SQL SERVER – DOT NET Chapter Wise Interview Questions

SQL SERVER – DOT NET Chapter Wise Interview Questions

Question 1:
What is normalization?
Note: A regular .NET programmer working on projects often stumbles in this question, which is but obvious. Bad part is sometimes interviewer can take this as a very basic question to be answered and it can be a turning point for the interview. So let’s cram it.

It is set of rules that have been established to aid in the design of tables that are meant to be connected through relationships. This set of rules is known as Normalization.
Benefits of normalizing your database will include:

  • Avoiding repetitive entries
  • Reducing required storage space
  • Preventing the need to restructure existing tables to accommodate new data.
  • Increased speed and flexibility of queries, sorts, and summaries.

Note: During interview candidates are expected to answer maximum of three normal forms.

Following are the three normal forms:
First Normal Form
For a table to be in first normal form, data must be broken up into the smallest units possible (See Figure 10.1). In addition to breaking data up into the smallest meaningful values, tables in first normal form should not contain repetitions groups of fields.
In the example as shown in Figure 10.1 City1 and City2 are repeating. In order that this table to be in First normal form you have to modify the table structure as follows. Also note that the Customer Name is now broken down to first name and last name (First normal form data should be broken down to smallest unit) (See Table 10.2).
Second Normal form
The second normal form states that each field in a multiple field primary key table must be directly related to the entire primary key (See Figure 0.3). Or in other words, each non-key field should be a fact about all the fields in the primary key.
In the above table of customer, City is not linked to any primary field (See Figure 10.4).
That takes our database to a second normal form.
Third Normal form
A non-key field should not depend on other non-key field. The field “Total” is dependent on “Unit price” and “qty” (See Figure 10.5).
So now the “Total” field is removed and is multiplication of Unit price * Qty.

Note: Fourth and Fifth Normal forms are left as a homework to users.

Question 2:
What is denormaiization?
Denormalization is the process of putting one fact in numerous places (its vice-versa of normalization). Only one valid reason exists for denormalizing a relational design – to enhance performance or if we are doing data warehousing and data mining. The sacrifice to performance is that you increase redundancy in database.

Question 3:
What are the different types of joins? What is the difference between them?
INNER JOIN: Inner join shows matches only when they exist in both tables. Example, in the below SQL there are two tables Customers and Orders and the inner join in made on Customers Customerid and Orders Customerid. So this SQL will only give you result with customers who have orders. If the customer does not have order, it will not display that record.

LEFT OUTER JOIN: Left join will display all records in left table of the SQL statement. In SQL below customers with or without orders will be displayed. Order data for customers without orders appears as NULL values. For example, you want to determine the amount ordered by each customer and you need to see who has not ordered anything as well. You can also see the LEFT OUTER JOIN as a mirror image of the RIGHT OUTER JOIN (Is covered in the next section) if you switch the side of each table.

RIGHT OUTER JOIN: Right join will display all records in right table of the SQL statement. In SQL below all orders with or without matching customer records will be displayed. Customer data for orders without customers appears as NULL values. For example, you want to determine if there are any orders in the data with undefined CustomerlD values (say, after a conversion or something like it). You can also see the RIGHT OUTER JOIN as a mirror image of the LEFT OUTER JOIN if you switch the side of each table.

Question 4:
What is a candidate key?
A table may have more than one combination of columns that could uniquely identify the rows in a table; each combination is a candidate key. During database design you can pick up one of the candidate keys to be the primary key. For example, in the Supplier table supplierd and suppliername can be candidate key but you will only pick up supplierid as the primary key.

Question 5:
What are indexes and what is the difference between clustered and non-clustered?
Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quickly.
There are clustered and non-clustered indexes. A clustered index is a special type of index that reorders the way in which records in the table are physically stored. Therefore, table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.
A non-clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.

