Saturday, December 28, 2013

Answer -Question of the Week - 25/12/2013- Question 1


Question:- Suppose we have a table says tbl_students whose structure is given below:-

Create table tbl_students(Studentid int identity(1,1) , Studentname nvarchar(150))

Suppose it contains the following data:-

Studentid  Studentname
1               Vivek Johari
2               Chandra Singh
4               Avinash Dubey
6               Akhil
7               Sunny Rana
10             Saurabh

As one can see from the data that some rows are deleted from the table as some studentids are missing.

So write a query which gives the studentid which are deleted from this table


Answer:-


 Thanks all for answering that question. I really appreciate the effort of all the people who tried and gives the response.We get multiple answers for this question in which correct answer is given below:-

Correct answer in case the first coulmn is an identity column

Answer given by Arjan  Dingemans


WITH Numbers AS (
SELECT 1 AS Number
UNION ALL
SELECT Number + 1
FROM Numbers
WHERE Number < IDENT_CURRENT('tbl_students')
)
SELECT Number
FROM Numbers N
LEFT JOIN tbl_students S ON N.Number = S.StudentId
WHERE S.StudentId IS NULL


Answer given By Chandra Prakash Singh

select number from master..spt_values s
left join tbl_students on s.number = tbl_students.studentid
where s.type='P'
and s.number <= IDENT_CURRENT('tbl_students')
and tbl_students.studentid is null and number > 0


Correct answer in case the first coulmn is not an identity column 

Answer given By Avinash Dubey

WITH DeletedRows (missnum, maxid)
AS

SELECT 1 AS missnum, (select max(Studentid) from tbl_Students) UNION ALL SELECT missnum + 1, maxid FROM DeletedRows 
WHERE missnum < maxid
)
SELECT missnum FROM DeletedRows
LEFT OUTER JOIN tbl_Students e on e.Studentid = DeletedRows.missnum
WHERE e.Studentid is NULL OPTION (MAXRECURSION 0)


Answer given by Aviral Lamba

select number from master..spt_values s
left join tbl_students on s.number = tbl_students.studentid
where s.type='P'
and s.number < (select MAX(studentid) from tbl_students)
and tbl_students.studentid is null and number > 0 


Also I appreciate the effort done by chetan Deshpande in attempting the question

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1 comment:

  1. Is this correct??

    BEGIN TRAN
    DECLARE @MAX INT= (SELECT MAX(Studentid) FROM TBL_STUDENTS)
    CREATE TABLE #T1(ID INT)
    WHILE(@MAX>0) BEGIN
    INSERT INTO #T1 VALUES(@MAX)
    SET @MAX=@MAX-1
    END
    SELECT * FROM #T1 except (SELECT Studentid FROM TBL_STUDENTS)
    DROP TABLE #T1
    COMMIT TRAN

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