DBMS - Interview Questions and Answers
What is the difference between a correlated subquery and a regular subquery?
A correlated subquery appears deceptively similar to a regular subquery. The difference is that a regular subquery can be processed from the bottom up. In a regular subquery, results from the lowest query can be determined and used to evaluate the upper-level query. In contrast, in a correlated subquery, the processing is nested; that is, a row from an upper query statement is used in comparison with rows in a lower level query. The key distinction of a correlated subquery is that the lower-level select statements use columns from upper-level statements.
What is a dependency graph?
A dependency graph is a diagram that is used to portray the connections between database elements. For example, a change in a table can potentially impact relationships, views, indexes, triggers, stored procedures, and application programs. These impacts need to be known and accounted for before making database changes.
Explain how to add a NOT NULL column to a table.
First, add the column as NULL. Then use UPDATE to add data to every row. Finally use an ALTER TABLE . . . ALTER COLUMN statement to change the column constraint to NOT NULL.
You have two tables, EMPLOYEE and COMPUTER that are in a one-to-one relationship. The foreign key is EmpNumber in COMPUTER which references EmpNumber as the primary key of EMPLOYEE. Explain what must be done to convert the one-to-one EMPLOYEE-COMPUTER relationship to a one-to-many relationship where one employee can have more than one computer.
In the one-to-one relationship, there will be a constraint on EmpNumber as a foreign key in COMPUTER stating that EmpNumber must be unique. To convert the relationship to a one-to-many relationship, just drop this constraint.
Explain the difference between an exclusive lock and a shared lock.
An exclusive lock prohibits other users from reading the locked resource; a shared lock allows other users to read the locked resource, but they cannot update it.
Explain the difference between optimistic locking and pessimistic locking.
Optimistic locking assumes no transaction conflict will occur and deals with the consequences if it does. Pessimistic locking assumes that conflict will occur and so prevents it ahead of time with locks. In general, optimistic locking is preferred for the Internet and for many intranet applications.