Execute SQL with "SELECT #daynamic_variable_name" in SQL SERVER - sql-server

Question

Through the exec in SQL Server, I want to retrieve the parameter's value dynamically. Please look into the below Example:
declare #p1 varchar(50) = '10'
declare #p2 varchar(50) = '20'
declare #p_nm varchar(50)
set #p_nm = '#p1' -- OR set #p_nm = '#p2'
declare #sql varchar(50) = 'select #p1'
declare #ans_val varchar(50)
set #ans_val = exec #sql
How could I get the result 10 if using set #p_nm = '#p1' and 20 if using set #p_nm = '#p2'?
In my case set #ans_val = exec #sql giving syntax error.

Answer 1

A variable only has scope within the batch it is inside. When you run dynamic SQL, the dynamic SQL has a different scope, so variables cannot be accessed if they are in a different one. For example, both of the following statements will generate an error:
DECLARE #SQL nvarchar(MAX) = N'SELECT #i;';
DECLARE #i int = 1;
EXEC (#SQL);
GO
DECLARE #SQL nvarchar(MAX) = N'DECLARE #i int = 1;';
EXEC (#SQL);
SELECT #i;
GO
If you are running dynamic SQL, you need to pass details of any variables as parameters, using sp_executesql. For example:
DECLARE #SQL nvarchar(MAX) = N'SELECT #n;';
DECLARE #i int = 1;
EXEC sp_executesql #SQL, N'#n int', #n = #i;
If you want the return scalar values to a variable, use an OUTPUT parameter (personally, I recommend against the syntax {Variable} = EXEC {Expression/Stored Procedure}, one reason being you are therefore limited to one scalar value).
So, again, as an example:
DECLARE #SQL nvarchar(MAX) = N'SET #a = #n + 1;';
DECLARE #i int = 1, #b int;
EXEC sp_executesql #SQL, N'#n int, #a int OUTPUT', #n = #i, #a = #b OUTPUT;
PRINT #b;

Answer 2

I'll do it like:
EXECUTE sp_executesql
N'SELECT #Var AS Result',
N'#Var VARCHAR(5)',
#Var = '12345';
Results:
+----+--------+
| | Result |
+----+--------+
| 1 | 12345 |
+----+--------+
Or
DECLARE #Out VARCHAR(5);
EXECUTE sp_executesql
N'SELECT #Out = #Var',
N'#Var VARCHAR(5) , #Out VARCHAR(5) OUTPUT',
#Var = '12345',
#Out = #Out OUTPUT;
SELECT #Out As Result;
Finally, please don't forgot to visit sp_executesql