Back to basics

  • Danny Ocean (7/2/2013)


    raulggonzalez (7/2/2013)


    Danny Ocean (7/1/2013)Thanks raulggonzalez πŸ™‚

    You're welcome!

    but please note that the script would give you correct results only when freq_interval = 8

    http://msdn.microsoft.com/en-us/library/ms178644.aspx

    Cheers

  • JAZZ Master (7/2/2013)


    martin.whitton (7/2/2013)


    Danny Ocean (7/1/2013)


    Good question. But i never feel to use "Bitwise AND" (&) in real working scenario. It will good, if anyone come with some real working example. πŸ™‚

    It provides a neat way of sending multiple options as a single integer parameter.

    For example, let's say you have a table called "Locations" with 2 columns, LocationID and Location, containing the following values:

    LocationID Location

    1 London

    2 Dublin

    4 Paris

    8 Berlin

    16 New York

    etc...

    Then you could select any combination of Location values as follows:

    declare @Selection int;

    set @Selection=10; -- (this will select Dublin and Paris, because Dublin's LocationID plus Paris's LocationID equals 10)

    select

    Location

    from

    Locations

    where

    LocationId & @Selection>0;

    I'm confused. Doesn't 2 (Dublin) and 8 (Berlin) equal 10 not 2 (Dublin) and 4 (Paris)? What am I missing?

    Sorry :blush:

    You're absolutely right - I should have said Dublin and Berlin.

  • Patibandla (7/2/2013)


    What if i define @S as BIGINT

    Can you give me a real time scenario as in this operator would be useful. i am just a bit curious as i have never used used it.

    If you check out the Books Online entry for the sysschedules table (2012),

    http://msdn.microsoft.com/en-us/library/ms178644.aspx

    you’ll notice the following statement:

    freq_interval is one or more of the following:

    1 = Sunday

    2 = Monday

    4 = Tuesday

    8 = Wednesday

    16 = Thursday

    32 = Friday

    64 = Saturday

    It means that if my job is to run on Tuesday and Saturday, the value will be 68. On Tuesday you would test it with a mask 4, on Saturday with 64. Test on say Wednesday (mask 8) will result in zero, meaning "don't run."

  • L' Eomot InversΓ© (7/2/2013)


    Good question, genuinely back to basics, but as your int value was 32767 wouldn't 32512 have been a better distractor than 65280?

    Yes, it probably would. πŸ™‚

  • martin.whitton (7/2/2013)


    JAZZ Master (7/2/2013)


    martin.whitton (7/2/2013)


    Danny Ocean (7/1/2013)


    Good question. But i never feel to use "Bitwise AND" (&) in real working scenario. It will good, if anyone come with some real working example. πŸ™‚

    It provides a neat way of sending multiple options as a single integer parameter.

    For example, let's say you have a table called "Locations" with 2 columns, LocationID and Location, containing the following values:

    I'm confused. Doesn't 2 (Dublin) and 8 (Berlin) equal 10 not 2 (Dublin) and 4 (Paris)? What am I missing?

    Sorry :blush:

    You're absolutely right - I should have said Dublin and Berlin.

    Oh good. I was beginning to think I did not understand bitwise AND at all!

  • Here's another example of using the AND (&) operator to query an integer being used as a bit-map. You may have a set of integers, each representing the calendar for a month. You can set bits in each to indicate certain days, perhaps weekends and holidays. For this month, July 2013, here in the U.S. we would set the bits for the four weekends and for the holiday on the fourth of July. Then, we can use the & operator to query whether a particular date in the month is not a business day:

    declare @monthCalendar int

    set @monthCalendar = power(2,4) -- set bits for the holiday and the four weekends

    + power(2,6)

    + power(2,7)

    + power(2,13)

    + power(2,14)

    + power(2,20)

    + power(2,21)

    + power(2,27)

    + power(2,28)

    declare @dateInMonth int

    set @dateInMonth = 4 -- check this date in the month

    declare @dateBitValue int

    set @dateBitValue = power(2,@dateInMonth)

    Select @monthCalendar as MonthCalendar -- this integer may have been stored in your db somewhere

    ,Case when @monthCalendar & @dateBitValue > 0 then 'Yes' Else 'No' end as DateIsSelected

  • Nice question and discussion.....

  • I use this methodology to establish a system of Rights for Users in applications. Using BIGINT you can have a total of 63 different individual Rights assigned to a User all contained in a single BIGINT column value. Very compact and very fast for retrieval and using masks for determining if the User has a specific right or not.

