如果在觸發器中爲空，則設置變量

Question

我有一個用於排序類別的「訂單」列的類別表。 我想在觸發之前插入遞增設置順序：

CREATE TRIGGER `categories_set_order` BEFORE INSERT ON `categories` FOR EACH ROW BEGIN 
    DECLARE max_order INT DEFAULT 0; 
    SELECT MAX(categories.order) INTO max_order FROM categories; 
    SET NEW.order = max_order + 1; 
END; 

但是如果在數據庫中沒有記錄，訂單列被設置爲NULL。 我已經修改了觸發代碼：

CREATE TRIGGER `categories_set_order` BEFORE INSERT ON `categories` FOR EACH ROW BEGIN 
    DECLARE max_order INT DEFAULT 0; 
    SELECT MAX(categories.order) INTO max_order FROM categories; 
    IF (ISNULL(@max_order)) THEN BEGIN 
     SET max_order = 0; 
    END; 

    SET NEW.order = max_order + 1; 
END; 

And I'm getting the following error: 
Error Code: 1064. 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 '' at line 9 

線9：

IF（ISNULL（@max_order））然後開始

我試着刪除 「@」，但仍然得到相同的錯誤。如何解決它？

Answer 1

您需要更改分隔符。否則數據庫認爲你的觸發器定義在第一個;結束，這將是不完整的。

而且從IF語句刪除BEGIN並添加使用END IF結束你IF

 
delimiter | 
CREATE TRIGGER `categories_set_order` BEFORE INSERT ON `categories` 
FOR EACH ROW BEGIN 
    DECLARE max_order INT DEFAULT 0; 
    SELECT MAX(categories.order) INTO max_order FROM categories; 
    IF (ISNULL(@max_order)) THEN 
     SET max_order = 0; 
    END IF; 

    SET NEW.order = max_order + 1; 
END 
| 
delimiter ; 

Answer 2

除了分隔符，你需要一個END IF

DELIMITER // 
CREATE TRIGGER `categories_set_order` BEFORE INSERT ON `categories` FOR EACH ROW BEGIN 
    DECLARE max_order INT DEFAULT 0; 
    SELECT MAX(categories.order) INTO max_order FROM categories; 
    IF (ISNULL(@max_order)) THEN 
     SET max_order = 0; 
    END IF; 
    SET NEW.order = max_order + 1; 
END; // 
DELIMITER ;