Ans: There can be a key apart from primary key in a table that can also be a key. This key may or may not be a unique key. For example, in an employee table, empno is a primary key, EMP name is a alternate key that may not be unique but still helps in identifying a row of the table.

Ans: A column or combination of columns that identify a row of data in a table is Primary Key.
A key in a table that identifies records in other table in called a foriegn key.

Primary key and unique key
Both are used to provide uniqueness of the column.
Primary key creates clustered key on the column by default whereas unique key creates non-clustered index by default.
Primary doesn't allow NULL values whereas unique key can allow one NULL value.

Ans: Referential integrity is a RDBMS concept and ensures the relationships among tables remain consistent. Data integration (correctness of data) and preventing inconsistent data are ensured by referential integrity. This concept has two keys namely primary key and foreign key. Among the tables, one table should contain primary key and other(s), foreign key. Foreign key specifies that the values in that column must not be other than the values available in the table which has primary key.

Ans: ER diagram is a conceptual and abstract representation of data and entities. ER is a data modeling process, which is to produce a conceptual schema of a system, often an RDBMS and the needs in top down fashion. ER diagrams are derived by using this model

Ans: RDBMS stands for Relational Database Management System. It organizes data into related rows and columns.

Features
It stores data in tables.
Tables have rows and column.
These tables are created using SQL.
And data from these tables are also retrieved using SQL

Ans: Indexes improve query performance but it slows down data modification operations.
Indexes consume disk space.

Ans: Indexes can be clustered and non-clustered indexes.
Clustered index keeps all the records in the database in the order of clustered index key.
There can be only one clustered index per table.
Non-clustered indexes are stored as B-tree structure in their own storage separate from data storage in the table.

Ans: Index is the way to order the records in a database according to the field values. It is the way to have fast access to the particular information. Indexes are created to the columns that are queried frequently.

Ans: select emp_id, emp_name, Sal from employee a where &n = (select count(distinct(salary)) from employee b where a.sal <=b.sal)

&n=22 ( U will be asked to enter the number and then enter for which record u want to retrieve)

a.sal <=b.sal (this can be used to retrieve max record)

a.sal >=b.sal( this can be used to retrieve min record).

Ans: START or @.