Question

CREATE TRIGGER bookAvailDelete after delete on Auction
for each row
Begin
if (Select count(OLD.isbn) from Auction a > 0)
update Book b
set available =false
where OLD.isbn = isbn
;

我有两张桌子，拍卖和书。拍卖有isbn关键，参考Book isbn主键。许多拍卖可以指向同一本书，所以我希望这个触发器检查每次拍卖删除，看看它是否是该书的最后一次拍卖isbn。如果是，则将Book中的“available”键设置为false。

Answer 1

请在下面编译测试。使用变量作为计数，如果最后也将结束。

delimiter //
CREATE TRIGGER bookAvailDelete after delete on Auction
for each row
Begin
DECLARE updatecount INT;
set updatecount = ( Select count(*) from Auction a where a.isbn = OLD.isbn);
if updatecount > 0 then
update Book b
set b.available =false
where b.isbn = OLD.isbn;
end if;
End;//

Answer 2

通常情况下，除非出现性能原因，否则您只需动态计算（而不是存储），查询如下：

SELECT b.*
   , EXISTS(SELECT * FROM Auction a WHERE a.isbn = b.isbn) AS available
FROM Book b

或（我的首选格式）：

SELECT b.*, (a.isbn IS NOT NULL) AS available
FROM Book b
LEFT JOIN (SELECT DISTINCT isbn FROM Auction) AS a ON b.isbn = a.isbn
;

Answer 3

假设可用的是int：

drop trigger bookAvailDelete;

DELIMITER //
CREATE TRIGGER bookAvailDelete after delete on Auction
    for each row begin
        if NOT EXISTS(Select * from Auction where isbn = old.isbn)
        then
            update Book b set b.available = 0 where b.isbn = old.isbn;
        end if; 
    end;

sql触发两个表不为空

3 个答案: