SQL SERVER – Interesting Interview Questions – Difficult SQL Puzzle

SQL SERVER - Interesting Interview Questions - Difficult SQL Puzzle puzzle Today at my organization, we had nearly 30 interviews scheduled of DBA and .NET developers. I really enjoyed participating in interviewing panel. It was really fun to ask questions and learn so many new things from developers. I will share a few of my experiences with candidates here. Let us see a difficult SQL puzzle.

Those who are scheduled to interview with me next week this is one question I will be sure asking you besides questions from my interview questions and answer series.

Question: If we have the following table, please write a script to display output below.

TableName: StudentsEnroll
Students Class1 Class2 Class3
Student1101
Student2111
Student3010
Student4101

Output:

Class1 has 3 Students
Class2 has 2 Students
Class3 has 3 Students

Solution:

Create same data –

USE DATABASE
GO
CREATE TABLE [StudentsEnroll] (
[Students] [varchar] (50) CONSTRAINT [DF_StudentsEnroll_Students] DEFAULT (''),
[Class1] [bit] NOT NULL CONSTRAINT [DF_StudentsEnroll_Class1] DEFAULT (0),
[Class2] [bit] NOT NULL CONSTRAINT [DF_StudentsEnroll_Class2] DEFAULT (0),
[Class3] [bit] NOT NULL CONSTRAINT [DF_StudentsEnroll_Class3] DEFAULT (0)
) ON [PRIMARY]
GO
INSERT INTO StudentsEnroll (Students, Class1, Class2, Class3)
VALUES ('Student1', 1, 0, 1)
GO
INSERT INTO StudentsEnroll (Students, Class1, Class2, Class3)
VALUES ('Student2', 1, 1, 1)
GO
INSERT INTO StudentsEnroll (Students, Class1, Class2, Class3)
VALUES ('Student3', 0, 1, 0)
GO
INSERT INTO StudentsEnroll (Students, Class1, Class2, Class3)
VALUES ('Student4', 1, 0, 1)
GO

Solution 1 – Using only SELECT statement

DECLARE @Col INT
SET @Col = 1
WHILE (@Col < 4)
BEGIN
EXEC('SELECT     ''Class'+@Col+' Has ''</pre>
<pre>+ CAST(COUNT(Students) AS VARCHAR(100)) + '' Students'' Results
FROM         studentsenroll
WHERE         Class'+@Col+' = 1
GROUP BY      Class'+@Col)
SET @Col = @Col + 1
CONTINUE
END

Solution 2 – Getting results in one resultset

SELECT 'Class1 has ' + CAST(COUNT(*) AS VARCHAR(10)) +' Students'
FROM StudentsEnroll
WHERE Class1 = 1
GROUP BY Class1
UNION ALL
SELECT 'Class2 has ' + CAST(COUNT(*) AS VARCHAR(10)) +' Students'
FROM StudentsEnroll
WHERE Class2 = 1
GROUP BY Class2
UNION ALL
SELECT 'Class3 has ' + CAST(COUNT(*) AS VARCHAR(10)) +' Students'
FROM StudentsEnroll
WHERE Class3 = 1
GROUP BY Class3

If you know any other method or script to achieve the same result, please share here with your friends.

Reference: Pinal Dave (https://blog.sqlauthority.com)

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36 Comments. Leave new

  • DECLARE @Class1 int, @Class2 int, @Class3 int,
    @Class1Total int, @Class2Total int, @Class3Total int
    SET @Class1Total = 0
    SET @Class2Total = 0
    SET @Class3Total = 0

    DECLARE student_CURSOR CURSOR FOR
    SELECT Class1, Class2, Class3
    FROM StudentsEnroll

    OPEN student_CURSOR

    FETCH next FROM student_CURSOR
    INTO @Class1, @Class2, @Class3

    WHILE @@FETCH_STATUS = 0
    BEGIN
    SET @Class1Total = @Class1Total + @Class1
    SET @Class2Total = @Class2Total + @Class2
    SET @Class3Total = @Class3Total + @Class3

    FETCH next FROM student_CURSOR
    INTO @Class1, @Class2, @Class3
    END

    CLOSE student_CURSOR
    DEALLOCATE student_CURSOR

    SELECT ‘Class1 has ‘ + cast(@Class1Total AS varchar(10)) + ‘ students’
    UNION ALL
    SELECT ‘Class2 has ‘ + cast(@Class2Total AS varchar(10)) + ‘ students’
    UNION ALL
    SELECT ‘Class3 has ‘ + cast(@Class3Total AS varchar(10)) + ‘ students’

    Do I get the job? Haaaa.

