Convert string with expression to decimal

I have a table which has a column 'Faktor' (varchar(50)) which contains expressions like:

1/3
2*9/5
0.567
0.23

No, I am searching a way to execute a select like

select Faktor from Artikel

which should return a column of type decimal with the values

回答1:

I'd go with a CLR, something like this (this has the advantage of working in SET based operations whereas the dynamic sql alternatives (i.e. Abduls answer) will not):

EDIT: Source Code for the CLR dll (Visual Studio 2008) posted here: http://www.heavencore.co.uk/filehub/uploaded/EvalFunction.zip

If you would prefer to compile the assembly yourself, here is the code:

Imports System
Imports System.Data
Imports System.Data.SqlClient
Imports System.Data.SqlTypes
Imports Microsoft.SqlServer.Server
Imports System.Runtime.InteropServices    

Partial Public Class UserDefinedFunctions
    <Microsoft.SqlServer.Server.SqlFunction()> _
    Public Shared Function EVAL(ByVal InputExpression As SqlString) As SqlDecimal
        Return Evaluate(InputExpression)
    End Function

    Private Shared Function Evaluate(ByVal expression As SqlString) As SqlDecimal
        Dim expressionStr As String = expression.ToString()
        Dim loDataTable = New DataTable()
        Dim loDataColumn = New DataColumn("Eval", GetType(Double), expressionStr)
        loDataTable.Columns.Add(loDataColumn)
        loDataTable.Rows.Add(0)
        Return ParseDecimal(loDataTable.Rows(0)("Eval").ToString(), 0)
    End Function

    Public Shared Function ParseDecimal(ByVal InputStr As String, Optional ByVal ReturnIfFail As Decimal = 0) As Decimal
        Dim ParseOutput As Decimal = 0
        If Decimal.TryParse(InputStr, ParseOutput) = False Then
            Return ReturnIfFail
        Else
            Return ParseOutput
        End If
    End Function
End Class

once installed and bound you could then do this:

SELECT Faktor, dbo.Eval(Faktor) as Result FROM Artikel

Edit: OK, i have just tested this and it works fine in set-based operations etc, installation was as follows:

-- Enable the CLR to run user-defined functions
EXEC sp_configure 
    'clr enabled' ,
    '1'
GO
RECONFIGURE
GO

-- Set the appropriate database security permission
ALTER DATABASE [TargetDatabase] SET TRUSTWORTHY ON
GO

-- Import the assembly
CREATE ASSEMBLY EvalFunction
FROM 'C:\bin\EvalFunction.dll'  
WITH PERMISSION_SET = EXTERNAL_ACCESS;
GO

-- Create the Eval function for easy use
CREATE FUNCTION dbo.Eval
    (
      @Expression NVARCHAR(255)
    )
RETURNS DECIMAL(18, 2)
AS EXTERNAL NAME 
    EvalFunction.[EvalFunction.UserDefinedFunctions].EVAL 
GO

回答2:

too bad. In the expression all numbers are int datatype. So, its difficult to calculate and show the result in decimal.

Here is a sample, hope it will help-

declare @expr nvarchar(10)
set @expr='2*9/5'
declare @query varchar(800)

BEGIN TRY
    SELECT CAST(@expr as decimal(5,2))
END TRY
BEGIN CATCH
    SET @query='SELECT CAST('+@expr+'.0 as decimal(5,2))'
    EXECUTE(@query)
END CATCH

回答3:

You could use the execute statment to compile the string to the result of the represented expression or you evaluate the expression in your application.