DBMS Interview Questions Answers

Table of Contents

What is indexing and the different kinds of indexing?

Indexing is a technique for determining how quickly specific data can be found. 

  1. Binary search style indexing
  2. B-Tree indexing
  3. Inverted list indexing
  4. Memory resident table
  5. Table indexing

What is the system catalog or catalog relation? How is it better known as?

An RDBMS maintains a description of all the data that it contains, information about every relation and index that it contains. This information is stored in a collection of relations maintained by the system called metadata. It is also called a data dictionary.

What is meant by query optimization?

The phase that identifies an efficient execution plan for evaluating a query that has the least estimated cost is referred to as query optimization.

What is durability in DBMS?

Once the DBMS informs the user that a transaction has successfully completed, its effects should persist even if the system crashes before all its changes are reflected on disk. This property is called durability.

What do you mean by atomicity and aggregation?

Atomicity: Either all actions are carried out or none are. Users should not have to worry about the effect of incomplete transactions. DBMS ensures this by undoing the actions of incomplete transactions.

Aggregation: A concept that is used to model a relationship between a collection of entities and relationships. It is used when we need to express a relationship among relationships.

What is a Phantom Deadlock?

In distributed deadlock detection, the delay in propagating local information might cause deadlock detection algorithms to identify deadlocks that do not really exist. Such situations are called phantom deadlocks and they lead to unnecessary aborts.

What is a checkpoint and When does it occur?

A Checkpoint is like a snapshot of the DBMS state. By taking checkpoints, the DBMS can reduce the amount of work to be done during restart in the event of subsequent crashes.

What are the different phases of a transaction?

Different phases are
1) Analysis phase
2) Redo Phase
3) Undo phase

What do you mean by a flat-file database?

It is a database in which there are no programs or user access languages. It has no cross-file capabilities but is user-friendly and provides user-interface management.

What is “transparent DBMS”?

It is one, which keeps its Physical Structure hidden from the user.

What is a query?

A query with respect to DBMS relates to user commands that are used to interact with a database. The query language can be classified into data definition language and data manipulation language.

What do you mean by Correlated subquery?

Subqueries, or nested queries, are used to bring back a set of rows to be used by the parent query. Depending on how the subquery is written, it can be executed once for the parent query or it can be executed once for each row returned by the parent query. If the subquery is executed for each row of the parent, this is called a correlated subquery.

A correlated subquery can be easily identified if it contains any references to the parent subquery columns in its WHERE clause. Columns from the subquery cannot be referenced anywhere else in the parent query. The following example demonstrates a non-correlated subquery.

Example: Select * From CUST Where ’10/03/1990′ IN (Select ODATE From ORDER Where CUST.CNUM = ORDER.CNUM)

What are the primitive operations common to all record management systems?

Addition, deletion, and modification.

Name the buffer in which all the commands that are typed in are stored?

‘Edit’ Buffer.

What are the unary operations in Relational Algebra?


Are the resulting relations of PRODUCT and JOIN operation the same?

PRODUCT- Concatenation of every row in one relation with every row in another.
JOIN- Concatenation of rows from one relation and related rows from another.


Two important pieces of RDBMS architecture are the kernel, which is the software, and the data dictionary, which consists of the system-level data structures used by the kernel to manage the database You might think of an RDBMS as an operating system (or set of subsystems), designed specifically for controlling data access; its primary functions are storing, retrieving, and securing data. An RDBMS maintains its own list of authorized users and their associated privileges; manages memory caches and paging; controls locking for concurrent resource usage; dispatches and schedules user requests; and manages space usage within its table-space structures.

Name the sub-systems of an RDBMS.

I/O, Security, Language Processing, Process Control, Storage Management, Logging and Recovery, Distribution Control, Transaction Control, Memory Management, Lock Management.

Which part of the RDBMS takes care of the data dictionary? How?

The data dictionary is a set of tables and database objects that are stored in a special area of the database and maintained exclusively by the kernel.

What is the job of the information stored in the data dictionary?

The information in the data dictionary validates the existence of the objects, provides access to them, and maps the actual physical storage location.

How do you communicate with an RDBMS?

You communicate with an RDBMS using Structured Query Language (SQL).

Define SQL and state the differences between SQL and other conventional programming Languages.

SQL is a nonprocedural language that is designed specifically for data access operations on normalized relational database structures. The primary difference between SQL and other conventional programming languages is that SQL statements specify what data operations should be performed rather than how to perform them.

Name the three major sets of files on disk that compose a database in Oracle.

There are three major sets of files on disk that compose a database. All the files are binary. These are

1) Database files 
2) Control files
3) Redo logs

The most important of these are the database files where the actual data resides. The control files and the redo logs support the functioning of the architecture itself. All three sets of files must be present, open, and available to Oracle for any data on the database to be useable. Without these files, you cannot access the database, and the database administrator might have to recover some or all of the database using a backup if there is one.

What is the database Trigger?

A database trigger is a PL/SQL block that can be defined to automatically execute for insert, update, and delete statements against a table. The trigger can e defined to execute once for the entire statement or once for every row that is inserted, updated, or deleted. For any one table, there are twelve events for which you can define database triggers. A database trigger can call database procedures that are also written in PL/SQL.