Question 6:
How can you increase SQL performance?
Following are tips which will increase your SQL performance:

  • Every index increases the time takes to perform INSERTS, UPDATES, and DELETES, so the number of indexes should not be too much. Try to use maximum 4-5 indexes on one table, not more. If you have read-only table, then the number of indexes may be increased.
  • Keep your indexes as narrow as possible. This reduces the size of the index and reduces the number of reads required to read the index.
  • Try to create indexes on columns that have integer values rather than character values.
  • If you create a composite (multi-column) index, the orders of the columns in the key are very important. Try to order the columns in the key as to enhance selectivity, with the most selective columns to the leftmost of the key.
  • If you want to join several tables, try to create surrogate integer keys for this purpose and create indexes on their columns.
  • Create surrogate integer primary key (identity for example) if your table will not have many insert operations.
  • Clustered indexes are more preferable than nonclustered, if you need to select by a range of values or you need to sort results set with GROUP BY or ORDER BY.
  • If your application will be performing the same query over and over on the same table, consider creating a covering index on the table.
  • You can use the SQL Server Profiler Create Trace Wizard with “Identify Scans of Large Tables” trace to determine which tables in your database may need indexes. This trace will show which tables are being scanned by queries instead of using an index.

Question 7:
What is DTS?
DTS (Data Transformation Services) is used to import data and while importing it helps us to transform and modify data. The name itself is self explanatory DTS.

Question 8:
What is fill factor?
The ‘fill factor’ option specifies how full SQL Server will make each index page. When there is no free space to insert new row on the index page, SQL Server will create new index page and transfer some rows from the previous page to the new one. This operation is called page splits. You can reduce the number of page splits by setting the appropriate fill factor option to reserve free space on each index page. The fill factor is a value from 1 through 100 that specifies the percentage of the index page to be left empty. The default value for fill factor is 0. It is treated similarly to a fill factor value of 100, the difference in that SQL Server leaves some space within the upper level of the index tree for FILLFACTOR = 0. The fill factor percentage is used only at the time when the index is created. If the table contains read-only data (or data that very rarely changed), you can set the ‘fill factor’ option to 100. When the table’s data is modified very often, you can decrease the fill factor to 70% or whatever you think is best.

Question 9:
What is RAID and how does it work?
Redundant Array of Independent Disks (RAID) is a term used to describe the technique of improving data availability through the use of arrays of disks and various data-striping methodologies. Disk arrays are groups of disk drives that work together to achieve higher data transfer and I/O rates than those provided by single large drives. An array is a set of multiple disk drives plus a specialized controller (an array controller) that keeps track of how data is distributed across the drives. Data for a particular file is written in segments to the different drives in the array rather than being written to a single drive.
For speed and reliability, it is better to have more disks. When these disks are arranged in certain patterns and are use a specific controller, they are called a (RAID) set. There are several numbers associated with RAID, but the most common are 1,5 and 10.
RAID 1 works by duplicating the same writes on two hard drives. Let us assume you have two 20 GB drives. In RAID 1, data is written at the same time to both the drives. RAID 1 is optimized for fast writes.
RAID 5 works by writing parts of data across all drives in the set (it requires at least three drives). If a drive failed, the entire set would be worthless. To combat this problem, one of the drives stores a “parity” bit. Think of a math problem, such as 3 + 7 = 10. You can think of the drives as storing one of the numbers, and the 10 is the parity part. By removing any one of the numbers, you can get it back by referring to the other two, like this: 3 + X = 10. Of course, losing more than one could be evil. RAID 5 is optimized for reads.
RAID 10 is a bit of a combination of both types. It does not store a parity bit, so it is faster, but it duplicates the data on two drives to be safe. You need at least four drives for RAID 10. This type of RAID is probably the best compromise for a database server.

Note: It is difficult to cover complete aspect of RAID in this book. It’s better to take some decent SQL Server book for in detail knowledge, but yes from interview aspect you can probably escape with this answer.

Question 10:
What is the difference between DELETE and TRUNCATE TABLE?
Following are difference between them:

  • DELETE table can have criteria while TRUNCATE cannot.
  • TRUNCATE table does not invoke trigger.
  • DELETE TABLE syntax logs the deletes thus make the delete operation slow. TRUNCATE table does not log any information but it logs information about deallocation of data page of the table so TRUNCATE table is faster as compared to delete table.

Note: Thanks to all the readers for pointing out my mistake for the above question in my first edition. I had mentioned that TRUNCATE table cannot be rolled back while delete can be.

