Mysql中的嵌套游标

Question

我有三张桌子.
项目(Id),属性(Id),project_attribute(Id,project_id,attribute_id).

我想创建一个记录project_attribute通过使用来自所有属性表属性表,从每个项目的项目表.

要创建此类记录,我使用以下存储过程.

DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `proj_attr`()
BEGIN   
    DECLARE proj_done, attribute_done BOOLEAN DEFAULT FALSE;    
    declare attributeId int(11) default 0;
    declare  projectId int(11) default 0;
    DECLARE curProjects CURSOR FOR SELECT id FROM project order by id;  
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET proj_done = TRUE;

    OPEN curProjects;
    cur_project_loop: LOOP
    FETCH FROM curProjects INTO projectId;

        IF proj_done THEN
        CLOSE curProjects;
        LEAVE cur_project_loop;
        END IF;

        BLOCK2: BEGIN
        DECLARE curAttribute CURSOR FOR SELECT id FROM attribute order by id;
        DECLARE CONTINUE HANDLER FOR NOT FOUND SET attribute_done = TRUE;
        OPEN curAttribute; 
        cur_attribute_loop: LOOP
        FETCH FROM curAttribute INTO attributeId;   
            IF attribute_done THEN
            CLOSE curAttribute;
            LEAVE cur_attribute_loop;
            END IF; 
            insert into project_attribute_value(project_id, attribute_id)
                values(projectId, attributeId); 
        END LOOP cur_attribute_loop;
        END BLOCK2;
    END LOOP cur_project_loop;


    END$$

DELIMITER ;

但是,此过程仅为project_attribute表中的1个项目创建记录,即使Project表中有50个项目.预期记录计数是count(projectId)*count(attributeId).

Answer 1

试试这个,这肯定会解决你的问题.

DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `proj_attr`()
BEGIN   
    DECLARE proj_done, attribute_done BOOLEAN DEFAULT FALSE;    
    declare attributeId int(11) default 0;
    declare  projectId int(11) default 0;
    DECLARE curProjects CURSOR FOR SELECT id FROM project order by id;  
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET proj_done = TRUE;

    OPEN curProjects;
    cur_project_loop: LOOP
    FETCH FROM curProjects INTO projectId;

        IF proj_done THEN
        CLOSE curProjects;
        LEAVE cur_project_loop;
        END IF;

        BLOCK2: BEGIN
        DECLARE curAttribute CURSOR FOR SELECT id FROM attribute order by id;
        DECLARE CONTINUE HANDLER FOR NOT FOUND SET attribute_done = TRUE;
        OPEN curAttribute; 
        cur_attribute_loop: LOOP
        FETCH FROM curAttribute INTO attributeId;   
            IF proj_done THEN
            set proj_done = false;
            CLOSE curAttribute;
            LEAVE cur_attribute_loop;
            END IF; 
            insert into project_attribute_value(project_id, attribute_id)
                values(projectId, attributeId); 
        END LOOP cur_attribute_loop;
        END BLOCK2;
    END LOOP cur_project_loop;


    END$$

DELIMITER ;

Answer 2

很坦率地说,嵌套游标(通常)是一个糟糕的主意.您可以使用法线直接获得所需内容,而无需使用光标CROSS JOIN.

INSERT INTO proj_attr (project, attribute)
    SELECT p.id AS projectid, a.id AS attributeid
    FROM project p CROSS JOIN attribute a;

Answer 3

在内部游标内的第一次迭代之后，“attribute_done”变量被设置为“true”。并且每次下一次迭代它都保持“真实”。

这会导致每次下一次迭代都跳过内部循环。

示例嵌套游标如下所示。

CREATE TABLE `parent` (
  `a` int(11) DEFAULT NULL
) ENGINE=InnoDB
CREATE TABLE `child` (
  `a` int(11) DEFAULT NULL,
  `b` varchar(20) DEFAULT NULL
) ENGINE=InnoDB

insert into parent values (1),(2),(3);

insert into child values (1,'a'),(1,'b'),(2,'a'),(2,'b'),(3,'a'),(3,'b');

----------------------------------
drop procedure if exists nestedCursor;
create procedure nestedCursor()
BEGIN   
    DECLARE done1, done2 BOOLEAN DEFAULT FALSE;  
    DECLARE parentId,childId int;
    DECLARE childValue varchar(30);

    DECLARE cur1 CURSOR FOR SELECT a FROM parent;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done1 = TRUE;

    open cur1;
    loop1: LOOP
    FETCH FROM cur1 INTO parentId;
    IF done1 THEN
        CLOSE cur1;
        LEAVE loop1;
    END IF;

    BLOCK1 : BEGIN
    DECLARE cur2 CURSOR FOR SELECT a,b FROM child where a = parentId;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done2 = TRUE;


    open cur2;
    loop2 : LOOP
    FETCH FROM cur2 INTO childId,childValue;  
        if done2 THEN
        CLOSE cur2;
        SET done2 = FALSE;
        LEAVE loop2;
        end if;
        select parentId,childId,childValue;

    END LOOP loop2;
    END BLOCK1;
    END loop loop1;
END;