What are stored-procedures? And what are the advantages of using them?

Stored procedures are database objects that perform a user-defined operation. A stored procedure can have a set of compound SQL statements. A stored procedure executes the SQL commands and returns the result to the client. Stored procedures are used to reduce network traffic.

What is Storage Manager?

It is a program module that provides the interface between the low-level data stored in the database, application programs and queries submitted to the system.

What is a Buffer Manager?

It is a program module, which is responsible for fetching data from disk storage into the main memory and deciding what data to be cached in memory.

What is a Transaction Manager?

It is a program module, which ensures that the database, remains in a consistent state despite system failures and concurrent transaction execution proceeds without conflicting.

What is File Manager?

It is a program module, which manages the allocation of space on disk storage and data structure used to represent information stored on a disk.

What is Authorization and Integrity manager?

It is the program module, which tests for the satisfaction of integrity constraint and checks the authority of the user to access data.

What are stand-alone procedures?

Procedures that are not part of a package are known as stand-alone because they independently defined. A good example of a stand-alone procedure is one written in an SQL*Forms application. These types of procedures are not available for reference from other Oracle tools. Another limitation of stand-alone procedures is that they are compiled at run time, which slows execution.

What are cursors give different types of cursors?

PL/SQL uses cursors for all database information accesses statements. The language supports the use of two types of cursors
1.) Implicit
2.) Explicit

What are cold backup and hot backup (in case of Oracle)?

Cold Backup: It is copying the three sets of files (database files, redo logs, and control files) when the instance is shut down. This is a straight file copy, usually from the disk directly to tape. You must shut down the instance to guarantee a consistent copy. If a cold backup is performed, the only option available in the event of data file loss is restoring all the files from the latest backup. All work performed on the database since the last backup is lost.

Hot Backup: Some sites (such as worldwide airline reservations systems) cannot shut down the database while making a backup copy of the files. The cold backup is not an available option.

What is meant by Proactive, Retroactive and Simultaneous Update?

Proactive Update: The updates that are applied to the database before it becomes effective in the real world.

Retroactive Update: The updates that are applied to the database after it becomes effective in the real world.

Simultaneous Update: The updates that are applied to the database at the same time when it becomes effective in the real world.

How the procedure is different from Function

How Exceptions can be handled?

Differentiate between Syntax and runtime errors.

A syntax error can be easily detected by a PL/SQL compiler. For eg, incorrect spelling.

A runtime error is handled with the help of the exception-handling section in a PL/SQL block. For eg, SELECT INTO statement, which does not return any rows.

Explain Commit, Rollback, and Savepoint.

For a COMMIT statement, the following is true:

  • Other users can see the data changes made by the transaction.
  • The locks acquired by the transaction are released.
  • The work done by the transaction becomes permanent.

A ROLLBACK statement gets issued when the transaction ends, and the following is true.

  • The work done in a transition is undone as if it was never issued.
  • All locks acquired by the transaction are released.

It undoes all the work done by the user in a transaction. With SAVEPOINT, only part of the transaction can be undone.

How many triggers can be applied to a table?

A maximum of 12 triggers can be applied to one table.

  •  Explain 3 basic parts of a trigger.
  • A triggering statement or event.
  • A restriction
  • An action
  • Show the cursor attributes of PL/SQL.

%ISOPEN: Checks if the cursor is open or not

%ROWCOUNT: The number of rows that are updated, deleted or fetched.

%FOUND: Checks if the cursor has fetched any row. It is true if rows are fetched

%NOT FOUND: Checks if the cursor has fetched any row. It is True if rows are not fetched.

How does ROWID help in running a query faster?

ROWID is the logical address of a row, it is not a physical column. It composes of data block number, file number and row number in the data block. Thus, I/O time gets minimized retrieving the row and results in a faster query.

What are the character functions?

INITCAP, UPPER, SUBSTR, LOWER and LENGTH are all character functions. Group functions give results based on groups of rows, as opposed to individual rows. They are MAX, MIN, AVG, COUNT, and SUM.

What do you understand by database and what is objects in oracle?

Ans: A database is defined as a collection of meaningful data. Objects in oracle mean Table, Views, Procedures, Triggers, Synonym, etc

What is an Oracle index?

Ans: An index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. The index can be created on one or more columns of a table. The index may also be considered as an ordered list of content of a column.

What is the fastest query method for a table?

Ans: By rowid

What is the difference between CHAR and VARCHAR?

Ans: CHAR is a fixed-length character type at the storage level, and that VARCHAR will be variable length.

What are the constraints and their types?

Integrity Constraint: An integrity constraint is a declarative way to define a business rule for a column of a table. An integrity constraint is a statement about a table’s data that is always true.

Types of integrity constraints: The following integrity constraints are supported by ORACLE:

1. NOT NULL: disallows nulls (empty entries) in a table’s column

2. UNIQUE: disallows duplicate values in a column or set of columns

3. PRIMARY KEY: disallows duplicate values and nulls in a column or set of columns

4. FOREIGN KEY: requires each value in a column or set of columns to match a value in a related table’s unique or Primary Key.

5. CHECK: disallows values that do not satisfy the logical expression of the constraint.


What is the maximum number Of columns, a table can have?

Ans: 254(Oracle)

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