    Reply
  • How about a 1 liner?

    select
    ‘Class1 has ‘ + cast(sum(cast([Class1] as int)) as varchar(10)) + ‘ students.\n’
    + ‘Class2 has ‘ + cast(sum(cast([Class2] as int)) as varchar(10)) + ‘ students.\n’
    + ‘Class3 has ‘ + cast(sum(cast([Class3] as int)) as varchar(10)) + ‘ students.\n’
    from [StudentsEnroll];

    Reply
  • In case there is a chance that the number of classes may change over time, here is a dynamic approach:

    declare @cls nvarchar(100);
    declare c cursor fast_forward for
    select
    c.name
    from
    sys.columns c
    inner join sys.tables t
    on c.object_id = t.object_id
    where
    t.[name] = ‘StudentsEnroll’
    and c.[name] ‘Students’;

    open c;
    fetch next from c into @cls;
    while @@fetch_status = 0
    begin
    declare @cmd nvarchar(500);
    set @cmd = ‘select ”’ + @cls + ‘ has ” + cast(count([Students]) as varchar(10)) + ” students.”’
    + ‘ from [StudentsEnroll] where [‘ + @cls + ‘] 0 group by [‘ + @cls + ‘]’;
    print @cmd;
    exec sp_sqlexec @cmd;
    fetch next from c into @cls;
    end
    close c;
    deallocate c;

    Reply
  • Zod and Eric

    Very nice.

    Eric – particularly the one liner approach is the one I am looking for from developers.

    When they get that one, I ask them follow up question as what happens if number of class is dynamic.

    Very good.

    Regards,
    Pinal

    Reply
  • Alexander Kintis
    December 7, 2008 10:51 pm

    Why not juse use a pivot? If you’d like, I can write the query.

    Reply
    • Absolutely, that is smart way to do this. If you can please provide query for all the blog readers.
      I was waiting for someone to suggest PIVOT.

      Kind Regards,
      Pinal Dave

      Reply
  • It amazes me that people are still using cursors as their first answer to everything… the are a very (very!) few situations were one may argue a cursor is applicable, this is most certainly not one of them. If Zod provided that as an answer, I would end the interview right there.
    Besides that, the schema in the original question is flawed. How about bonus points to the interviewee that points out that it is a bad design to have classes listed columnwise like this. Having to do dynamic sql to get around this design flaw is only a “good” answer if somehow you’re stuck with a bad schema.

    Reply
  • Imran Mohammed
    December 8, 2008 4:26 am

    @Pinal

    how about this ?

    select CLASS+ ‘ has ‘+convert(varchar(10),count(orders ))+’ Students ‘ ‘Output’ from (
    SELECT students, Class, Orders
    FROM
    (SELECT students, class1, class2,class3
    FROM studentsenroll ) p
    UNPIVOT
    (Orders FOR Class IN
    (class1, class2, class3 )
    )AS unpvt) X where orders = 1 group by class

    Thanks,
    IM

    Reply
  • If Zod provided that as an answer, I would end the interview right there.

    Uh, Pinal asked for other ways to do it, not the best way to do it. So, if you were giving the interview, I think I’d walk out right here since you didn’t understand Pinal’s question. ;-)

    Reply
  • DECLARE @opXml as XML

    SET @opXml = ‘<ClassStud value=”‘ + REPLACE(
    (SELECT
    ‘Class1 has ‘ + CAST(SUM(CAST(Class1 AS SMALLINT)) AS VARCHAR(4)) + ‘ students’ ,
    ‘,Class2 has ‘ + CAST(SUM(CAST(Class2 AS SMALLINT)) AS VARCHAR(4)) + ‘ students’ ,
    ‘,Class3 has ‘ + CAST(SUM(CAST(Class3 AS SMALLINT)) AS VARCHAR(4)) + ‘ students’
    FROM [StudentsEnroll]
    FOR XML PATH(”) ), ‘,’, ‘” /><ClassStud value=”‘) + ‘” />’

