4 Ways to Increase Your Salary (Using UPDATE)

  • arup chakraborty

    Hall of Fame

    Points: 3640

    Comments posted to this topic are about the item 4 Ways to Increase Your Salary (Using UPDATE)

  • Jochen Vleming

    Right there with Babe

    Points: 728

    Hi Arup,

    I'm fairly new to SQL so maybe it's a stupid question, but why do you use a temp-table in the last example? I think there's no use for that since both the emp-table and the temp-table share the same identity. The way the query is built I think it wouldn't even work if the id's in the emp-table would be different (for example 1,2,5,6,7 (after delete)).

    Jochen

  • Kevin Gill

    SSCrazy

    Points: 2316

    You say that salary * 115 / 100 and (salary * 115) / 100 return different results due to operator precedence - I can't see how operator precedence makes any difference in this scenario. Could you elaborate or provide an example?

    Basically :

    salary x (115/100) = (salary x 115)/100 so operator precedence should make no difference...

    Actually - thinking about it, is it the integer division that's the issue? What I say above is true in a pure mathematical sense, but if you divide 115/100 as integers in SQLServer, you get 1. If however you divide 115 as a decimal /100, then you get 1.150000, and the calculation will work in any layout. So technically it's the operator precedence causing the integer divide to happen first which is the issue I suppose... though it's the integer divide in its own right that means there is any confusion to be had in the first place.

    declare @percentage decimal(3,0)

    set @percentage = 115 -- Even works with an 'integer' decimal.

    select 3000 * @percentage / 100

    select (3000 * @percentage) / 100

    select 3000 * (@percentage / 100.0)

    Cheers

    -- Kev

    -------------------------------
    Oh no!

  • stuart.jeffery

    SSC Enthusiast

    Points: 165

    I have to agree, there is no real difference with the calculations except in the interpretation by the system. Int divide by Int = Int which causes incorrect values. Simpler would be to change one of the Int's or even remove one altogether.

    What is wrong with using Salary * 1.15? After all Salary is a float.

    Remember the key to performance is simplicity. Every level of complexity will exponentially affect your query performance.

  • Martin Hill-258755

    SSC Enthusiast

    Points: 124

    There is actually another way to archive a row by row processing. I am using it in different scenarios since quite a few years and it turned out to be very efficient.

    BEGIN

    Declare @ID int

    Select @ID = Min(ID) from emp

    WHILE @ID Is Not Null

    BEGIN

    UPDATE emp SET salary = (salary * 115) /100 WHERE emp.id = @ID

    Select @ID = Min(ID) from emp where ID > @ID

    END

    END

    In relation to the actual performance of this method I listed the times/cost on my server below. Please note that I also included the cost calculated by the execution plan:

    READ ESTIMATED EXECUTION COST

    Direct SQL 5 0.0132935 100%

    Cursor 69 0.1462060 1099%

    Temp table with TOP 254 0.2531007 1904%

    Temp table with IDENTITY column 121 0.0994881 748%

    Min Loop 39 0.0861955 648%

    ๐Ÿ™‚

  • Jason-299789

    SSC-Insane

    Points: 21601

    Martins answer is a lot sweeter and more robust if you need to loop through a data set, as there is no guarantee that the numbers are consecutive eg record number 4 is deleted for some reason (yes it can happen), so the last record will never get updated, assuming a 1% row deletion count, then in an organisation with 1000 members, 10 wont get pay rises.

    As always the advice is use set based queries rather than Loops and Cursors, though in some cases you have to revert to them they should be rare.

    As an aside, one of my colleagues ran some internal tests on a somewhat larger data set (1000 rows, same number of columns) and found that the Cursor ran significantly faster than the last option.

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  • Jochen Vleming

    Right there with Babe

    Points: 728

    nice one Martin,

    That's a nice and easy way to do the updates.

    ...And independent of identity values in the table.

  • honza.mf

    SSCertifiable

    Points: 5519

    The last example uses assumption on the identity column values in the main table.

    You can just add these values to the temp table.

