mysql - mysql dynamic query inside stored procedure

Question

I wrote the following dynamic query in my stored procedure,

FETCH cur1 INTO a1, ldt1, b1;
    WHILE DONE = 0 DO

        SET @s=CONCAT('UPDATE ',fname , ' SET connectedDateTime = ldt1,opid = b1 
        WHERE hq_conferee_seqno=a1 AND (LoggedDateTime <=ldt1 AND connectedDateTime IS NULL)');
        FETCH cur1 INTO a1, ldt1, b1;
        PREPARE stmt FROM @s;
        EXECUTE stmt;
    END WHILE; 
    CLOSE cur1;

When I compile the stored I am not getting any error. But When I run this code I got an error like You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NULL' at line 1

What will be the problem. Please help me. Thanks in advance.

Answer 1

如手册所述：

局部变量仅在存储程序执行期间才在作用域内。在准备好的语句中不允许引用它们，因为它们对于当前会话是全局的，并且在执行语句时变量可能已经超出范围。

相反，您应该将光标获取到用户变量中，并从准备好的语句中引用那些（它们在连接期间保持在范围内），或者（更好）将它们作为参数传递给准备好的语句：

SET @s=CONCAT('UPDATE ',fname , ' SET connectedDateTime = ?,opid = ? 
WHERE hq_conferee_seqno=? AND (LoggedDateTime <=? AND connectedDateTime IS NULL)');
FETCH cur1 INTO @a1, @ldt1, @b1;
PREPARE stmt FROM @s;
EXECUTE stmt USING @ldt1, @b1, @a1, @ldt1;