Variable in a dynamic query - sql

Question

I have a problem once I want to run the next query:
declare #j int = 1;
declare #column varchar(255);
set #column = 'last_name';
declare #tmp varchar(255);
declare #query nvarchar(255) = N'select ' + #column + N' from TEST where id = #j'
declare #tbl table(tmp varchar(255))
insert into #tbl
exec sp_executesql #query
select top 1 #tmp = tmp from #tbl
select #tmp
select * from #tbl;
The problem is that if I change the variable #j to a numeric value in the declaration of the #query variable, like this
declare #query nvarchar(255) = N'select ' + #column + N' from TEST where id = 1'
the query is running successfully, if I left the #j variable there, like this
declare #query nvarchar(255) = N'select ' + #column + N' from TEST where id = #j'
I got an error message:
"Must declare the scalar variable #j."
Why? And how can I solve that my query would work with the variable #j?

Answer 1

in the place of ==== where id = #j'
change like this may it works
Convert(nvarchar(2), #j);

Answer 2

You need to cast the #j variable to string. AS its type is INT you should cast it as
CAST(#j AS VARCHAR(12))
Also, you can pass parameters to dynamic SQL statement when it is executed using sp_executesql. In your case it will be something like this:
declare #j int = 1;
declare #column varchar(255);
set #column = 'last_name';
declare #tmp varchar(255);
declare #query nvarchar(255) = N'select ' + #column + N' from TEST where id = #j'
declare #tbl table(tmp varchar(255))
insert into #tbl
exec sp_executesql #query, N'#j INT', #j = #j
select top 1 #tmp = tmp from #tbl
select #tmp
select * from #tbl;
In the following line:
exec sp_executesql #query, N'#j INT', #j = #j
#query is your dynamic T-SQL statement
N'#j INT' is declaration of parameters
#j = #j is parameters assignments
sp_executsql documentation

Answer 3

You can use parameters in the sp_executesql statement like this:
CREATE TABLE TempTable (
TempID INT IDENTITY(1,1) NOT NULL,
SomeDescription VARCHAR(255) NOT NULL,
PRIMARY KEY(TempID))
INSERT INTO TempTable (SomeDescription)
VALUES ('Description 1'),
('Description 2'),
('Description 3')
DECLARE #sql NVARCHAR(500) = 'SELECT * FROM TempTable WHERE TempID = #TempVar',
#params NVARCHAR(500) = '#TempVar int',
#j INT = 2;
EXEC sp_executesql #sql, #params, #TempVar = #j;

Answer 4

declare #j int = 1;
declare #column varchar(255);
set #column = 'last_name';
declare #tmp varchar(255);
declare #query nvarchar(255) = N'select ' + #column + N' from TEST where id ='+ CAST(#j AS VARCHAR(5))
declare #tbl table(tmp varchar(255))
insert into #tbl
exec (#query)
select top 1 #tmp = tmp from #tbl
select #tmp
select * from #tbl;
enter code here