Database Interview Questions

1.    What is the Difference between DBMS and RDBMS?

A DBMS has to be persistent, that is it should be accessible when the program created the data ceases to exist or even the application that created the data restarted. A DBMS also has to provide some uniform methods independent of a specific application for accessing the information that is stored.

RDBMS is a Relational Data Base Management System Relational DBMS. This adds the additional condition that the system supports a tabular structure for the data, with enforced relationships between the tables. This excludes the databases that don't support a tabular structure or don't enforce relationships between tables.

2.    What are constraints? Explain different types of constraints.

Constraints enable the RDBMS to enforce the integrity of the database automatically, without need to create triggers, rule or defaults. Such constraints can be specified when the table when the table is first created via the CREATE TABLE statement, or after the table is already created via the ALTER TABLE statement.

Common types of constraints

1.    NOT NULL Constraint: Ensures that a column cannot have NULL value.

2.    DEFAULT Constraint: Provides a default value for a column when NONE is specified.

3.    UNIQUE Constraint: Ensures that all values in a column are different.

4.    CHECK Constraint: Makes sure that all values in a column satisfy certain criteria.

5.    Primary Key Constraint: Used to uniquely identify a row in the table.

6.    Foreign Key Constraint: Used to ensure REFERENTIAL INTEGRITY of the data.

a. Foreign Key: A foreign key is a field (or fields) that points to the primary key of another table. The purpose of the foreign key is to ensure referential integrity of the data. In other words, only values that are supposed to appear in the database are permitted.

For example, say we have two tables, a CUSTOMER table that includes all customer data, and an ORDERS table that includes all customer orders. The constraint here is that all orders must be associated with a customer that is already in the CUSTOMER table. In this case, we will place a foreign key on the ORDERS table and have it relate to the primary key of the CUSTOMER table. This way, we can ensure that all orders in the ORDERS table are related to a customer in the CUSTOMER table. In other words, the ORDERS table cannot contain information on a customer that is not in the CUSTOMER table.

CREATE TABLE ORDERS
(Order_ID integer primary key,
Order_Date date,
Customer_SID integer references CUSTOMER (SID),
Amount double);

b. Primary Key: A primary key is used to uniquely identify each row in a table. It creates Cluster index on a column and dose not allow Null value. A primary key can consist of one or more fields on a table. When multiple fields are used as a primary key, they are called a composite key.

Primary keys can be specified either when the table is created (using CREATE TABLE) or by changing the existing table structure (using ALTER TABLE).

CREATE TABLE Customer
(SID integer PRIMARY KEY,
Last_Name varchar(30),
First_Name varchar(30));

c. Unique Key: The UNIQUE constraint ensures that all values in a column are distinct.

For example, in the following CREATE TABLE statement,

CREATE TABLE Customer
(SID integer Unique,
Last_Name varchar (30),
First_Name varchar (30));

A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys.

A key formed by combining at least two or more columns is called composite key.

3.    Definition of View, Sub Query, System Catalog Tables, Schema

a. View: A view is a virtual table. A view consists of rows and columns just like a table. The difference between a view and a table is that views are definitions built on top of other tables (or views), and do not hold data themselves. If data changes in the underlying table, the same change is reflected in the view. A view can be built on top of a single table or multiple tables. It can also be built on top of another view.

Views offer the following advantages:

·         EASE OF USE: A view hides the complexity of the database tables from end users. Essentially we can think of views as a layer of abstraction on top of the database tables.

·         SPACE SAVINGS: Views takes very little space to store, since they do not store actual data.

·         ADDITIONAL DATA SECURITY: Views can include only certain columns in the table so that only the non-sensitive columns are included and exposed to the end user. In addition, some databases allow views to have different security settings, thus hiding sensitive data from prying eyes

b. Sub Query: It is possible to embed a SQL statement within another. When this is done on the WHERE or the HAVING statements, we have a subquery construct.

The syntax is as follows:

SELECT "column_name1"
FROM "table_name1"
WHERE "column_name2" [Comparison Operator]
(SELECT "column_name3"
FROM "table_name2"
WHERE [Condition])

c. System Catalog Tables

A set of system catalog tables is created and maintained for each database. These tables contain information about the definitions of the database objects (for example, tables, views, indexes, and packages), and security information about the type of access that users have to these objects. These tables are stored in the SYSCATSPACE table space.

These tables are updated during the operation of a database; for example, when a table is created. You cannot explicitly create or drop these tables, but you can query and view their content. When the database is created, in addition to the system catalog table objects, the following database objects are defined in the system catalog:

d. SQL Schema: Table is a class for objects representing a database table. The methods of an instantiated object do allow to access the information within a database's data dictionary and to represent them as SQL create statements and the like using the proper SQL dialect.

