我可以在实际触发语句之前执行MySQL触发器吗？

Question

我有一个子表上的触发器更新父表上的计数器。无论是CREATE TRIGGER ... BEFORE INSERT还是AFTER INSERT，触发器中的SQL总是在执行后执行。

有没有办法强制它运行之前？

CREATE TABLE items (
    id int(11) unsigned NOT NULL AUTO_INCREMENT, 
    quantity_sold int(11) unsigned DEFAULT 0, 
    PRIMARY KEY (id) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci; 

CREATE TABLE line_items (
    id int(11) unsigned NOT NULL AUTO_INCREMENT, 
    item_id int(11) unsigned DEFAULT NULL, 
    quantity int(11) unsigned DEFAULT 1, 
    PRIMARY KEY (id), 
    CONSTRAINT fk FOREIGN KEY (item_id) REFERENCES items (id) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci; 


INSERT INTO items (id) VALUES (1); 

DROP TRIGGER IF EXISTS line_item_trigger; 
delimiter $$ 
CREATE TRIGGER line_item_trigger BEFORE INSERT ON line_items 
FOR EACH ROW 
BEGIN 
    UPDATE items SET quantity_sold = quantity_sold + NEW.quantity WHERE id = NEW.item_id; 
END 
$$ 

运行：

INSERT INTO line_items (item_id) VALUES (1); 

...结果：

INSERT INTO line_items (item_id) VALUES (1); 
UPDATE items SET quantity_sold = quantity_sold + NEW.quantity WHERE id = NEW.item_id; 

我希望能有UPDATE第一发生。

（为什么呢？这是所有在交易完成，并且INSERT被锁定items表时它的外键检查，从而导致死锁所有的地方。）

预先感谢任何见解。

Answer 1

CREATE TRIGGER line_item_trigger BEFORE INSERT ON line_items 
FOR EACH ROW 

该触发器被强制在插入之前调用。没有什么比这更需要做的了。

请检查您的承诺频率根据需要确保您提交交易。

Answer 2

你有没有演示这种行为的例子？

以下示例更新row_count_line_items列中的items表，从BEFORE TRIGGER开始，对line_items表中的行数进行计数。因为它是一个BEFORE TRIGGER，row_count_line_items列为零（0）（INSERT没有在那一刻做）：

mysql> START TRANSACTION; 
Query OK, 0 rows affected (0.00 sec) 

mysql> DROP TRIGGER IF EXISTS `line_item_trigger`; 
Query OK, 0 rows affected (0.00 sec) 

mysql> DROP TABLE IF EXISTS `line_items`, `items`; 
Query OK, 0 rows affected (0.01 sec) 

mysql> CREATE TABLE IF NOT EXISTS `items` (
    -> `id` int(11) unsigned NOT NULL AUTO_INCREMENT, 
    -> `quantity_sold` int(11) unsigned DEFAULT 0, 
    -> `row_count_line_items` int, 
    -> PRIMARY KEY (`id`) 
    ->) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci; 
Query OK, 0 rows affected (0.00 sec) 

mysql> CREATE TABLE IF NOT EXISTS `line_items` (
    -> `id` int(11) unsigned NOT NULL AUTO_INCREMENT, 
    -> `item_id` int(11) unsigned DEFAULT NULL, 
    -> `quantity` int(11) unsigned DEFAULT 1, 
    -> PRIMARY KEY (`id`), 
    -> CONSTRAINT `fk` FOREIGN KEY (`item_id`) REFERENCES items (`id`) 
    ->) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci; 
Query OK, 0 rows affected (0.00 sec) 

mysql> INSERT INTO `items` (`id`) VALUES (1); 
Query OK, 1 row affected (0.01 sec) 

mysql> CREATE TRIGGER `line_item_trigger` BEFORE INSERT ON `line_items` 
    -> FOR EACH ROW 
    -> UPDATE `items` 
    -> SET `quantity_sold` = `quantity_sold` + NEW.`quantity`, 
    ->  `row_count_line_items` = (SELECT COUNT(`id`) 
    ->         FROM `line_items`) 
    -> WHERE `id` = NEW.`item_id`; 
Query OK, 0 rows affected (0.00 sec) 