    Bill

  • In SQL you have several options to associate a set with a row for a multi-valued attribute of an entity. The most obvious solution is a separate table that stores a foreign key to the original row along with a single (reference to a) value of the attribute in each row. You can add an XML column and store several attribute values inside it (even for different attributes) but storing and retrieving their value introduces some overhead and additional complexity to your queries. If the number of values for a single row is rather limited, a numbered column collection is also an option ([TelNr_1], [TelNr_2], [TelNr_3], ...) though I would personally never recommend this solution. If the total number of possible values is very limited you might use a bitmap where each bit represents a value and that bit is set if the multi-valued attribute contains that value. A separate table should map the individual bits to the actual values.

    If you can live with the limitations of a bitmap (no foreign key and at most 64 values) it will save some storage but more important it allows you to do all kinds of set operations using simple bitwise operations. Checking wether the intersection of two sets is not empty requires no more than a bitwise and and a comparison. A bitwise or creates the union of two sets, an exclusive or the symmetric difference and an and with a not the difference. Allowing fast set operations makes these bitmaps extremely useful in authorization schemes. You should add the bitmap to your tool belt but use it with care, because it obscures the actual relations between tables and might cause serious trouble as soon as the number of possible values (roles for example) suddenly grows above that magic limit of 64.

    Although there is no physical relation between the table with the bitmap attribute and the table that maps the individual bits (guarded by a foreign key), a logical relation between these tables does exist. Fortunately SQL Server allows us to define any join predicate we like, so instead of an eqi-join we may use a bitwise and as well: [OriginalTable] INNER JOIN [ValueTable] ON ([OriginalTable].[Bitmap] & [ValueTable].[IndividualBit]) <> 0. A subquery with a FOR XML clause and a few REPLACEs around it can turn your bitmap into a human-readable comma-separated list of attribute values.

  • Wow... One thing about Bitwise operaters is certain.

    It is easy to read these posts and know who understands them completely and who is lost.

    This would make for a great Article... Especialy the is BIGINT supported part....

    Hrmmmmm :smooooth:

  • I think that the best example here is @@OPTIONS.You do need the Bitwise AND operator if you're using it.

  • PHYData DBA (7/3/2013)


    Wow... One thing about Bitwise operaters is certain.

    It is easy to read these posts and know who understands them completely and who is lost.

    This would make for a great Article... Especialy the is BIGINT supported part....

    Hrmmmmm :smooooth:

    I wrote this QotD when I found that one of the members of my team did not understand that you could do AND on operands of a different length. I thought that the SQLServerCentral audience could benefit, and I hope we all did.

    πŸ˜‰

  • It was a good basic question. πŸ™‚

  • raulggonzalez (7/2/2013)


    Danny Ocean (7/1/2013)


    Good question. But i never feel to use "Bitwise AND" (&) in real working scenario. It will good, if anyone come with some real working example. πŸ™‚

    Hi, another example where BIT comparison is useful, msdb..sysschedules keeps the freq_interval in bitwise value πŸ™‚

    use [msdb]

    go

    create table #DaysOfWeekBitWise(

    [bitValue] [tinyint] NOT NULL PRIMARY KEY,

    [name] [varchar](10) NULL,

    )

    go

    insert into #DaysOfWeekBitWise ([bitValue], [name])

    values (1, N'Sunday')

    , (2, N'Monday')

    , (4, N'Tuesday')

    , (8, N'Wednesday')

    , (16, N'Thursday')

    , (32, N'Friday')

    , (64, N'Saturday')

    go

    select j.name

    , case when j.enabled = 1 then 'Yes' else 'No' end as enabled

    , jsch.next_run_date

    , jsch.next_run_time

    --, jst.*

    , s.freq_interval

    , ISNULL( STUFF( (SELECT N', ' + name FROM #DaysOfWeekBitWise AS B WHERE B.bitValue & s.freq_interval = B.bitValue FOR XML PATH('') ), 1, 2, '' ), 'None' ) AS backup_schedule

    from msdb.dbo.sysjobs as j

    left join msdb.dbo.sysjobschedules as jsch

    on jsch.job_id = j.job_id

    left join msdb.dbo.sysschedules as s

    on s.schedule_id = jsch.schedule_id

    order by j.name

    go

    drop table #DaysOfWeekBitWise

    go

    Cheers

    Another good example is with Reporting Services and Subscriptions

    Jason...AKA CirqueDeSQLeil
    _______________________________________________
    I have given a name to my pain...MCM SQL Server, MVP
    SQL RNNR
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