    But Martin's solution is better



    See, understand, learn, try, use efficient
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  • jarred.nicholls

    SSC Enthusiast

    Points: 140

    Jason Lees-299789 (11/26/2009)


    As an aside, one of my colleagues ran some internal tests on a somewhat larger data set (1000 rows, same number of columns) and found that the Cursor ran significantly faster than the last option.

    Cursors and temp tables are very similar, since a cursor creates a temp table in tempdb (spooling all data to disk before being read back)...just like a temp table does. A cursor has an upfront hit when allocating itself, but that overhead doesn't occur with each row fetch and each row fetch is quite speedy.

    The CPU cycles is much greater on each row's operation in the identity temp table method (it is doing a COUNT(*) on the temp table each iteration). With a large data set, a cursor will (always) startup slower but finish faster and with less CPU. On a small data set, a cursor will still startup slower but finish slower as well.

    Bottom line is, the last method makes certain assumptions about the identity column that can't always be made and thus isn't a universal solution. And of course, 99% of the time there is a set-based solution that will run circles around any procedural solution. I once tuned a stored procedure that took over 2 hours to complete by replacing a cursor with a set-based solution and brought the execution time down to just around 15 seconds. If you are forced into a procedural situation, first post your situation onto a forum and let someone find a set-based solution to the problem (there's a great chance there is one), and if none is found, use a method that suits the size of the data set. Martin's solution is the appropriate one here, no matter the size of the data set - it doesn't use a temp table.

  • juancvaz

    SSC Enthusiast

    Points: 109

    Hi All,

    Iยดm test for 24200 rows, the cursor done in 35 seconds and the tabla with identity in 84 seconds.

    But when i modify the WHILE control for an constant the final time is 35 seconds, equal to cursor.

    BEGIN

    CREATE TABLE #temp(id INT IDENTITY(1,1), name VARCHAR(32), salary float)

    INSERT INTO #temp

    SELECT name, salary FROM #emp

    DECLARE @i INT, @last INT

    SELECT @Last = COUNT(id) FROM #temp

    SET @i = 1

    WHILE (@i <= @last )

    BEGIN

    UPDATE #emp

    SET salary = (salary * 115)/100

    WHERE #emp.id = @i

    SET @i = @i + 1

    END

    END

  • John Lee-439997

    Grasshopper

    Points: 15

    I've been using Martin's solution for years, however, slightly modified.

    BEGIN

    Declare @min int, @max int

    Select @min = Min(ID),

    @max=Max(ID) -- OR a user-defined max

    from emp

    WHILE @min <= @max

    BEGIN

    UPDATE emp SET salary = (salary * 115) /100 WHERE emp.id = @min

    Select @min = Min(ID) from emp where ID >@min

    END

  • djgubber

    SSC Rookie

    Points: 48

    Since \ and * share the same precidence, does that not mean that if we have salary*115/100 that the salary*115 part will happen first? Or is there no guarentee of the ordering?

    Edit:

    The presidence article that was linked to says this "When two operators in an expression have the same operator precedence level, they are evaluated left to right based on their position in the expression."

  • nickblethyn

    SSC Journeyman

    Points: 91

    I found a very efficient way to DECREASE my salary.

    I stayed in a government job for longer than I should have, and inflation did the rest ๐Ÿ™

  • Robert Davis

    One Orange Chip

    Points: 28027

    Actually, the author and the first two replyers are wrong about the order of precedence. If you fully understand tha order of precedence, multiplication is evaluated before division. The order is as follows:

    Paranthesis -> Exponents -> Multiplication -> Division -> Addition -> Subtraction

    This can be remembered using the following nemonic: Please excuse my dear aunt Sally. That's how I learned it in school.

    But don't believe me. Try it out:

    Declare @Salary int

    Set @Salary = 85199

    Select @Salary*115/100, (@Salary*115)/100

    ----------- -----------

    97978 97978

    (1 row(s) affected)


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  • jarred.nicholls

    SSC Enthusiast

    Points: 140

    Despite the misinformed on operator precedence, just multiply by 1.15! Keep things DRY and simple ๐Ÿ™‚

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