SQL interview questions and answers
By admin | July 14, 2008
1. What are two methods of retrieving SQL?
We can retrieve the sql by using SELECT and CURSOR
2. What cursor type do you use to retrieve multiple recordsets?
For multiple records we use explicit cursors
3. 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.
4. 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.
5. 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 …)
6. 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.
7. What is a "join"? - 'join' used to connect two or more tables logically with or without common field.
8. 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.
9. 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.
10. 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.
11. 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.
12. 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.
13. 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.
14. 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.
15. What is "index covering" of a query? - Index covering means that "Data can be found only using indexes, without touching the tables"
16. What types of join algorithms can you have?
17. 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.
8. While doing an ascending order sort on a column having NULL values, where does
the NULLs show up in the result set? At the beginning or at the end?
Ascending order sort - NULLs come last because Oracle treats NULLs are the largest possible values
Descending order sort - NULLs come first
* How to make NULLs come last in descending order sort?
Add NULLS LAST to the order by desc clause
Eg: select col1 from table1 order by col1 desc NULLS LAST
9. Time of execution of an SQL Statement
Eg: set timing on select * from EMP
After execution of each query we get the time take for it.
10. What is the Datatype of NULL in Oracle?
Datatype of NULL is "char(0)" and size is '0'
11. DATE Functions
Gives date
select trunc(sysdate,'MM') from dual;
select last_day(trunc(sysdate)) from dual
Gives day
select to_char(trunc(sysdate,'MM'),'DAY') from dual;
select to_char(last_day(trunc(sysdate)),'DAY') from dual;
12. Maximum length of all Oracle Objects :
Maximum length of all Oracle Objects should not exceed 30 characters.
13. Query to find the nNth Highest salary
select min(sal) from (select distinct sal from emp order by sal desc) where rownum < n;
14. Oracle Functions - Replace versus Trim
SQL> select replace('jose. antony@ yahoo.com',' ', null) as Replace1 from dual;
REPLACE1
--------------------
jose.antony@yahoo.com --Removes all spaces from in-between
SQL> select trim('jose. antony@ yahoo.com') as Trim1 from dual;
TRIM1
----------------------
jose. antony@ yahoo.com --Removes spaces from both sides only.
15. In Oracle can we do any DMLs in a function which is called from a select statement?
No. Oracle Throws Error
ORA-14551: cannot perform a DML operation inside a query
ORA-06512: at "EMP.FNC_VACATION_UPDATE", line 11
16. In Oracle, what happens if there is no statement inside an Anonymous block?
For Eg:
Declare
Begin
End;
Ans: It will throw Error.
Correct:
Declare
Begin
NULL;
End;
17. In Oracle, where does NULLs appear in an ascending order sort? First or Last? And why?
NULLs appear Last in an ascending order sort. Oracle treats NULLs as the largest possible values in the Database. So, they appear last
18. How does NULLs work with Indexes? What are the workarounds?
Indexes wont help in giving proper search results if there are NULL values in the indexed columns. So, it is always advisable to create the NVL() function based indexes for such columns.
19. In an Outer Join, what happens to columns that are not matched?
All unmatched columns (cell values) returns NULLS.
20. What are Index Organized Tables (IOT)?
Index organized tables are indexes which actually hold the data which is being indexed, unlike the
indexes which are stored somewhere else ans have links to actual data.
21. User_Source table contains code for which all Objects?
a. Functions
b. Packages - both spec and body
c. Procedures
d. Triggers
23. Explain ROWID in Oracle.
ROWID is a unique hexadecimal value which Oracle inserts to identify each record being inserted. It is
used for all Full Table scans.
Structure:
OOOOOOFFFBBBBBBRRR
OOOOOO - First six characters is the Object Number which idenities the Data Segment
FFF - Next 3 characters is the Database File number
BBBBBB - Next 6 characters shows the DataBlock number
RRR -Next 3 characters identified the Row within the block
COOL INTERVIEWS
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What is the difference between oracle,sql and sql server ?
- Oracle is based on RDBMS.
- SQL is Structured Query Language.
- SQL Server is another tool for RDBMS provided by MicroSoft.
why you need indexing ? where that is stroed and what you mean by schema object? For what purpose we are using view?
We cant create an Index on Index.. Index is stoed in user_index table.Every object that has been created on Schema is Schema Object like Table,View etc.If we want to share the particular data to various users we have to use the virtual table for the Base table...So tht is a view.
indexing is used for faster search or to retrieve data faster from various table. Schema containing set of tables, basically schema means logical separation of the database. View is crated for faster retrieval of data. It's customized virtual table. we can create a single view of multiple tables. Only the drawback is..view needs to be get refreshed for retrieving updated data.
Difference between Store Procedure and Trigger?
