Creating cursor with Dynamic SQL in MySQL

Question

I am writing a stored procedure, which opens a cursor to a table, and then iterate through all records. In the iterating process I create a dynamic query based on the results of the first cursor.

I need to open the cursor on dynamic SQL, but MySQL is not allowing me to do so. According to the official doc of MySQL: "Cursors must be declared before declaring handlers. Variables and conditions must be declared before declaring either cursors or handlers".

Here is the script:

DELIMITER $$

DROP PROCEDURE IF EXISTS sp_test$$

CREATE PROCEDURE `sp_test`()
BEGIN
    -- Declarations
    
    DECLARE prepared_sql VARCHAR(1000);
    DECLARE index_count INT;

    -- Cursors
    DECLARE cursor1 CURSOR FOR SELECT * from table1;
    -- Continue Handler for Cursor
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET no_more_rows = TRUE;
    -- Open cursors
    OPEN cursor1;

    -- Business Logic
    all_alerts_loop: LOOP
        -- Fetch record from cursor1 and create a dynamic sql
                
        -- Check if cursor has reached to end than leave the loop
        IF no_more_rows THEN
            LEAVE all_alerts_loop;
        END IF;
        
        
        WHILE @some_other_variable <> 0
        DO
                              -- I want to open cursor 2 on this sql
            -- set @prepared_sql =  'create dynamic sql here';  
                    END WHILE;
        
                    -- This works fine
        PREPARE stmt FROM @prepared_sql;
        EXECUTE stmt;

                    -- But can't define cursor here? so what is the solution
                    -- Gives syntax error, I have tried with @prepared_sql also rather than stmt
        DECLARE cursor2 CURSOR FOR stmt;
        
    END LOOP;
    
    -- closing cursors
    CLOSE cursor1;
    END$$

DELIMITER ;

Any idea on how to create cursor for a dynamic query? in MySQL?

Answer 1

It's not allowed a DEFINE cur CURSOR FOR prepared_statement, you must define a valid SQL statement. The good news is that you can define the cursor on a view that can be dynamically created later. For example...

DROP PROCEDURE IF EXISTS my_dynamic_proc;
DELIMITER //
CREATE PROCEDURE my_dynamic_proc(tablename varchar(64), fieldname varchar(64), country VARCHAR(64))
BEGIN
    DECLARE adr_value varchar(500);
    DECLARE done BOOLEAN DEFAULT FALSE;
    -- Cursor definition
    DECLARE cur1 CURSOR FOR SELECT address FROM tmp_view_address;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
    -- Dynamic view definition and creation
    SET @v = concat('CREATE OR REPLACE VIEW tmp_view_address as SELECT `',fieldname,'` as address FROM ',tablename,' WHERE country_name = "',country,'" group by 1 order by count(1) desc');
    PREPARE stm FROM @v;
    EXECUTE stm;
    DEALLOCATE PREPARE stm;
    -- Open cursor
    OPEN cur1;
    read_loop: LOOP
      FETCH cur1 INTO adr_value;
      IF done THEN
        LEAVE read_loop;
      END IF;
      -- Basic output result
      SELECT concat("My address is ",adr_value);
      -- Use every result in a dynamic update
      SET @u = concat('update ',tablename,' set new_field_address = "',adr_value,'" where country_name = "',country,'" and new_field_address is null');
      PREPARE stm FROM @u;
      EXECUTE stm;
      DEALLOCATE PREPARE stm;
  END LOOP;
  CLOSE cur1;
END//
DELIMITER ;

Answer 2

Create an another Procedure and write the code of cursor in this new procedure and then call the procedure from where u want to declare a cursor...