如何检查变量是否为空，如果为空则将其设置为零

Question

我有两个表，分别是表Item List和表Rekod。我尝试创建一个触发器，该触发器将在更新记录的任何地方更新表item_list中特定项目的数量。但是，输入第一条记录时我遇到了问题。触发器将不起作用。仅当加和减记录已经存在时，它才起作用。我尝试了此操作，但似乎我也无法再更新rekod表

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

如果有很好的理由为什么在更新rekod时应该同步所有时间清单，例如“当我们更新rekod时我们一次更改10000行”

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;

或者用于逐行保持项目列表同步，例如“我们一次只能更新一个项目”

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;

不确定您对插入的处理方式-我以为rekod就像一个历史表一样，只有很长的加和减序列

我个人只是想将每一个我想知道余额的尖齿加起来，而不是尝试使项目列表保持同步

Answer 2

应该足以测试每一行的状态并确定是否要加或减

赞

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 ;