Cursor-fetched variable in prepared statement within a stored procedure?

Question

Hullo,

Can any help me shed some light on what's going on when I try to prepare a statement from a string that contains a cursor-fetched variable?

Below is the procedure that I'm trying to create.

When I create it without the prepared statement, and replace those three lines with SELECT @export_sql_stmt, the output verifies that the SQL statement is being constructed correctly; but with the prepared statement, the value of @export_sql_stmt is NULL. That confuses me...

Any help would be most gratefully received!

DELIMITER //
CREATE PROCEDURE generate_district_files()
BEGIN

-- Declare variables
DECLARE district_val VARCHAR(50); -- The name of the current district
DECLARE no_more_rows BOOLEAN; -- Loop exit condition; set by NOT FOUND handler 

-- Declare the cursor
DECLARE districts_cur CURSOR FOR SELECT district FROM staking_jobs GROUP BY district;

-- Declare handler to set loop exit condition
DECLARE CONTINUE HANDLER FOR NOT FOUND SET no_more_rows = TRUE;

-- Get the districts
OPEN districts_cur;

-- Loop through each district, creating a CSV file for each
the_loop: LOOP

    -- Get the district name
    FETCH districts_cur INTO district_val;

    -- exit the loop if there were no records, or we've processed them all
    IF no_more_rows THEN
        CLOSE districts_cur;
        LEAVE the_loop;
    END IF;

    -- Output the CSV file for this district
    --  
    -- Set up the statement for exporting (required because of dynamic filename)
    SET @export_sql_stmt = CONCAT('SELECT district, \'s\', pick_id',
        ' FROM staking_jobs WHERE district = \'', district_val, '\'',
        ' INTO OUTFILE \'/tmp/to_be_staked_', district_val, '.csv\'',
        ' FIELDS TERMINATED BY \',\' OPTIONALLY ENCLOSED BY \'"\'');

    PREPARE stmt1 FROM @export_sql_stmt;
    EXECUTE stmt1;
    DEALLOCATE PREPARE stmt1;

END LOOP the_loop;

END //
DELIMITER ;

Thank you!