    SELECT x.value(‘@value’, ‘varchar(100)’) AS [output]
    FROM @opXml.nodes(‘/ClassStud’) as p(x)

    Reply
  • How about this:

    declare @tbl table (i int)
    insert into @tbl (i)
    select 1
    union
    select 2
    union
    select 3

    select
    case when i = 1 then ‘Class 1 has ‘ + cast(sum(cast (class1 as int)) as varchar(100)) + ‘ Students’ else
    case when i = 2 then ‘Class 2 has ‘ + cast(sum(cast (class2 as int)) as varchar(100)) + ‘ Students’ else
    case when i = 3 then ‘Class 3 has ‘ + cast(sum(cast (class3 as int)) as varchar(100)) + ‘ Students’ else ” end
    end
    end
    from [StudentsEnroll]
    join @tbl t on i <= 3
    group by i

    Reply
  • I’d be concerned about any organisation that asked a question like that. The table design is terrible, and if it was more normalized, there wouldn’t be any need to come up with clever solutions.

    Of course, if the question asked how you would improve the schema and then return the output that would be acceptable.

    Reply
  • Dear Readers,

    Take this question as T-SQL practice exam rather than schema improvement problem.

    In my organization we do not ask this simple question for Schema improvement. Normalization is important but in data warehousing and BI de-normalization is equally important.

    Hope this clear now and waiting for more T-SQL solution to this problem.

    Everybody who participated here – Thanks I really see good suggestions.

    Kind Regards,
    Pinal Dave

    Reply
  • Hi all,
    Good to see you here and very very thanks for sharing the knowledge.

    So anyone can u please tell me about the difference between table variables and the cursors..

    which is good one and where??

    Thanks
    Regards Prakash

    Reply
  • select ‘class1 has ‘ + cast((select count(*) from StudentsEnroll where class1=1 group by class1) as char(3)) + ‘Students’
    union
    select ‘class2 has ‘ + cast((select count(*) from StudentsEnroll where class2=1 group by class2) as char(3)) + ‘Students’
    union
    select ‘class3 has ‘ + cast((select count(*) from StudentsEnroll where class3=1 group by class3) as char(3)) + ‘Students’

    Reply
  • Thanks for the nice tricks.

    Reply
  • Guys, this is great participation from all of you, I will be writing in depth another article where I will have all this answers included.

    Keep up the good work.

    If you are interested participate in second question :
    https://blog.sqlauthority.com/2008/12/10/sql-server-interesting-interview-questions-part-2-puzzle/

    Regards,
    Pinal Dave ( )

    Reply
  • Imran is correct in that PIVOT is not the answer (as surmised) but UNPIVOT.

    Here’s a bit cleaner version of UNPIVOT that is closer to the asked for end result…

    SELECT Class,
    CAST(‘has’ as char(3)) as ‘has’,
    SUM(CAST(Breakout as tinyint)) as ‘X’,
    CAST(‘students.’ as char(9)) as ‘students.’
    FROM (

    SELECT Class, Breakout, Students FROM
    (
    SELECT Students, Class1, Class2, Class3
    FROM StudentsEnroll
    ) se_inner
    UNPIVOT (Breakout FOR Class IN (Class1, Class2, Class3)) AS denorm
    ) se_outer
    GROUP BY Class ORDER BY Class

    Reply
  • hi i am new to sql server and its very intresting to see such good questions.
    i would participate int hese discussions and post my comments for the queries.

    Thanks
    jagadesh

    Reply
  • hi pinal,
    I am a beginner in sql server…can u give me any suggestions in his regard..

    Thanks
    Jagadesh

    Reply
  • SELECT ‘Class1 has ‘+ CONVERT(nvarchar(10),sum(CASE class1 WHEN 1 THEN 1 ELSE 0 end))+’ students’
    FROM StudentsEnroll se union ALL
    SELECT ‘Class2 has ‘+ CONVERT(nvarchar(10),sum(CASE class2 WHEN 1 THEN 1 ELSE 0 end))+’ students’
    FROM StudentsEnroll se union ALL
    SELECT ‘Class3 has ‘+ CONVERT(nvarchar(10),sum(CASE class3 WHEN 1 THEN 1 ELSE 0 end))+’ students’
    FROM StudentsEnroll se

    Reply

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