mysql> INSERT INTO `line_items` (`item_id`) VALUES (1); 
Query OK, 1 row affected (0.01 sec) 

mysql> COMMIT; 
Query OK, 0 rows affected (0.00 sec) 

mysql> SELECT `id`, `quantity_sold`, `row_count_line_items` 
    -> FROM `items`; 
+----+---------------+----------------------+ 
| id | quantity_sold | row_count_line_items | 
+----+---------------+----------------------+ 
| 1 |    1 |     0 | 
+----+---------------+----------------------+ 
1 row in set (0.00 sec) 

mysql> SELECT `id`, `item_id`, `quantity` 
    -> FROM `line_items`; 
+----+---------+----------+ 
| id | item_id | quantity | 
+----+---------+----------+ 
| 1 |  1 |  1 | 
+----+---------+----------+ 
1 row in set (0.00 sec) 

见db-fiddle。

如果我们改变了事件AFTER（AFTER TRIGGER），然后row_count_line_items列将有一个（1）（因为，INSERT已经完成）值：

mysql> START TRANSACTION; 
Query OK, 0 rows affected (0.00 sec) 

mysql> DROP TRIGGER IF EXISTS `line_item_trigger`; 
Query OK, 0 rows affected (0.00 sec) 

mysql> DROP TABLE IF EXISTS `line_items`, `items`; 
Query OK, 0 rows affected (0.00 sec) 

mysql> CREATE TABLE IF NOT EXISTS `items` (
    -> `id` int(11) unsigned NOT NULL AUTO_INCREMENT, 
    -> `quantity_sold` int(11) unsigned DEFAULT 0, 
    -> `row_count_line_items` int, 
    -> PRIMARY KEY (`id`) 
    ->) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci; 
Query OK, 0 rows affected (0.00 sec) 

mysql> CREATE TABLE IF NOT EXISTS `line_items` (
    -> `id` int(11) unsigned NOT NULL AUTO_INCREMENT, 
    -> `item_id` int(11) unsigned DEFAULT NULL, 
    -> `quantity` int(11) unsigned DEFAULT 1, 
    -> PRIMARY KEY (`id`), 
    -> CONSTRAINT `fk` FOREIGN KEY (`item_id`) REFERENCES items (`id`) 
    ->) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci; 
Query OK, 0 rows affected (0.00 sec) 

mysql> INSERT INTO `items` (`id`) VALUES (1); 
Query OK, 1 row affected (0.00 sec) 

mysql> CREATE TRIGGER `line_item_trigger` AFTER INSERT ON `line_items` 
    -> FOR EACH ROW 
    -> UPDATE `items` 
    -> SET `quantity_sold` = `quantity_sold` + NEW.`quantity`, 
    ->  `row_count_line_items` = (SELECT COUNT(`id`) 
    ->         FROM `line_items`) 
    -> WHERE `id` = NEW.`item_id`; 
Query OK, 0 rows affected (0.00 sec) 

mysql> INSERT INTO `line_items` (`item_id`) VALUES (1); 
Query OK, 1 row affected (0.08 sec) 

mysql> SELECT `id`, `quantity_sold`, `row_count_line_items` 
    -> FROM `items`; 
+----+---------------+----------------------+ 
| id | quantity_sold | row_count_line_items | 
+----+---------------+----------------------+ 
| 1 |    1 |     1 | 
+----+---------------+----------------------+ 
1 row in set (0.00 sec) 

mysql> COMMIT; 
Query OK, 0 rows affected (0.00 sec) 

mysql> SELECT `id`, `item_id`, `quantity` 
    -> FROM `line_items`; 
+----+---------+----------+ 
| id | item_id | quantity | 
+----+---------+----------+ 
| 1 |  1 |  1 | 
+----+---------+----------+ 
1 row in set (0.00 sec) 

见db-fiddle。