SQL Interview Questions and Answers:
Part-I
1) What is SQL?
SQL stands for Structured
Query Language , and it is used to communicate with the Database. This is a
standard language used to perform tasks such as retrieval, updating, insertion
and deletion of data from a database..
2) What is a
Database?
Database is nothing
but an organized form of data for easy access, storing, retrieval and managing
of data. This is also known as structured form of data which can be accessed in
many ways.
Example: School Management Database,
Bank Management Database.
3) What are tables and
Fields?
A table is a set of
data that are organized in a model with Columns and Rows. Columns can be
categorized as vertical, and Rows are horizontal. A table has specified number of
column called fields but can have any number of rows which is called record.
Example:
Table:
Employee.
Field:
Emp ID, Emp Name, Date of Birth.
Data:
201456, David, 11/15/1960.
4) What is a primary key?
A primary key is a
combination of fields which uniquely specify a row. This is a special kind of
unique key, and it has implicit NOT NULL constraint. It means, Primary key
values cannot be NULL.
5) What is a unique key?
A Unique key constraint uniquely
identified each record in the database. This provides uniqueness for the column
or set of columns.
A Primary key
constraint has automatic unique constraint defined on it. But not, in the case
of Unique Key.
There can be many
unique constraint defined per table, but only one Primary key constraint defined
per table.
6) What is a foreign key?
A foreign key is one
table which can be related to the primary key of another table. Relationship
needs to be created between two tables by referencing foreign key with the
primary key of another table.
7) What is a join?
This is a keyword used to query data
from more tables based on the relationship between the fields of the tables.
Keys play a major role when JOINs are used.
8) What are the types
of join and explain each?
There are various
types of join which can be used to retrieve data and it depends on the
relationship between tables.
Inner join: Inner join return
rows when there is at least one match of rows between the tables.
Right Join: Right
join return rows which are common between the tables and all rows of Right hand
side table. Simply, it returns all the rows from the right hand side table even
though there are no matches in the left hand side table.
Left Join: Left
join return rows which are common between the tables and all rows of Left hand
side table. Simply, it returns all the rows from Left hand side table even
though there are no matches in the Right hand side table.
Full Join: Full
join return rows when there are matching rows in any one of the tables. This
means, it returns all the rows from the left hand side table and all the rows
from the right hand side table.
9) What is normalization?
Normalization is the
process of minimizing redundancy and dependency by organizing fields and table
of a database. The main aim of Normalization is to add, delete or modify field
that can be made in a single table.
10) What is Denormalization?
Denormalization is a
technique used to access the data from higher to lower normal forms of
database. It is also process of introducing redundancy into a table by
incorporating data from the related tables.
11) What is a View?
A view is a virtual table which
consists of a subset of data contained in a table. Views are not virtually
present, and it takes less space to store. View can have data of one or more
tables combined, and it is depending on the relationship.
12) What is an Index?
An index is performance tuning method
of allowing faster retrieval of records from the table. An index creates an
entry for each value and it will be faster to retrieve data.
13) What are all the different
types of indexes?
There are three types of indexes -.
Unique Index: This
indexing does not allow the field to have duplicate values if the column is
unique indexed. Unique index can be applied automatically when primary key is
defined.
Clustered Index: This
type of index reorders the physical order of the table and search based on the
key values. Each table can have only one clustered index.
Non Clustered Index: Non Clustered Index
does not alter the physical order of the table and maintains logical order of
data. Each table can have 999 non clustered indexes.
14) What is a Cursor?
A database Cursor is
a control which enables traversal over the rows or records in the table. This
can be viewed as a pointer to one row in a set of rows. Cursor is very much
useful for traversing such as retrieval, addition and removal of database
records.
15) What is a
relationship and what are they?
Database Relationship
is defined as the connection between the tables in a database. There are
various data basing relationships, and they are as follows:.
- One to One Relationship.
- One to Many Relationships.
- Many to One Relationship.
- Self-Referencing Relationship.
16) What is a query?
A DB query is a code
written in order to get the information back from the database. Query can be
designed in such a way that it matched with our expectation of the result set.
Simply, a question to the Database.
17) What is subquery?
A sub query is a
query within another query. The outer query is called as main query, and inner
query is called subquery. SubQuery is always executed first, and the result of
subquery is passed on to the main query.
18) What are the types of
subquery?
There are two types
of subquery – Correlated and Non-Correlated.
A Correlated subquery cannot be considered
as independent query, but it can refer the column in a table listed in the FROM
the list of the main query.
A Non-Correlated sub query can be
considered as independent query and the output of subquery are substituted in
the main query.
19) What is a stored procedure?
Stored Procedure is a
function consists of many SQL statement to access the database system. Several
SQL statements are consolidated into a stored procedure and execute them
whenever and wherever required.
