Setting variable to its own value's value dynamically - Possible? - sql

Question

Using TSQL
Trying to set the #DynamicValue = 23 which is the parameter it is "equal" to in the below but can't figure it out.
DECLARE #DynamicValue VARCHAR(MAX) = '#PersonID';
DECLARE #PersonID INT = 23;
DECLARE #PlaceID INT = 48;
DECLARE #ThingID INT = 29;
DECLARE #nSQL NVARCHAR(MAX);
SET #nSQL = N'SELECT #Parameter2 = ' + CAST(#DynamicValue AS VARCHAR(MAX));
-- This is where the error is. If I used #PersonID here it would be fine but I want this to be dynamic
EXEC sp_executesql #nSQL,
N'#Parameter2 NVARCHAR(MAX) OUTPUT',
#Parameter2 = #DynamicValue OUTPUT;
SELECT #DynamicValue;
Any ideas? Different approaches?

Answer 1

You just have to pass all the parameters into the dynamic batch that performs the assignment.
Like this:
DECLARE #DynamicValue VARCHAR(MAX) = '#PersonID';
DECLARE #PersonID INT = 23;
DECLARE #PlaceID INT = 48;
DECLARE #ThingID INT = 29;
DECLARE #nSQL NVARCHAR(MAX);
SET #nSQL = N'set #Parameter2 = ' + CAST(#DynamicValue AS VARCHAR(MAX));
EXEC sp_executesql #nSQL,
N'#PersonID int, #PlaceID int, #ThingID int, #Parameter2 NVARCHAR(MAX) OUTPUT',
#PersonID = #PersonID, #PlaceID = #PlaceID, #ThingID = #ThingID, #Parameter2 = #DynamicValue OUTPUT;
SELECT #DynamicValue;

Answer 2

The only way I see:
DECLARE #DynamicValue VARCHAR(MAX) = '#PersonID';
DECLARE #PersonID INT = 23;
DECLARE #PlaceID INT = 48;
DECLARE #ThingID INT = 29;
SELECT #DynamicValue = CAST(CASE #DynamicValue
WHEN '#PersonID' THEN #PersonID
WHEN '#PlaceID' THEN #PlaceID
WHEN '#ThingID' THEN #ThingID END as VARCHAR(MAX))
SELECT #DynamicValue

Answer 3

EzLo's comment above that a variable declared outside of dynamic SQL can't be accessed made me rewrite your code in the following way, seems to work:
SET #nSQL = N'DECLARE #PersonID INT = ' + CAST(#PersonID as varchar)
+' DECLARE #PlaceID INT = ' + CAST(#PlaceID as varchar)
+' DECLARE #ThingID INT =' + CAST(#ThingID as varchar)
+' SELECT #Parameter2 = ' + CAST(#DynamicValue AS VARCHAR(MAX));

Answer 4

There is another way to do the same trick, but you still would have to list all your parameters:
DECLARE #DynamicValue VARCHAR(MAX) = '#PersonID';
DECLARE #PersonID INT = 23;
DECLARE #PlaceID INT = 48;
DECLARE #ThingID INT = 29;
DECLARE #DynamicValues TABLE (Parameter VARCHAR(MAX), Value INT)
INSERT INTO #DynamicValues VALUES
('#PersonID',#PersonID),
('#PlaceID',#PlaceID),
('#ThingID',#ThingID);
SELECT #DynamicValue = CAST(Value as VARCHAR(MAX))
FROM #DynamicValues WHERE Parameter = #DynamicValue
SELECT #DynamicValue