Question 11:
If locking is not implemented, what issues can occur?
Following are the problems that occur if you do not implement locking properly in SQL Server.

  • Lost Updates: Lost updates occur if you let two transactions modify the same data at the same time, and the transaction that completes first is lost. You need to watch out for lost updates with the READ UNCOMMITTED isolation level. This isolation level disregards any type of locks, so two simultaneous data modifications are not aware of each other. Suppose that a customer has due of 2000$ to be paid. He/she pays 1000$ and again buys a product of 500$. Lets say that these two transactions are now been entered from two different counters of the company. Now both the counter user starts making entry at the same time 10: 00 AM. Actually speaking at 10: 01 AM the customer should have 2000$- 1000$+500 = 1500$ pending to be paid. But as said in lost updates the first transaction is not considered and the second transaction overrides it. So the final pending is 2000$+50Q$ = 2500$………… I hope the company does not loose the customer.
  • Non-Repeatable Read: Non-repeatable reads occur if a transaction is able to read the same row multiple times and gets a different value each time. Again, this problem is most likely to occur with the READ UNCOMMITTED isolation level. Because you let two transactions modify data at the same time, you can get some unexpected results. For instance, a customer wants to book flight, so the travel agent checks for the flights availability. Travel agent finds a seat and goes ahead to book the seat. While the travel agent is booking the seat, some other travel agent books the seat. When this travel agent goes to update the record, he gets error saying that “Seat is already booked”. In short, the travel agent gets different status at different times for the seat.
  • Dirty Reads: Dirty reads are a special case of non-repeatable read. This happens if you run a report while transactions are modifying the data that you are reporting on. For example, there is a customer invoice report, which runs on 1: 00 AM (ante meridien) in afternoon and after that all invoices are sent to the respective customer for payments. Let us say one of the customer has 1000$ to be paid. Customer pays 1000$ at 1: 00 AM and at the same time report is run. Actually, customer has no money pending but is still issued an invoice.
  • Phantom Reads: Phantom reads occur due to a transaction being able to read a row on the first read, but not being able to modify the same row due to another transaction deleting rows from the same table. Lets say you edit a record in the mean time somebody comes and deletes the record, you then go for updating the record which does not exist…Panicked.

Interestingly, the phantom reads can occur even with the default isolation level supported by SQL Server: READ COMMITTED. The only isolation level that does not allow phantoms is SERIALIZABLE, which ensures that each transaction is completely isolated from others. In other words, no one can acquire any type of locks on the affected row while it is being modified.

Question 12:
What are different transaction levels in SQL Server?
Transaction Isolation level decides how is one process isolated from other process. Using transaction levels, you can implement locking in SQL Server.
There are four transaction levels in SQL Server:

  • READ COMMITTED: The shared lock is held for the duration of the transaction, meaning that no other transactions can change the data at the same time. Other transactions can insert and modify data in the same table, however, as long as it is not locked by the first transaction.
  • READ UNCOMMITTED: No shared locks and no exclusive locks are honored. This is the least restrictive isolation level resulting in the best concurrency but the least data integrity.
  • REPEATABLE READ: This setting disallows dirty and non-repeatable reads. However, even though the locks are held on read data, new rows can still be inserted in the table, and will subsequently be interpreted by the transaction.
  • SERIALIZABLE: This is the most restrictive setting holding shared locks on the range of data. This setting does not allow the insertion of new rows in the range that is locked; therefore, no phantoms are allowed.

Following is the syntax for setting transaction level in SQL Server.

