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| SQl Interview Qustions For Fresher's |
1. What is the difference between a
"where" clause and a "having" clause?
- "Where" is a kind of restiriction statement. You use where
clause to restrict all the data from DB.Where clause is using before result
retrieving. But Having clause is using after retrieving the data.Having clause
is a kind of filtering command.
2. What is the basic form of a SQL
statement to read data out of a table?
-The basic form to
read data out of table is ‘SELECT * FROM table_name; ‘ An answer: ‘SELECT *
FROM table_name WHERE xyz= ‘whatever’;’ cannot be called basic form because of
WHERE clause.
3. What structure can you implement for
the database to speed up table reads?
- Follow the rules of DB tuning we have
to: 1] properly use indexes ( different types of indexes) 2] properly locate
different DB objects across different tablespaces, files and so on.3] create a
special space (tablespace) to locate some of the data with special datatype (
for example CLOB, LOB and …)
4. What are the tradeoffs with having
indexes?
- 1. Faster selects, slower updates.
2. Extra storage space to store
indexes. Updates are slower because in addition to updating the table you have
to update the index.
5. What is a "join"?
- ‘join’ used to
connect two or more tables logically with or without common field.
6. What is "normalization"?
"Denormalization"? Why do you sometimes want to denormalize?
- Normalizing data means eliminating redundant information from a table
and organizing the data so that future changes to the table are easier.
Denormalization means allowing redundancy in a table. The main benefit of
denormalization is improved performance with simplified data retrieval and
manipulation. This is done by reduction in the number of joins needed for data
processing.
7. What is a "constraint"?
- A constraint allows you to apply simple referential integrity checks
to a table. There are four primary types of constraints that are currently
supported by SQL Server: PRIMARY/UNIQUE - enforces uniqueness of a particular
table column. DEFAULT - specifies a default value for a column in case an
insert operation does not provide one. FOREIGN KEY - validates that every value
in a column exists in a column of another table. CHECK - checks that every
value stored in a column is in some specified list. Each type of constraint
performs a specific type of action. Default is not a constraint. NOT NULL is
one more constraint which does not allow values in the specific column to be
null. And also it the only constraint which is not a table level constraint.
8. What types of index data structures can
you have?
- An index helps to faster search values in tables. The three most
commonly used index-types are: - B-Tree: builds a tree of possible values with
a list of row IDs that have the leaf value. Needs a lot of space and is the
default index type for most databases. - Bitmap: string of bits for each
possible value of the column. Each bit string has one bit for each row. Needs
only few space and is very fast.(however, domain of value cannot be large, e.g.
SEX(m,f); degree(BS,MS,PHD) - Hash: A hashing algorithm is used to assign a set
of characters to represent a text string such as a composite of keys or partial
keys, and compresses the underlying data. Takes longer to build and is
supported by relatively few databases.
9. What is a "primary key"?
- A PRIMARY INDEX or PRIMARY KEY is something which comes mainly from database theory. From its behavior is almost the same as an UNIQUE INDEX, i.e.
there may only be one of each value in this column. If you call such an INDEX
PRIMARY instead of UNIQUE, you say something about
your table design, which I am not able to explain in few words. Primary Key is
a type of a constraint enforcing uniqueness and data integrity for each row of
a table. All columns participating in a primary key constraint must possess the
NOT NULL property.
10.What is a "functional
dependency"? How does it relate to database table design?
- Functional
dependency relates to how one object depends upon the other in the database.
for example, procedure/function sp2 may be called by procedure sp1. Then we say
that sp1 has functional dependency on sp2.
11.What is a "trigger"?
- Triggers are stored
procedures created in order to enforce integrity rules in a database. A trigger
is executed every time a data-modification operation occurs (i.e., insert,
update or delete). Triggers are executed automatically on occurance of one of
the data-modification operations. A trigger is a database object directly
associated with a particular table. It fires whenever a specific statement/type
of statement is issued against that table. The types of statements are
insert,update,delete and query statements. Basically, trigger is a set of SQL
statements A trigger is a solution to the restrictions of a constraint. For
instance: 1.A database column cannot carry PSEUDO columns as criteria where a
trigger can. 2. A database constraint cannot refer old and new values for a row
where a trigger can.
12.Why can a "group by" or
"order by" clause be expensive to process?
- Processing of
"group by" or "order by" clause often requires creation of
Temporary tables to process the results of the query. Which depending of the
result set can be very expensive.
13.What is "index covering" of a
query?
- Index covering means that "Data can be found only using
indexes, without touching the tables"
14.What is a SQL view?
- An output of a
query can be stored as a view. View acts like small table which meets our
criterion. View is a precomplied SQL query which is used to select data from
one or more tables. A view is like a table but it doesn’t physically take any
space. View is a good way to present data in a particular format if you use
that query quite often. View can also be used to restrict users from accessing
the tables directly.
Thank you Readers.If you have more Question or suggestions for Sql Interview Qustions for freshers then please comment on below box.
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