To add a local variable value to a dynamic query result set - sql-server

Question

I have a dynamic query where in I want to add the value of a local variable to every row of the result set. I have a simplified version of the example below.
While this query works fine:
DECLARE #purchaseDate AS DATE
SET #purchaseDate = '12/23/2020'
SELECT Name, #purchaseDate
FROM Fruits
The similar one in dynamic SQL does not work:
DECLARE #query AS NVARCHAR(MAX),
#purchaseDate AS DATE
SET #purchaseDate = '12/23/2020'
SET #query = 'SELECT Name, #purchaseDate FROM Fruits'
EXEC sp_executesql #query
I get the error
Must declare the scalar variable "#purchaseDate".
So I assumed I might need to declare my purchaseDate inside the query as the dynamic SQL query cannot access the variable. So I tried this:
DECLARE #query AS NVARCHAR(MAX)
SET #query = 'DECLARE #purchaseDate AS DATE' +
'SET #purchaseDate = ' + '12/23/2020 ' +
'SELECT Name, #purchaseDate FROM Fruits'
EXEC sp_executesql #query
But I get the same error message.
How do I go about fixing it?

Answer 1

You can't reference a variable declared outside of a dynamic statement inside a dynamic statement. You need to add the declaration in the parameters and pass the value:
DECLARE #query AS nvarchar(MAX),
#purchaseDate AS date;
SET #purchaseDate = '20201223';
SET #query = 'SELECT Name,#purchaseDate FROM Fruits;';
EXEC sp_executesql #query, N'#purchaseDate date', #purchaseDate = #purchaseDate;

Answer 2

The recommended way to put a variable in dynamic query with a prepared statement:
if OBJECT_ID('Fruits') is not null drop table Fruits
create table Fruits(
name varchar(100)
)
insert into Fruits
values('apple')
DECLARE #purchaseDate AS DATE
SET #purchaseDate = '12/23/2020'
DECLARE #P1 int;
EXEC sp_prepare #P1 output,
N'#purchaseDate date',
N'SELECT Name, #purchaseDate FROM Fruits';
EXEC sp_execute #P1, #purchaseDate;
EXEC sp_unprepare #P1;
Take a look at the Microsoft Doku.
If you are lazy or just want a simple adhoc solution (not recommended):
if OBJECT_ID('Fruits') is not null drop table Fruits
create table Fruits(
name varchar(100)
)
insert into Fruits
values('apple')
DECLARE #query AS NVARCHAR(MAX),
#purchaseDate AS DATE
SET #purchaseDate = '12/23/2020'
set #query = 'SELECT Name, ' + convert(nvarchar(10),#purchaseDate, 12) + ' FROM Fruits'
exec sp_executesql #query