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 first cursor. I need to open the cursor on dynamic sql, but MySQL is not allowing to do me so, as accoriding 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 how to create cursor for 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 ;