What is normalization? Explain different levels of normalization?

Check out the article Q100139 from Microsoft knowledge base and of
course, there's much more information available in the net. It'll be a
good idea to get a hold of any RDBMS fundamentals text book,
especially the one by C. J. Date. Most of the times, it will be okay
if you can explain till third normal form.

What is denormalization 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.

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.

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

Both 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 nonclustered index by
default. Another major difference is that, primary key doesn't allow
NULLs, but unique key allows one NULL only.



What are user defined datatypes and when you should go for them?

User defined datatypes let you extend the base SQL Server datatypes 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 datatype called
Flight_num_type of varchar(8) and use it across all your tables.

What is bit datatype and what's the information that can be stored
inside a bit column?

Bit datatype is used to store boolean information like 1 or 0 (true or
false). Untill SQL Server 6.5 bit datatype could hold either a 1 or 0
and there was no support for NULL. But from SQL Server 7.0 onwards,
bit datatype can represent a third state, which is NULL.

Define candidate key, alternate key, composite key.

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.

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.

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, Durability. These are the properties of a transaction.

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.

CREATE INDEX myIndex ON myTable(myColumn)

What type of Index will get created after executing the above statement?

Non-clustered index. Important thing to note: By default a clustered
index gets created on the primary key, unless specified otherwise.

What's the maximum size of a row?

8060 bytes.

What is lock escalation?

Lock escalation is the process of converting a lot of low level locks
(like row locks, 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 it's dynamically managed by SQL Server.

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 deallocation of the data pages of the
table, which makes it faster. Of course, TRUNCATE TABLE can be rolled
back.

Explain the storage models of OLAP

Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more
infomation.

What are constraints? Explain different types of constraints.

Constraints enable the RDBMS enforce the integrity of the database
automatically, without needing you to create triggers, rule or defaults.

Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY

What is RAID

RAID stands for Redundant Array of Inexpensive Disks, used to provide
fault tolerance to database servers. There are six RAID levels 0
through 5 offering different levels of performance, fault tolerance.

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.

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.

What are the different ways of moving data/databases between servers
and 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, dettaching and attaching databases, replication, DTS,
BCP, logshipping, INSERT...SELECT, SELECT...INTO, creating INSERT
scripts to generate data.

What is database replicaion? What are the different types of
replication you can set up in SQL Server?

Replication is the process of copying/moving data between databases on
the same or different servers. SQL Server supports the following types
of replication scenarios:

* Snapshot replication
* Transactional replication (with immediate updating subscribers,
with queued updating subscribers)
* Merge replication


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

Cursors allow row-by-row prcessing of the resultsets.

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 rowundtrip, however large the resultset is. Cursors are
also costly because they require more resources and temporary storage
(results in more IO operations). Furthere, 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.

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.

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 which contains rows for normal employees as well as
managers. So, 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'



SELECT t1.empname [Employee], t2.empname [Manager]
FROM emp t1, emp t2
WHERE t1.mgrid = t2.empid



Here's an advanced query using a LEFT OUTER JOIN that even returns the
employees without managers (super bosses)



SELECT t1.empname [Employee], COALESCE(t2.empname, 'No manager') [Manager]
FROM emp t1
LEFT OUTER JOIN
emp t2
ON
t1.mgrid = t2.empid

sql

Explain an outer join?

An outer join includes rows from tables when there are no matching values in the tables.

What is a subselect? Is it different from a nested select?
A subselect is a select which works in conjunction with another select. A nested select is a kind of subselect where the inner select passes to the where criteria for the outer select.

What is the difference between group by and order by?
Group by controls the presentation of the rows, order by controls the presentation of the columns for the results of the SELECT statement.

What keyword does an SQL SELECT statement use for a string search?
The LIKE keyword allows for string searches. The % sign is used as a wildcard.

What are some SQL aggregates and other built-in functions?
The common aggregate, built-in functions are AVG, SUM, MIN, MAX, COUNT and DISTINCT.

How is the SUBSTR keyword used in SQL?
SUBSTR is used for string manipulation with column name, first position and string length used as arguments. E.g. SUBSTR (NAME, 1 3) refers to the first three characters in the column NAME.


Explain the EXPLAIN statement?
The explain statement provides information about the optimizer's choice of access path of the SQL.

What is referential integrity?
Referential integrity refers to the consistency that must be maintained between primary and foreign keys, i.e. every foreign key value must have a corresponding primary key value.

What is a NULL value? What are the pros and cons of using NULLS?
A NULL value takes up one byte of storage and indicates that a value is not present as opposed to a space or zero value. It's the DB2 equivalent of TBD on an organizational chart and often correctly portrays a business situation.Unfortunately, it requires extra coding for an application program to handle this situation.

What is a synonym? How is it used?
A synonym is used to reference a table or view by another name. The other name can then be written in the application code pointing to test tables in the development stage and to production entities when the code is migrated.
The synonym is linked to the AUTHID that created it.

What is an alias and how does it differ from a synonym?
An alias is an alternative to a synonym, designed for a distributed environment to avoid having to use the location qualifier of a table or view. The alias is not dropped when the table is dropped.



When can an insert of a new primary key value threaten referential integrity?
Never. New primary key values are not a problem. However, the values of foreign key inserts must have corresponding primary key values in their related tables. And updates of primary key values may require changes in foreign key values to maintain referential integrity.



What is the difference between static and dynamic SQL?
Static SQL is hard-coded in a program when the programmer knows the statements to be executed. For dynamic SQL the program must dynamically allocate memory to receive the query results.


Compare a subselect to a join?
Any subselect can be rewritten as a join, but not vice versa. Joins are usually more efficient as join rows can be returned immediately, subselects require a temporary work area for inner selects results while processing the outer select.

What is the difference between IN subselects and EXISTS subselect?
If there is an index on the attributes tested an IN is more efficient since DB2 uses the index for the IN. (IN for index is the mnemonic).

What is a Cartesian product?
A Cartesian product results from a faulty query. It is a row in the results for every combination in the join tables.


What is a tuple?
A tuple is an instance of data within a relational database.

What is the difference between static and dynamic SQL?
Static SQL is compiled and optimized prior to its execution; dynamic is compiled and optimized during execution.

What are common SQL abend codes? (e.g. : 0,100 etc.,)

-818 time stamp mismatch

-180 wrong data moved into date field



What is meant by dynamic SQL?

Dynamic SQL are SQL statements that are prepared and executed within a program while the program is executing.

The SQL source is contained in host variables rather than being hard coded into the program. The SQL statement may

change from execution to execution.



What is meant by embedded SQL?

They are SQL statements that are embedded with in application program and are prepared during the program

preparation process before the program is executed. After it is prepared, the statement itself does not change(although

values of host variables specified within the statement might change).