Interview Basic Question for SQL

1) The Difference between Count and Count (*)?

Count:-

Counts the number of non-null values.

Count (*):-

Counts the number of rows in the table, including null values and duplicates.

2) What is a ROWID?

ROWID is the logical address of a row, and it is unique within the database.

3) What is a UNIQUE KEY?

A UNIQUE KEY is one or more columns that must be unique for each row of the table.

4) What is a PRIMARY KEY?

The PRIMARY KEY is the column(s) used to uniquely identify each row of a table.

5) Can Primary key is a Foreign Key on the same table?

Yes

6) Write a query to select the 5th highest salary from a table.

SELECT min (salary) AS high5 FROM employee WHERE salary IN (SELECT DISTINCT TOP 5 salary FROM orders ORDER BY salary DESC)