- we can call stored procedure explicitly.
- but trigger is automatically invoked when the action defined in trigger is done.
ex: create trigger after Insert on - this trigger invoked after we insert something on that table.
- Stored procedure can't be inactive but trigger can be Inactive.
- Triggers are used to initiate a particular activity after fulfilling certain condition.It need to define and can be enable and disable according to need.
What is the advantage to use trigger in your PL?
Triggers are fired implicitly on the tables/views on which they are created. There are various advantages of using a trigger. Some of them are:
- Suppose we need to validate a DML statement(insert/Update/Delete) that modifies a table then we can write a trigger on the table that gets fired implicitly whenever DML statement is executed on that table.
- Another reason of using triggers can be for automatic updation of one or more tables whenever a DML/DDL statement is executed for the table on which the trigger is created.
- Triggers can be used to enforce constraints. For eg : Any insert/update/ Delete statements should not be allowed on a particular table after office hours. For enforcing this constraint Triggers should be used.
- Triggers can be used to publish information about database events to subscribers. Database event can be a system event like Database startup or shutdown or it can be a user even like User loggin in or user logoff.
What the difference between UNION and UNIONALL?
Union will remove the duplicate rows from the result set while Union all does'nt.
What is the difference between TRUNCATE and DELETE commands?
Both will result in deleting all the rows in the table .TRUNCATE call cannot be rolled back as it is a DDL command and all memory space for that table is released back to the server. TRUNCATE is much faster.Whereas DELETE call is an DML command and can be rolled back.
Which system table contains information on constraints on all the tables created ?
yes,
USER_CONSTRAINTS,
system table contains information on constraints on all the tables created
Explain normalization?
Normalisation means refining the redundancy and maintains stablisation. there are four types of normalisation :
first normal forms, second normal forms, third normal forms and fourth Normal forms.
How to find out the database name from SQL*PLUS command prompt?
Select * from global_name;
This will give the datbase name which u r currently connected to.....
What is the difference between SQL and SQL Server ?
SQLServer is an RDBMS just like oracle,DB2 from Microsoft
whereas
Structured Query Language (SQL), pronounced "sequel", is a language that provides an interface to relational database systems. It was developed by IBM in the 1970s for use in System R. SQL is a de facto standard, as well as an ISO and ANSI standard. SQL is used to perform various operations on RDBMS.
What is diffrence between Co-related sub query and nested sub query?
Correlated subquery runs once for each row selected by the outer query. It contains a reference to a value from the row selected by the outer query.
Nested subquery runs only once for the entire nesting (outer) query. It does not contain any reference to the outer query row.
For example,
Correlated Subquery:
select e1.empname, e1.basicsal, e1.deptno from emp e1 where e1.basicsal = (select max(basicsal) from emp e2 where e2.deptno = e1.deptno)
Nested Subquery:
select empname, basicsal, deptno from emp where (deptno, basicsal) in (select deptno, max(basicsal) from emp group by deptno)
WHAT OPERATOR PERFORMS PATTERN MATCHING?
Pattern matching operator is LIKE and it has to used with two attributes
1. % and
2. _ ( underscore )
% means matches zero or more characters and under score means mathing exactly one character
1)What is difference between Oracle and MS Access?
2) What are disadvantages in Oracle and MS Access?
3) What are feratures&advantages in Oracle and MS Access?
Oracle's features for distributed transactions, materialized views and replication are not available with MS Access. These features enable Oracle to efficiently store data for multinational companies across the globe. Also these features increase scalability of applications based on Oracle.
What is database?
A database is a collection of data that is organized so that itscontents can easily be accessed, managed and updated. open this url : http://www.webopedia.com/TERM/d/database.html
What is cluster.cluster index and non cluster index ?
Clustered Index:- A Clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table may have only one clustered index.Non-Clustered Index:- 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 in the disk. The leaf nodes of a non-clustered index does not consists of the data pages. instead the leaf node contains index rows.
How can i hide a particular table name of our schema?
you can hide the table name by creating synonyms.
e.g) you can create a synonym y for table x
create synonym y for x;
What is difference between DBMS and RDBMS?
The main difference of DBMS & RDBMS is RDBMS have Normalization. Normalization means to refining the redundant and maintain the stablization.
the DBMS hasn't normalization concept.
What are the advantages and disadvantages of primary key and foreign key in SQL?
Primary key Advantages
1) It is a unique key on which all the other candidate keys are functionally dependent
Disadvantage 1) There can be more than one keys on which all the other attributes are dependent on.
Foreign Key Advantage
1)It allows refrencing another table using the primary key for the other table
Which date function is used to find the difference between two dates?
datediff
for Eg: select datediff (dd,'2-06-2007','7-06-2007')
output is 5
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