SQL Query to use column as a formula to calculate value - sql

Question

I am working on a request that i need to calculate value based on formula specified in another column
Below is my table:
I need to write the query to get value which will be based on FORMULA column. eg I need result as
As formula could be anything consisting of my columns PRICE and SIZE, how do i write the query to achieve this?

Answer 1

Dynamic query is the (only) way to go and it's not that complicated:
DECLARE #query NVARCHAR(MAX) = '';
SELECT #query = #query + '
UNION
SELECT ItemID, Price, Size, Formula, ' + Formula + ' AS CalcValue FROM YourTable WHERE Formula = ''' + Formula + ''' '
FROM YourTable;
SET #query = STUFF(#query,1,8,'');
PRINT #query;
EXEC (#query);
SQLFiddle DEMO
But you must be aware how prone to errors this is. If value of Formula column is not valid formula query breaks.
edit: going with UNION instead of UNION ALL because of same formula appearing in multiple rows
edit2: Plan B - Instead of running bunch of same select queries and making distinct of results, better to make distinct formulas at beginning:
DECLARE #query NVARCHAR(MAX) = '';
WITH CTE_DistinctFormulas AS
(
SELECT DISTINCT Formula FROM YourTable
)
SELECT #query = #query + '
UNION ALL
SELECT ItemID, Price, Size, Formula, ' + Formula + ' AS CalcValue FROM YourTable WHERE Formula = ''' + Formula + ''' '
FROM CTE_DistinctFormulas;
SET #query = STUFF(#query,1,12,'');
PRINT #query;
EXEC (#query);
SQLFiddle DEMO 2 - added few more rows

Answer 2

Another alternative which is relatively easy to do is with a CLR. You can take advantage of the Compute Method of DataTable to give a simple one line code in C#.
[Microsoft.SqlServer.Server.SqlFunction]
public static double Evaluate(SqlString expression)
{
return double.Parse((new DataTable()).Compute(expression.ToString(), "").ToString());
}
Then add the assembly to SQL Server and create the wrapper function:
CREATE FUNCTION [dbo].[Evaluate](#expression [nvarchar](4000))
RETURNS [float] WITH EXECUTE AS CALLER
AS
EXTERNAL NAME [YourAssemblyName].[YourClassName].[Evaluate]
GO
Now you can call the function as part of a simple select statement:
SELECT itemid, price, size, formula,
dbo.Evaluate(REPLACE(REPLACE(formula, 'PRICE', FORMAT(price,'0.00')),
'SIZE', FORMAT(size, '0'))) as calcvalue FROM YourTable

Answer 3

I did something similar to this and if you play in a smaller pool of operations it is not that hard. I went with a series where I had X and Y columns and an operator. Then I just did a large case when statement on identifying the operator and performing logic based on that. You may have to change your structure slightly. The core of the problem is that SQL is a result set based engine so anything where you have to do an operation to determine dynamic is going to be slower. EG:
DECLARE #Formula TABLE
(
FormulaId INT IDENTITY
, FormulaName VARCHAR(128)
, Operator VARCHAR(4)
);
DECLARE #Values TABLE
(
ValueId INT IDENTITY
, FormulaId INT
, Price MONEY
, Size INT
)
INSERT INTO #Formula (FormulaName, Operator)
VALUES ('Simple Addition', '+'), ( 'Simple Subtraction', '-'), ('Simple Multiplication', '*'), ('Simple Division', '/'), ('Squared', '^2'), ('Grow by 20 percent then Multiply', '20%*')
INSERT INTO #Values (FormulaId, Price, Size)
VALUES (1, 10, 5),(2, 10, 5),(3, 10, 5),(4, 10, 5),(5, 10, 5),(6, 10, 5),(1, 16, 12),(6, 124, 254);
Select *
From #Values
SELECT
f.FormulaId
, f.FormulaName
, v.ValueId
, Price
, Operator
, Size
, CASE WHEN Operator = '+' THEN Price + Size
WHEN Operator = '-' THEN Price - Size
WHEN Operator = '*' THEN Price * Size
WHEN Operator = '/' THEN Price / Size
WHEN Operator = '^2' THEN Price * Price
WHEN OPerator = '20%*' THEN (Price * 1.20) * Size
END AS Output
FROM #Values v
INNER JOIN #Formula f ON f.FormulaId = v.FormulaId
With this method my operation is really just a pointer reference to another table that has an operator that is really for all intents and purposes just a token I use for my case statement. You can even compound this potentially and if you wanted to do multiple passed you could add a 'Group' column and a 'Sequence' and do one after the other. It depends how difficult your 'formulas' become. Because if you get into more than 3 or 4 variables that change frequently with frequent operator changes, then you probably would want to do dynamic sql. But if they are just a few things, that should not be that hard. Just keep in mind the downside of this approach is that it is hard coded at a certain level, yet flexible that the parameters put into it can apply this formula over and over.