DELIMITER $$
DROP PROCEDURE IF EXISTS `do_thing` $$
CREATE PROCEDURE `do_thing`()
BEGIN
        DECLARE how_many INT;
        SELECT COUNT(*) FROM things INTO how_many;
        -- Prepare roleback.
        DECLARE `_rollback` BOOL DEFAULT 0;
        DECLARE CONTINUE HANDLER FOR SQLEXCEPTION SET `_rollback` = 1;
        -- Start transaction.
        START TRANSACTION;
                -- Do all the things.
        IF `_rollback` THEN
                ROLLBACK;
        ELSE
                COMMIT;
        END IF;
END $$
DELIMITER ;

DELIMITER $$
DROP PROCEDURE IF EXISTS `do_thing` $$
CREATE PROCEDURE `do_thing`()
BEGIN
       -- DECLARE STATEMENTS first
        DECLARE how_many INT;
        DECLARE `_rollback` BOOL DEFAULT 0;
        DECLARE CONTINUE HANDLER FOR SQLEXCEPTION SET `_rollback` = 1;
        --
        SELECT COUNT(*) INTO how_many FROM things;

        -- Start transaction.
        START TRANSACTION;
                -- Do all the things.
        IF `_rollback` THEN
                ROLLBACK;
        ELSE
                COMMIT;
        END IF;
END $$
DELIMITER ;