Question 13:
What are the different locks in SQL Server?
Depending on the transaction level, six types of lock can be acquired on data:

  • Intent: The intent lock shows the future intention of SQL Server’s lock manager to acquire locks on a specific unit of data for a particular transaction. SQL Server uses intent locks to queue exclusive locks, thereby ensuring that these locks will be placed on the data elements in the order the transactions were initiated. Intent locks come in three types: Intent Shared (IS), Intent Exclusive (IX), and Shared with Intent Exclusive (SIX).
    IS locks indicate that the transaction will read some (but not all) resources in the table or page by placing shared locks.
    IX locks indicate that the transaction will modify some (but not all) resources in the table or page by placing exclusive locks.
    SIX locks indicates that the transaction will read all resources, and modify some (but not all) of them. This will be accomplished by placing the shared locks on the resources read and exclusive locks on the rows modified. Only one SIX lock is allowed per resource at one time; therefore, SIX locks prevent other connections from modifying any data in the resource (page or table), although they do allow reading the data in the same resource.
  • Shared: Shared(S) locks allow transactions to read data with SELECT statements. Other connections are allowed to read the data at the same time; however, no transactions are allowed to modify data until the shared locks are released.
  • Update: Update(U) locks are acquired just prior to modifying the data. If a transaction modifies a row, then the update lock is escalated to an exclusive lock; otherwise, it is converted to a shared lock. Only one transaction can acquire update locks to a resource at one time Using update locks prevents multiple connections from having a shared lock that want to eventually modify a resource using an exclusive lock. Shared locks are compatible with other shared locks, but are not compatible with Update locks.
  • Exclusive: Exclusive(X) locks completely lock the resource from any type of access including reads. They are issued when data is being modified through INSERT, UPDATE and DELETE statements.
  • Schema: Schema modification (Sch-M) locks are acquired when data definition language statements, such as CREATE TABLE, CREATE INDEX, ALTER TABLE, and so on are being executed. Schema stability locks (Sch-S) are acquired when store procedures are being compiled.
  • Bulk Update: Bulk Update (BU) locks are used when performing a bulk-copy of data into a table with TABLOCK hint. These locks improve performance while bulk copying data into a table; however, they reduce concurrency by effectively disabling any other connections to read or modify data in the table.

Question 14:
Can we suggest locking hints to SQL Server?
We can give locking hints that helps you over ride default decision made by SQL Server. For instance, you can specify the ROWLOCK hint with your UPDATE statement to convince SQL Server to lock each row affected by that data modification. Whether it is prudent to do so is another story; what will happen if your UPDATE affects 95% of rows in the affected table? If the table contains 1000 rows, then SQL Server will have to acquire 950 individual locks, which is likely to cost a lot more in terms of memory than acquiring a single table lock. So think twice before you bombard your code with ROWLOCKS.

Question 15:
What is LOCK escalation?
Lock escalation is the process of converting of low-level locks (like rowlocks, page locks) into higher- level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards SQL Server dynamically manages it.

Question 16:
What are the different ways of moving data between databases in SQL Server?
There are lots of options available; you have to choose your option depending upon your requirements. Some of the options you have are BACKUP/RESTORE, detaching and attaching databases, replication, DTS (Data Transformation Services), BCP (Bulk Copy Program), logshipping, INSERT…SELECT, SELECT…INTO, creating INSERT scripts to generate data.

Question 17:
What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
You can use Having Clause with the GROUP BY function in a query and WHERE Clause is applied to each row before, they are part of the GROUP BY function in a query.

Question 18:
What is the difference between UNION and UNION ALL SQL syntax?
UNION SQL syntax is used to select information from two tables. But it selects only distinct records from both the table, while UNION ALL selects all records from both the tables.

Note: Selected records should have same datatype or else the syntax will not work.

Question 19:
How can you raise custom errors from stored procedure?
The RAISERROR statement is used to produce an ad hoc error message or to retrieve a custom message that is stored in the sysmessages table. You can use this statement with the error handling code presented in the previous section to implement custom error messages in your applications. The syntax of the statement is shown here.

A description of the components of the statement follows.

msg_id: The ID for an error message, which is stored in the error column in sysmessages.
msg_str: A custom message that is not contained in sysmessages.
Severity: The severity level associated with the error. The valid values are 0-25. Severity levels 0-18 can be used by any user, but 19-25 are only available to members of the fixed-server role sysadmin. When levels 19-25 are used, the WITH LOG option is required.
State A value that indicates the invocation state of the error. The valid values are 0-127. This value is not used by SQL Server.
One or more variables that are used to customize the message. For example, you could pass the current process ID (@@spid) so it could be displayed in the message.
WITH option, . . .
The three values that can be used with this optional argument are described here.
LOG: Forces the error to log in the SQL Server error log and the NT application log.
NOWAIT: Sends the message immediately to the client.
SETERROR: Sets @@error to the unique ID for the message or 50, 000.
The number of options available for the statement makes it seem complicated, but it is actually easy to use. The following shows how to create an ad hoc message with a severity of 10 and a state of 1:
RAISERROR (‘An error occurred updating the Nonfatal table’, 10,1)
An error occurred updating the non-fatal table.
The statement does not have to be used in conjunction with any other code, but for our purposes, it will be used with the error handling code presented earlier. The following alters the ps_NonFatal_iNSERT procedure to use raiserror:

