mysql - 在 MySQL 中使用游标时从 DECLARE 语句调用存储过程

Question

我试图在 MySQL 中使用游标多次调用存储过程。我想多次调用它，因为 my_id 的值存在于某个临时表中，并遍历这些 ID 并连接结果。

无论如何，我在这部分过程中遇到了问题:

DECLARE curs CURSOR FOR  
  SELECT something FROM somewhere;

我不想从某个地方选择一些东西。我想要类似的东西

DECLARE curs CURSOR FOR  
  CALL storedproc(@an_id);

DECLARE 语句可以用来调用存储过程吗？或者它是否必须仅与 SELECT 相关联？谷歌了一下，恐怕是后者吧。

Answer 1

使用游标需要一些标准的样板代码来包围它。

使用游标为表中的每组值调用存储过程需要基本相同的样板。您 SELECT 您想要传递的值，无论您从哪里获取它们(可以是临时表、基表或 View ，并且可以包括对存储函数的调用)，然后调用过程具有这些值。

我在下面编写了该样板代码的语法有效示例，并附有注释以解释每个组件的作用。没有什么比被要求“只是因为”做某事更让我不喜欢的了——所以一切都(希望)得到了解释。

您提到用多个值调用过程，所以这个例子使用 2。

请注意，由于某种原因，她发生的事件按特定顺序排列。变量必须首先声明，游标必须在它们的继续处理程序之前声明，循环必须遵循所有这些事情。这给人的印象是这里有一些相当极端的僵化，但事实并非如此。您可以通过在过程主体内的 BEGIN ... END block 中嵌套附加代码来重置顺序；例如，如果在循环内需要第二个游标，只需在循环内声明它，在另一个 BEGIN ... END 内。

DELIMITER $$

DROP PROCEDURE IF EXISTS `my_proc` $$
CREATE PROCEDURE `my_proc`(arg1 INT) -- 1 input argument; you might not need one
BEGIN

-- from http://stackoverflow.com/questions/35858541/call-a-stored-procedure-from-the-declare-statement-when-using-cursors-in-mysql

-- declare the program variables where we'll hold the values we're sending into the procedure;
-- declare as many of them as there are input arguments to the second procedure,
-- with appropriate data types.

DECLARE val1 INT DEFAULT NULL;
DECLARE val2 INT DEFAULT NULL;

-- we need a boolean variable to tell us when the cursor is out of data

DECLARE done TINYINT DEFAULT FALSE;

-- declare a cursor to select the desired columns from the desired source table1
-- the input argument (which you might or might not need) is used in this example for row selection

DECLARE cursor1 -- cursor1 is an arbitrary label, an identifier for the cursor
 CURSOR FOR
 SELECT t1.c1, 
        t1.c2
   FROM table1 t1
  WHERE c3 = arg1; 

-- this fancy spacing is of course not required; all of this could go on the same line.

-- a cursor that runs out of data throws an exception; we need to catch this.
-- when the NOT FOUND condition fires, "done" -- which defaults to FALSE -- will be set to true,
-- and since this is a CONTINUE handler, execution continues with the next statement.   

DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

-- open the cursor

OPEN cursor1;

my_loop: -- loops have to have an arbitrary label; it's used to leave the loop
LOOP

  -- read the values from the next row that is available in the cursor

  FETCH NEXT FROM cursor1 INTO val1, val2;

  IF done THEN -- this will be true when we are out of rows to read, so we go to the statement after END LOOP.
    LEAVE my_loop; 
  ELSE -- val1 and val2 will be the next values from c1 and c2 in table t1, 
       -- so now we call the procedure with them for this "row"
    CALL the_other_procedure(val1,val2);
    -- maybe do more stuff here
  END IF;
END LOOP;

-- execution continues here when LEAVE my_loop is encountered;
-- you might have more things you want to do here

END $$

DELIMITER ;