mysql语法问题

Question

我正在尝试在 MySQL 上创建触发器，但我遇到了语法问题，我无法找到。如果有更多经验的人可以帮助我，那就太好了（这是我第一次使用 MySQL！）...

我创建此触发器的原因是为了删除所有与“service_descriptor”具有多对多关系的孤立“标签”（这两个实体由 service_labels 链接）。

我的代码是：

CREATE TRIGGER `trg_delete_orphan_label` AFTER DELETE 
FOR EACH ROW ON `restdb`.`service_labels`  
  DELETE FROM `restdb`.`labels`
   WHERE EXISTS (SELECT *
                   FROM old D 
              LEFT_JOIN `restdb`.`service_labels` SL ON SL.`id_label` = D.`id_label` 
                                                    AND D.`id_service` = SL.`id_service`
                  WHERE SL.`id_label` IS NULL
                        `restdb`.`labels`.`id` = D.SL.`id_label`); 

提前致谢！

Answer 1

你错过了AND

试试这个代码

CREATE TRIGGER `trg_delete_orphan_label` AFTER DELETE FOR EACH ROW ON `restdb`.`service_labels`   
    DELETE 
    FROM 
        `restdb`.`labels` 
    WHERE 
        EXISTS (SELECT 
                    * 
                FROM 
                    old D LEFT_JOIN 
                    `restdb`.`service_labels` SL ON 
                        SL.`id_label` = D.`id_label` AND 
                        D.`id_service` = SL.`id_service` 
                WHERE 
                    SL.`id_label` IS NULL AND
                    `restdb`.`labels`.`id` = D.SL.`id_label`                         
                    );  

Answer 2

您的触发器存在一些问题，最明显的是“FOR EACH ROW”的位置，以及您将 OLD 视为表格的方式，而实际上它只是一行。

这应该适合你：

DROP TRIGGER IF EXISTS `trg_delete_orphan_label`;

CREATE TRIGGER `trg_delete_orphan_label` AFTER DELETE ON `service_labels` 
FOR EACH ROW
    DELETE FROM `labels`
    WHERE `id` = OLD.`id_label`
    AND NOT EXISTS (
        SELECT NULL
        FROM `service_labels` SL 
        WHERE SL.`id_label` = `labels`.`id`                        
    );                     

Answer 3

谢谢大家...由于您的帮助，我终于解决了...

最后的工作是：

删除后创建触发器trg_delete_orphan_labelrestdb.service_descriptor
每一行 从restdb.labels 删除 在哪里id不在（ 选择restdb.service_labels.id_label 来自restdb.service_labels);