USE tempdb

When an error-producing call is made to the procedure, the custom message is passed to the client.

Question 20:
What is ACID fundamental?
A transaction is a sequence of operations performed as a single logical unit of work. A logical unit of work must exhibit four properties, called the ACID (Atomicity, Consistency, Isolation, and Durability) properties, to qualify as a transaction:

  • Atomicity: A transaction must be an atomic unit of work; either all of its data modifications are performed or none of them is performed.
  • Consistency: When completed, a transaction must leave all data in a consistent state. In a relational database, all rules must be applied to the transaction’s modifications to maintain all data integrity.
  • Isolation: Modifications made by concurrent transactions must be isolated from the modifications made by any other concurrent transactions. A transaction either see data in the state it was before another concurrent transaction modified it, or it sees the data after the second transaction ha’s completed, but it does not see an intermediate state. This is referred to as serializability because it results in the ability to reload the starting data and replay a series of transactions to end up with the data in the same state it was in after the original transactions were performed.
  • Durability: After a transaction has completed, its effects are permanently in place in the system. The modifications persist even in the event of a system failure.

Question 21:
What are the different types of triggers in SQI Server?
There are two types of triggers:

  • INSTEAD OF triggers: INSTEAD OF triggers fire in place of the triggering action. For example, if an INSTEAD OF UPDATE trigger exists on the Sales table and an UPDATE statement is executed against the Salestable, the UPDATE statement will not change a row in the sales table. Instead, the UPDATE statement causes the INSTEAD OF UPDATE trigger to be executed.
  • AFTER triggers: AFTER triggers execute following the SQL action, such as an insert, update, or delete. This is the traditional trigger which existed in SQL Server.

INSTEAD OF triggers are executed automatically before the Primary Key and the Foreign Key constraints are checked, whereas the traditional AFTER triggers is executed after these constraints are checked.
Unlike AFTER triggers, INSTEAD OF triggers can be created on views.

Question 22:
If we have multiple AFTER Triggers, can we specify sequence?
If a table has multiple AFTER triggers, then you can specify which trigger should be executed first and which trigger should be executed last using the stored procedure sp_settriggerorder.

Question 23:
What is SQL injection?
It is a Form of attack on a database-driven Website in which the attacker executes unauthorized SQL commands by taking advantage of insecure code on a system connected to the Internet, bypassing the firewall. SQL injection attacks are used to steal information from a database from which the data would normally not be available and/or to gain access to an organization’s host computers through the computer that is hosting the database.
SQL injection attacks typically are easy to avoid by ensuring that a system has strong input validation.
As name suggest we inject SQL which can be relatively dangerous for the database. For example, this is a simple SQL

Now somebody does not put “x” as the input but puts “x ; drop table members;”. So the actual SQL which will execute is:

Think what will happen to your database.

Question 24:
What is the difference between Stored Procedure and User-Defined Function?
Following are some major differences between a Stored Procedure (SP) and User-Defined Functions (UDF):   ”

  • The main purpose of UDF was to increase reuse in stored procedure. So you can call UDF from a stored procedure and not vice versa.
  • You cannot change any data using UDF while you can do everything with a stored procedure.
  • UDF does not return output parameters while SP’s return output parameters.

Note: SQL Server product is equivalently important from interview point of view. I have dedicated a complete book “SQL Server Interview questions” to crack SQL Server. If you are interested in buying the book mail bpb@bol.net.in /bpb@vsnl.com or call the nearest BPB book stall for my book. For shop phone numbers you can either see the back or front page of the book.


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