Join
-Right Outer Join
-Full Outer Join
Hash clusters are useful in cases where
(i) There is a uniform, even and predictable no. of key values
(ii) Queries using equality predicates
(iii) The table is NOT growing constantly, and the keys are rarely updated.
A sequence is a database object created by a user that can be used to generate unique integers. A typical usage of sequences is to generate primary key values which are unique for each row.
It is generated and incremented (or decremented) by an internal Oracle routine. It can be used by multiple users and for multiple tables too. A sequence can be used instead of writing an application code for sequence-generating routine.