Thursday, December 22, 2011

Converting CLng in MS Access SQL to T-SQL (SQL Server)

The problem I need to solve is how do I reproduce the exact functionality of the CLng (function built-in to MS Access SQL) in MS SQL Sever. The answer is not so difficult, but knowing what the question to ask was not so obvious.

After much frustration I finally realized that how CLng in MS Access SQL works is actually quite unintuitive. My situation can be explained quite simply. I have a query in MS Access that I need to convert to run completely on SQL Server. In parts of the MS Access queries the CLng function is used. CLng is similar to Round() in SQL Server, but is not the same particularly when it comes to values that end in .5. Initially I thought it would just truncate the decimals of a double. For example, I thought CLng(.5) should be 1, but MS Access says it is 0. Interestingly, CLng(3.5) is 4, not 3 and CLng(2.5) is 2, not 3. This behavior is very inconsistent to me, but I need to reproduce it on SQL Server using T-SQL so that I can compare the queries.

After some reading about issues related to rounding in general on Wikipedia I found out that there are many ways to round. After reading a technical article from Microsoft I found out that indeed VBA (and appears to be MS Access SQL) do not round the same as MS SQL Server. From this I learned the key to the solution; the CLng function in MS Access implements rounding using the Banker’s Rounding method described in both of the above sources. I did find an implementation of Banker’s Rounding once I knew what to search for. I found a good solution here. I have copy and pasted the solution below.

Here is that function you can execute on SQL Server that will do the same thing as Clng on MS Access SQL because it is an implementation of Banker’s Rounding.


create function MSAccessClng(@x as float)
returns int
as
BEGIN
Declare @DecimalPlaces as tinyint
Set @DecimalPlaces = 0

set @x = @x * power(10, @DecimalPlaces)

return
  case when @x = floor(@x) then @x
  else
    case sign(ceiling(@x) - 2*@x + floor(@x))
    when 1 then floor(@x)
    when -1 then ceiling(@x)
    else 2*round(@x/2,0) end
  end / power(10, @DecimalPlaces)
END

Now you have a function you can use on SQL Server just like you have on MS Access. Keep in mind this is a Scalar-valued function and this means there is a performance penalty for using it, but if you need this functionality you need the functionality.

Below are different examples of what I used to test that the functionality is the same on MS Access and MS SQL Server (T-SQL). Notice, the ones that say SAME means that CLng() has the same functionality as cast(). The ones that say DIFFERENCE means that the special logic was needed to implement differences. You can also see the usage of the function.

-- SAME
select clng(-3.3) -- -3
select cast(-3.3 as int) -- -3
select Round(-3.3, 0) -- 3
select dbo.MSAccessClng(-3.3) -- -3

-- SAME
select clng(-0.3) -- 0
select cast(-0.3 as int) -- 0
select round(-.3,0) -- 0
select dbo.MSAccessClng(-0.3) -- 0

-- DIFFERENCE
select clng(-3.5) -- -4
select cast(-3.5 as int) -- -3
select Round(-3.5,0) -- -4
select dbo.MSAccessClng(-3.5) -- -4

-- SAME
select clng(-0.5) -- 0
select cast(-0.5 as int) -- 0
select round(-0.5, 0) -- error
select dbo.MSAccessClng(-0.5) -- -0

-- DIFFERENCE
select clng(-0.51) -- -1
select cast(-0.51 as int) -- 0
select round(-0.51, 0) -- error
select dbo.MSAccessClng(-0.51) -- -1

-- DIFFERENCE
select clng(-0.8) -- -1
select cast(-0.8 as int) -- 0
select round(-0.8, 0) -- error
select dbo.MSAccessClng(-0.8) -- -1

-- SAME
select clng(-1) -- -1
select cast(-1 as int) -- -1
select round(-1, 0) -- -1
select dbo.MSAccessClng(-1) -- -1

-- SAME
select clng(-100000) -- -10000
select cast(-100000 as int) -- -10000
select round(-100000, 0) -- -10000
select dbo.MSAccessClng(-100000) -- -10000


-- SAME
select clng(3.3) -- 3
select cast(3.3 as int) -- 3
select Round(3.3, 0) -- 3
select dbo.MSAccessClng(3.3) -- 3

-- SAME
select clng(0.3) -- 0
select cast(0.3 as int) -- 0
select round(0.3,0) -- 0
select dbo.MSAccessClng(0.3) -- 0

-- DIFFERENCE
select clng(3.5) -- 4
select cast(3.5 as int) -- 3
select Round(3.5,0) -- 4
select dbo.MSAccessClng(3.5) -- 4

-- SAME
select clng(0.5) -- 0
select cast(0.5 as int) -- 0
select round(0.5, 0) -- error
select dbo.MSAccessClng(0.5) -- 0

-- DIFFERENCE
select clng(0.51) -- 1
select cast(0.51 as int) -- 0
select round(0.51, 0) -- error
select dbo.MSAccessClng(0.51) -- 1

-- DIFFERENCE
select clng(0.8) -- 1
select cast(0.8 as int) -- 0
select round(0.8, 0) -- error
select dbo.MSAccessClng(0.8) -- 1

-- SAME
select clng(1) -- 1
select cast(1 as int) -- 1
select round(1, 0) -- 1
select dbo.MSAccessClng(1) – 1

-- DIFFERENCE
select clng(34.5) -- 34
select cast(34.5 as int) -- 34
select Round(34.5,0) -- 35
select dbo.MSAccessClng(34.5) -- 4

-- DIFFERENCE
select clng(0) -- 0
select cast(0 as int) -- 0
select round(0, 0) -- 0
select dbo.MSAccessClng(0) – 0

-- SAME
select clng(100000) -- 10000
select cast(100000 as int) -- 10000
select round(100000, 0) -- 10000
select dbo.MSAccessClng(100000) -- 10000

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