Tuesday, September 29, 2009

Difference between Subquery, Nested Subquery and Correlated Subquery



Query: - Query can be defined as a way to inquire the data from the database. It is used to extract the data from one table or multiple tables depending upon the user needs.

Suppose we have a two tables Student and courses whose structure is given below:-

create table Student (Studentid int identity(1,1), Firstname nvarchar(200), Lastname nvarchar(200),Email nvarchar(100))

create table Course (Courseid int identity(1,1), Coursename nvarchar(250), CourseAdmin int)

Now suppose we insert the following data into these tables:-

For table Student

insert into Student values ('Atul','Bajaj', 'atul@abc.com' )

insert into Student values ('Vivek','Johari', 'vivek@abc.com' )

insert into Student values ('Ankur','Johari', 'ankur@abc.com' )

insert into Student values ('Tarveen', 'Kaur', 'Tarveen@abc.com')

For table Course

Insert into Course values('Oracle',2)

Insert into Course values('Automation',4)

Insert into Course values('Java',2)

Insert into Course values('QTP',4)

Now the query to see all the data from the table student and course is given below:-

Select * from student









Select * from Course








Subquery:-If a sql statement contains another sql statement then the sql statement which is inside another sql statement is called Subquery. It is also known as nested query. The Sql Statement which contains the other sql statement is called Parent Statement.

For example, if we want to find the name of the course Admin of the course “Oracle”, then the following subquery will be used:-

select Firstname+' '+Lastname from student where studentid in (select courseadminid from course where coursename ='Oracle')

Result:-





In this example, the sql statement select courseadminid from course where coursename ='Oracle'is a subquery.

Nested Subquery:-If a Subquery contains another subquery, then the subquery inside another subquery is called nested subquery.

Let us suppose we have another table called “StudentCourse” which contains the information, which student is connected to which Course. The structure of the table is:-

create table StudentCourse( StudentCourseid int identity(1,1), Studentid int, Courseid int)

The Query to insert data into the table “Studentcourse” is

Insert into StudentCourse values(1,3)
Insert into StudentCourse values(2,1)
Insert into StudentCourse values(3,2)
Insert into StudentCourse values(4,4)

Note: - We don’t need to insert data for the column StudentCourseid since it is an identity column.

Now, if we want to get the list of all the student which belong to the course “Oracle”, then the query will be,

select Firstname, lastname from student where studentid in (select studentid from studentcourse where courseid in (select courseid from course where coursename='Oracle'))

Result:-





In this example we use the nested subquery since the subquery “select courseid from course where coursename='Oracle'” is itself contained in the another subquery(Parent Subquery) “select studentid from studentcourse where courseid in (select courseid from course where coursename='Oracle')”.

Correlated Subquery:-If the outcome of a subquery is depends on the value of a column of its parent query table then the Sub query is called Correlated Subquery.

Suppose we want to get the details of the Courses (including the name of their course admin) from the Course table, we can use the following query:-

select Coursename ,Courseadminid,(select Firstname+' '+Lastname  from student where studentid=Course.courseadminid)as CourseAdminName from course

Result:-








Here in this example the “select Firstname+' '+Lastname  from student where studentid=Course.courseadminid” is called the correlated subquery since the outcome of this subquery is depends on the column courseadminid of the parent query. This means that the correlated subquery will be executed for each row selected by the parent query.

It is not necessary that the column on which the correlated query is depended is included in the selected columns list of the parent query. For example the   below query will also works even the column courseadminid on which the correlated query is depends , is not included in the selected columns list of the parent query.

select Coursename ,(select Firstname+' '+Lastname  from student where studentid=Course.courseadminid)as CourseAdminName from course

Results:-



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55 comments:

  1. Good one Bro.. U made it simple to understand

    Keep Going
    -Winkey

    ReplyDelete
    Replies
    1. Author writes as...."eg of For example, if we want to find the name of the course Admin of the course “Oracle”, then the following subquery will be used:-


      select Firstname+' '+Lastname from student where studentid in (select courseadminid from course where coursename ='Oracle')
      "

      But i say in the above query he is retrieving the name of the student first name + last name and not the coureadmin name.

      Delete
    2. 100's of ppl will browse and try to learn something new. So please before posting something on ur blogs pls correct it, veryify it.As already someone pointed out ur insert statement too. What if a newbie trying to learn something out of this blog. Please Ensure before posting anything.

      Delete
    3. Hi Anonymous,
      First of all thanks for visiting my blog and spent time in writing the comments as comments motivate me to do more better. Second if you read my article carefully u got to know that, in my article, I store the course admin details also in the student table. So by the query which u mention as
      select Firstname+' '+Lastname from student where studentid in (select courseadminid from course where coursename ='Oracle')
      gives me the name of the COURSE ADMIN NOT ANY STUDENT. if you run that query and article carefully u will know that very well.

      So next time read blog carefully before writing such comments.
      Keep visiting my blog.:-)

      Delete
    4. Hi Anonymous again,
      Yes, you are right hundreds of people visits my blogs daily and give me valuable comments which motivate me to improve my articles quality better. So before giving wrong comments, reading the articles carefully on not just on my blog but on any blog as UR WRONG COMMENTS WILL GOING TO CONFUSE THE OTHER READERS
      sO Please Ensure before posting ANY COMMENTS THAT YOU READ THE FULL ARTICLES .

      Keep visiting my blog and giving me your valuable comments.