20) What is a trigger?
A DB trigger is a code or programs that
automatically execute with response to some event on a table or view in a
database. Mainly, trigger helps to maintain the integrity of the database.
Example: When a new student is added to
the student database, new records should be created in the related tables like
Exam, Score and Attendance tables.
21) What is the difference
between DELETE and TRUNCATE commands?
DELETE command is used to remove
rows from the table, and WHERE clause can be used for conditional set of
parameters. Commit and Rollback can be performed after delete statement.
TRUNCATE removes all rows
from the table. Truncate operation cannot be rolled back.
22) What are local and global variables and their
differences?
Local variables are the variables
which can be used or exist inside the function. They are not known to the other
functions and those variables cannot be referred or used. Variables can be
created whenever that function is called.
Global variables are the variables
which can be used or exist throughout the program. Same variable declared in
global cannot be used in functions. Global variables cannot be created whenever
that function is called.
23) What is a constraint?
Constraint can be
used to specify the limit on the data type of table. Constraint can be
specified while creating or altering the table statement. Sample of constraint
are.
- NOT NULL.
- CHECK.
- DEFAULT.
- UNIQUE.
- PRIMARY KEY.
- FOREIGN KEY.
24) What is data Integrity?
Data Integrity defines the accuracy and
consistency of data stored in a database. It can also define integrity
constraints to enforce business rules on the data when it is entered into the
application or database.
25) What is Auto Increment?
Auto increment keyword allows the user
to create a unique number to be generated when a new record is inserted into
the table. AUTO INCREMENT keyword can be used in Oracle and IDENTITY keyword
can be used in SQL SERVER. Mostly this keyword can be used whenever PRIMARY KEY
is used.
26) What is the difference between Cluster and
Non-Cluster Index?
A
Clustered index
is used for easy retrieval of data from the database by altering the way that
the records are stored. Database sorts out rows by the column which is set to
be clustered index.
A
nonclustered index does not alter the way it was stored
but creates a complete separate object within the table. It point back to the
original table rows after searching.
27) What is Self-Join?
Self-join is set to
be query used to compare to itself. This is used to compare values in a column
with other values in the same column in the same table. ALIAS ES can be used
for the same table comparison.
28) What is Cross-Join?
Cross join defines as
Cartesian product where number of rows in the first table multiplied by number
of rows in the second table. If suppose, WHERE clause is used in cross join
then the query will work like an INNER JOIN.
29) What is user defined functions?
User defined
functions are the functions written to use that logic whenever required. It is
not necessary to write the same logic several times. Instead, function can be
called or executed whenever needed.
30) What are all
types of user defined functions?
Three types of user
defined functions are.
·
Scalar Functions.
·
Inline Table valued functions.
·
Multi statement valued functions.
Scalar
returns unit, variant defined the return clause. Other two types return table
as a return.
31) Advantages and
Disadvantages of Stored Procedure?
Stored procedure can
be used as a modular programming – means create once, store and call for
several times whenever required. This supports faster execution instead of
executing multiple queries. This reduces network traffic and provides better
security to the data.
Disadvantage is that it can be executed
only in the Database and utilizes more memory in the database server.
32) What is CLAUSE?
SQL clause is defined
to limit the result set by providing condition to the query. This usually
filters some rows from the whole set of records.
Example
– Query that has WHERE condition. Query that
has HAVING condition.
33) What is recursive
stored procedure?
A stored procedure
which calls by itself until it reaches some boundary condition. This recursive
function or procedure helps programmers to use the same set of code any number
of times.
34) What is Union, minus and Interact commands?
UNION operator is used to combine the
results of two tables, and it eliminates duplicate rows from the tables.
MINUS operator is used to return rows from
the first query but not from the second query. Matching records of first and
second query and other rows from the first query will be displayed as a result
set.
INTERSECT operator is used to
return rows returned by both the queries.
35) What is an ALIAS command?
ALIAS name can be given to a table or
column. This alias name can be referred in WHERE clause to identify the table
or column.
Example:
Select st.StudentID, Ex.Result from
student st, Exam as Ex where st.studentID = Ex. StudentID
Here, st refers to alias name for
student table and Ex refers to alias name for exam table.
36) What is the difference
between TRUNCATE and DROP statements?
TRUNCATE
removes all the rows from the table, and it cannot be rolled
back.
DROP
command removes a table from the database and operation cannot be rolled back.
37) What are aggregate and scalar
functions?
Aggregate functions
are used to evaluate mathematical calculation and return single values. This
can be calculated from the columns in a table. Scalar functions return a single
value based on the input value.
Example -
Aggregate
– max (), count – Calculated with respect to numeric.
Scalar
– UCASE (), NOW () – Calculated with respect to strings.
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