如何从准备好的声明中获得标量结果？

Question

是否可以将预准备语句的结果设置为变量？我试图创建以下存储过程,但它失败了:

第31行的错误1064(42000):SQL语法中有错误; 检查与MySQL服务器版本对应的手册,以便在'stmt USING @m,@ c,@ a附近使用正确的语法;

DROP PROCEDURE IF EXISTS deleteAction;

DELIMITER $$
CREATE PROCEDURE deleteAction(
    IN modul CHAR(64),
    IN controller CHAR(64),
    IN actn CHAR(64))

MODIFIES SQL DATA

BEGIN

    PREPARE stmt FROM 'SELECT id 
                         FROM actions 
                        WHERE `module` = ? 
                          AND `controller` = ? 
                          AND `action` = ?';

    SET @m = modul;
    SET @c = controller;
    SET @a = actn;

    SET @i = EXECUTE stmt USING @m, @c, @a;

    DEALLOCATE PREPARE stmt;

    DELETE FROM acl WHERE action_id = @i;
    DELETE FROM actions WHERE id = @i; 

END 
$$
DELIMITER ;

Answer 1

这可能看起来很奇怪,但您可以直接在预准备语句字符串中分配变量:

PREPARE stmt FROM 'SELECT @i := id FROM ...';

-- ...

EXECUTE stmt USING @m, @c, @a;

-- @i will hold the id returned from your query.

测试用例:

CREATE TABLE actions (id int, a int);

INSERT INTO actions VALUES (1, 100);
INSERT INTO actions VALUES (2, 200);
INSERT INTO actions VALUES (3, 300);
INSERT INTO actions VALUES (4, 400);
INSERT INTO actions VALUES (5, 500);

DELIMITER $$
CREATE PROCEDURE myProc(
    IN p int
)

MODIFIES SQL DATA

BEGIN

    PREPARE stmt FROM 'SELECT @i := id FROM actions WHERE `a` = ?';

    SET @a = p;

    EXECUTE stmt USING @a;

    SELECT @i AS result;

    DEALLOCATE PREPARE stmt;

END 
$$
DELIMITER ;

结果:

CALL myProc(400);

+---------+
| result  |
+---------+
|       4 |
+---------+
1 row in set (0.00 sec)