4.    What is de-normalization and when would you go for it?

As the name indicates, denormalization is the reverse process of normalization. It's the controlled introduction of redundancy in to the database design. It helps improve the query performance as the number of joins could be reduced.

5.    How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables?

One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table.

6.    What's the difference between a primary key and a unique key?

Primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a no clustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

7.    What are user defined data types and when you should go for them?

User defined data types let you extend the base SQL Server data types by providing a descriptive name, and format to the database. Take for example, in your database, there is a column called Flight_Num which appears in many tables. In all these tables it should be varchar (8).

In this case you could create a user defined data type called Flight_num_type of varchar (8) and use it across all your tables.

8.    What is bit data type and what's the information that can be stored inside a bit column?

Bit data type is used to store Boolean information like or 0 (true or false). Until SQL Server 5 bit data type could hold either a or 0 and there was no support for NULL. But from SQL Server 0 onwards, bit data type can represent a third state, which is NULL.

9.    What are defaults? Is there a column to which a default can't be bound?

A default is a value that will be used by a column, if no value is supplied to that column while inserting data. IDENTITY columns and timestamp columns can't have defaults bound to them.

10. What is a transaction and what are ACID properties?

A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation and Durability. These are the properties of a transaction.

11. Explain different isolation levels

An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation): Read Uncommitted, Read Committed, Repeatable Read, Serializable. See SQL Server books online for an explanation of the isolation levels. Be sure to read about SET TRANSACTION ISOLATION LEVEL, which lets you customize the isolation level at the connection level.

12. What's the difference between DELETE TABLE and TRUNCATE TABLE commands?

DELETE TABLE is a logged operation, so the deletion of each row gets logged in the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a table, but it won't log the deletion of each row, instead it logs the de-allocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled back.

13. What is an index? What are the types of indexes? How many clustered indexes can be created on a table? I create a separate index on each column of a table. What are the advantages and disadvantages of this approach?

Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quicker.

Indexes are of two types: Clustered indexes and non-clustered indexes

When you create a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Non-clustered indexes have their own storage separate from the table data storage. Non-clustered indexes are stored as B-tree structures (so do clustered indexes), with the leaf level nodes having the index key and it's row locater. The row located could be the RID or the Clustered index key, depending up on the absence or presence of clustered index on the table.

If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same t ime, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used.

14. What are the steps you will take to improve performance of a poor performing query?

This is a very open ended question and there could be a lot of reasons behind the poor performance of a query. But some general issues that you could talk about would be: No indexes, table scans, missing or out of date statistics, blocking, excess recompilations of stored procedures, procedures and triggers without SET NOCOUNT ON, poorly written query with unnecessarily complicated joins, too much normalization, excess usage of cursors and temporary tables.

Some of the tools/ways that help you troubleshooting performance problems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performance monitor, Graphical execution plan in Query Analyzer.

15. What is a deadlock and what is a live lock? How will you go about resolving deadlocks?

Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other's piece. Each process would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user's process.

A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely.

16. What is blocking and how would you troubleshoot it?

Blocking happens when one connection from an application holds a lock and a second connection requires a conflicting lock type. This forces the second connection to wait, blocked on the first.

17. What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?

Cursors allow row-by-row processing of the result sets.

Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information.

Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip; where as a normal SELECT query makes only one roundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Further, there are restrictions on the SELECT statements that can be used with some types of cursors.

Most of the times, set based operations can be used instead of cursors. Here is an example:

If you have to give a flat hike to your employees using the following criteria:

Salary between 30000 and 40000 -- 5000 hike

Salary between 40000 and 55000 -- 7000 hike

Salary between 55000 and 65000 -- 9000 hike

In this situation many developers tend to use a cursor, determine each employee's salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below:

UPDATE tbl_emp SET salary =

CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000

WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000

WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000

END

Another situation in which developers tend to use cursors: You need to call a stored procedure when a column in a particular row meets certain condition. You don't have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row. For examples of using WHILE loop for row by row processing, check out the 'My code library' section of my site or search for WHILE.

Write down the general syntax for a SELECT statement covering all the options.

Here's the basic syntax: (Also checkout SELECT in books online for advanced syntax).

SELECT select_list

[INTO new_table_]

FROM table_source

[WHERE search_condition]

[GROUP BY group_by__expression]

[HAVING search_condition]

[ORDER BY order__expression [ASC | DESC] ]

18. What is a join and explain different types of joins.

Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table.

Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.

19. What is a self join? Explain it with an example.

Self join is just like any other join, except that two instances of the same table will be joined in the query. Here is an example:

Employees table that contains details of employees and managers so, we have to find out the managers of all the employees, you need a self join.

