如何检查变量是否为空，如果为空则将其设置为零

Question

我有两个 table，分别是 Table Item List 和 Table Rekod 。我尝试创建一个触发器，该触发器将更新 table item_list 中特定项目的数量，无论记录是否更新。但是我在输入第一条记录时遇到了问题。触发器将不起作用。它仅在 add 和 minus 记录都已存在时才有效。我尝试了这个，但似乎我也无法再更新我的 rekod table

CREATE TRIGGER update_quantity AFTER UPDATE ON rekod
FOR EACH ROW
BEGIN
    DECLARE add INT;
    DECLARE minus INT;
    DECLARE final INT;
    SET add = 
SELECT (SELECT SUM(rekod.quantity) from rekod where rekod.status = "add" &&rekod.SKU=item_list.SKU )
FROM item_list ;
    SET minus = 
SELECT (SELECT SUM(rekod.quantity) from rekod where rekod.status = "minus" &&rekod.SKU=item_list.SKU )
FROM item_list ;
    IF add IS NULL
        THEN 
        SET add = 0;
    END IF;
    IF minus IS NULL
        THEN
        SET minus = 0;
    END IF;
    SET final = add - minus;
    UPDATE item_list SET Quantity = final;
END;
//
delimiter ;```

Answer 1

如果有充分的理由在对 rekod 进行更新时同步所有时间列表，例如 "when we update rekod we change 10000 rows at a time"

CREATE TRIGGER update_quantity AFTER UPDATE ON rekod
BEGIN

    UPDATE 
      item_list
      INNER JOIN (SELECT sku, SUM(CASE WHEN status = 'add' THEN quantity ELSE -quantity END) as s FROM rekod) x
      ON x.sku = item_list.sku
    SET item_list.quantity = x.s;
END;
//
delimiter;

或者为了保持项目列表逐行同步，例如 "we only ever update one item at a time in rekod"

CREATE TRIGGER update_quantity AFTER UPDATE ON rekod
FOR EACH ROW
BEGIN

    UPDATE 
      item_list
    SET quantity = (SELECT SUM(CASE WHEN status = 'add' THEN quantity ELSE -quantity END) as s FROM rekod WHERE sku = new.sku) 
    WHERE sku = new.sku;

END;
//
delimiter;

不确定您对插入做了什么 - 我认为 rekod 的工作方式类似于历史记录 table，只有一长串加和减

就我个人而言，我只是想知道余额而不是尝试保持项目列表同步

Answer 2

测试每一行的状态并确定是否添加或减去就足够了

喜欢

DELIMITER //
CREATE TRIGGER update_quantity AFTER UPDATE ON rekod
FOR EACH ROW
BEGIN    
    IF NEW.status = "add"  THEN 
       SET  @add = NEW.quantity;
    ELSE
        SET  @add = - NEW.quantity;
    END IF;

    UPDATE item_list SET Quantity = Quantity + @add WHERE sku = new.sku;
END;
//
delimiter ;

如何检查变量是否为空，如果为空则将其设置为零

how to check if Variable is null and set it to zero if it is null

mysql

null

database-trigger