      Delete
    5. hi thanks for responding
      You still did not catch my point... your firstname + lastname gives you the name same as courseadminname but the value is from the studenttable not from the courseadmindetails.. since both are same u are thinking that it is courseadmin name... your courseadmin table is only in your condition and u r not retrieving anything from. please check once again. if possible remove the firstrow in the courseadmindetails and check

      Delete
  2. hey thanks it was helpful. if u could add certain exapmles for al kind of joins,it would b great...!!

    thanks and cheers...!!

    ReplyDelete
  3. Thanks for your response. You can find joins with example on one of my article "Sql Joins- Inner Joins, Self Joins, Outer Joins, Cross Joins"

    http://vivekjohari.blogspot.com/2010/01/sql-joins-inner-joins-self-joins-outer.html

    ReplyDelete
  4. well done dude ..its easy to understand ........

    ReplyDelete
  5. Thanks Ajeet for your response...:-)

    ReplyDelete
  6. Replies
    1. Thanks Puja for your valuable comments :-)

      Delete
    2. well donee tute

      Delete
  7. superb sir.The explanation is very easy to understand.

    ReplyDelete
  8. Hi Sir,
    Could you explain me what is generally top n query in oracle.

    Select e1.emp_id from emp e1 where 2 = ( select count(distinct e2.emp_id) from emp_1 e2 where e1.emp_id = e2.emp_id);

    what does here 2 = stands for, i tried with few examples.When 1= given instead of 2 answer changes..could you please explain me..

    ReplyDelete
    Replies
    1. Hi
      Thanks for your valuable comments.
      Can you please gives more details of your problem as it will help me in explaining.

      Delete
  9. thank you sir for the wonderful and easy examples. It was very well explained. Thank you once again.

    ReplyDelete
  10. nice explanation. Thanks

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  11. Hey Vivek.... Please correct you Insert query.. :(
    this your table structure.

    create table Student (Studentid int identity(1,1), Firstname nvarchar(200), Lastname nvarchar(200),Email nvarchar(100))



    And this your insert query..
    insert into Student values ('Atul','Bajaj', 'atul@abc.com' )

    IF you insert like that ... Oracle through the error. bcoz you try to insert the varchar value in INT variable. That not right..

    your insert query should like that..
    INSERT INTO Student
    (Firstname, Lastname,Email)
    VALUES
    ('Atul','Bajaj', 'atul@abc.com');

    In that case Oracle insert the value in exact column. :)

    ReplyDelete
    Replies
    1. Hi Sumeet,
      Many thanks for your post. This article is written for SQL Server and in SQL Server, this code work fine.:-). However ur suggestion is good, from now on wards, I will try to mention the alternate command for Oracle too.:-)

      Delete
  12. Thanks.......,the examples has gave me a clear idea..

    ReplyDelete
    Replies
    1. Thanks Reddy for your valuable comments.....

      Delete
  13. nice explanation..

    ReplyDelete
  14. could plz explain about indexed views

    ReplyDelete
  15. Thanks Vivek for making it so simple to understand

    ReplyDelete
    Replies
    1. Thanks Siddharth for your valuable comments

      Delete
  16. kindly put datawarehousing questions and answers and put sql quiz and puzzles it will look more precious to see for SQL lovers

    ReplyDelete
    Replies
    1. Hi Ashok,
      Thanks for your invaluable comments. I will looking forward to add more sql tips puzzles and Example in near future.

      Delete
    2. Thanking you. am also expect more SQL test quiz from your side........thankxxxxxxxxxx

      Delete
  17. now i understand wt actualy d difference izzz............. thnkzzz

    ReplyDelete
  18. simply superb... gr8 article.
    how do i improve my sql skills? any thoughts ? thanks.

    ReplyDelete
  19. It could have been more elaborate. This just a very basic type of explaination. Examples also are not upto the standard.

    ReplyDelete
  20. After completing so many certifications if your explanation stuff is so weak like you have provided. I doubt your dedication to the technology & authentication of the certification candidate. Very poor it shows improper professional development with years of experience

    ReplyDelete
    Replies
    1. Thanks Anonymous for your comments as its motivate me to improve my article quality. But it is better if you give comments based on my articles not on my certificates as it like that you are more concern about my certificate than my article.
      Also some time it is hard to understand why people hide their identity while criticizing someone as sometimes it put a question mark on the person intention.

      Thanks again
      Vivek

      Delete
  21. Very useful Vivek ji.... Helped me a lot.....

    ReplyDelete
  22. Hello Vivek, juz wondering if the below query can be alternate query for correlated query example.
    select A.Coursename , B.Firstname+' '+B.Lastname as CourseAdminName from A.course AND B.STUDENT where b.studentid=a.courseadminid

    Ajay.

    ReplyDelete
  23. Mrinal ChakravortyAugust 26, 2013 at 3:24 PM

    Good One Vivek..

    ReplyDelete
  24. GOOD Atricle , Thanks for your effort.

    ReplyDelete
  25. In one Of Interview I hve been asked , What is Co-related Query and how it works internally in oracle.
    Also it has been extended wht is diffrenece between co related query and temp table in SQL query.

    ReplyDelete
  26. an excellent example and I really cleared my doubts very clearly...Keep it Guru..

    ReplyDelete
  27. Easily understandable, very good article

    ReplyDelete
  28. Hello Sir, you have explained the above aricle very well. Based on the above article I want to ask you that, I want to create a query in which I am comparing a field in two different tables and I want to get the record from the table which is having the higher value after that earlier comparision. I am not able to do this. Waiting for your response.

    ReplyDelete
  29. You made it very simple...
    Thanks..

    ReplyDelete