CREATE TABLE emp

(

empid int,

mgrid int,

empname char(10)

)

INSERT emp SELECT 1, 2,'Vyas'

INSERT emp SELECT 2, 3,'Mohan'

INSERT emp SELECT 3, NULL,'Shobha'

INSERT emp SELECT 4, 2,'Shridhar'

INSERT emp SELECT 5, 2,'Sourabh'

QUERY 1:

SELECT t1.empname [Employee], t2.empname [Manager]

FROM emp t1, emp t2

WHERE t1.mgrid = t2.empid

QUERY 2:

SELECT t1.empname [Employee], COALESCE (t2.empname, 'No manager') [Manager]

FROM emp t1

LEFT OUTER JOIN emp t2

ON t1.mgrid = t2.empid

20. What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?

Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table.

In SQL Server 5 you could define only 3 triggers per table, one for INSERT, one for UPDATE and one for DELETE. From SQL Server 0 onwards, this restriction is gone, and you could create multiple triggers per each action. But in 0 there's no way to control the order in which the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires last using sp_settriggerorder

Triggers can't be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE and DELETE) happens on the table on which they are defined.

Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster.

Till SQL Server 0, triggers fire only after the data modification operation happens. So in a way, they are called post triggers. But in SQL Server 2000 you could create pre triggers also. Search SQL Server 2000 books online for INSTEAD OF triggers.

Also check out books online for 'inserted table', 'deleted table' and COLUMNS_UPDATED ()

There is a trigger defined for INSERT operations on a table, in an OLTP system. The trigger is written to instantiate a COM object and pass the newly insterted rows to it for some custom processing. What do you think of this implementation? Can this be implemented better?

Instantiating COM objects is a time consuming process and since you are doing it from within a trigger, it slows down the data insertion process. Same is the case with sending emails from triggers. This scenario can be better implemented by logging all the necessary data into a separate table, and have a job which periodically checks this table and does the needful.

23. How to check a trigger is fired or not, while doing Database testing?

It can be verified by querying the common audit log where we can able to see the triggers fired. 

24. What is way of writing test cases for database testing? 1. Understand the functional requirement of the application thoroughly. 

2. Find out the back end tables used, joined used between the tables, cursors used (if any), triggers used(if any), stored procedures used (if any), input parameter used and output parameters used for developing that requirement.

3. After knowing all these things you have to write the test case with different input values for checking all the paths of SP.
Writing of test cases for backend testing is not same as functional testing. We have to use white box testing techniques. We write the test cases based on the specification or use cases. Specification consist what data will store in what column in what tables

24. What are the different stages involved in Database Testing

     I.        Verify the data in the database with respect to frontend transactions

   II.        Verify the constraint (Primary key, Foreign key)

  III.        Verify the performance of the procedures

  IV.        Verify the triggers (execution of triggers)

    V.        Verify the transactions (begin, commit, rollback)

25. What we normally check for in the Database Testing?

     I.        The field size validation

   II.        Check constraints.

  III.        Indexes are done or not (for performance related issues)

  IV.        Stored procedures

    V.        The field size defined in the application is matching with that in the db.

26. What is Database testing?

a.    Data Validity testing

b.    Data Integrity testing

c.    Performance related to data base.

d.    Testing of Procedure, triggers and functions.

For doing data validity testing you should be good in SQL queries.
For data integrity testing you should know about referential integrity and different constraint.
For performance related things you should have idea about the table structure and design.
For testing Procedure triggers and functions you should be able to understand the same.

27. What steps does a tester take in testing Stored Procedures?

1. Go through the requirement, as to why the particular stored procedure is written for.
2. Check whether all the required indexes, joins, updates, deletions are correct comparing with the tables mentions in the Stored Procedure. And also he has to ensure whether the Stored Procedure follows the standard format like comments, updated by, etc.
3. Then check the procedure calling name, calling parameters, and expected responses for different sets of input parameters.
4. Then run the procedure yourself with database client programs like TOAD, or mysql, or Query Analyzer

28. How to test data loading in Data base testing?

You have to do the following things while you are involving in Data Load testing.
1. You have know about source data (table(s), columns, data types and Constraints)
2. You have to know about Target data (table(s), columns, data types and Constraints)
3. You have to check the compatibility of Source and Target.
4. You have to open corresponding DTS package in SQL Enterprise Manager and run the DTS package (If you are using SQL Server).
5. You should compare the column's data of Source and Target.
6. You have to check the number to rows of Source and Target.
7. Then you have to update the data in Source and see the change is reflecting in Target or not.
8. You have to check about junk character and NULLs.

29. Example of Queries:
1. Finding Duplicate Records

SELECT email, COUNT (email) AS NumOccurrences

FROM users GROUP BY email

HAVING (COUNT (email) > 1)

2. Query for retrieving the 2nd highest value

Select Max (Salary) from tblSalary 
Where Salary < (Select Max (Salary) from tblSalary)