我目前正在经历试图了解函数和触发器的成长之痛。我试图从我正在阅读的书中解决问题,但我不知道如何做某些部分。触发器和函数的麻烦
使用该表
create table movies (
id integer primary key,
title varchar(255) not null,
year integer
);
insert into movies values (1, 'The Croods', 2013);
insert into movies values (2, 'Now You See Me', 2013);
insert into movies values (3, 'Argo', 2012);
insert into movies values (4, 'Jurassic World', 2015);
create table discs (
id integer primary key,
movie_id integer not null references movies(id),
type_id integer references disc_types(id),
price decimal(10,2),
available boolean
);
insert into discs values (1, 1, 1, 1.59, 't');
insert into discs values (2, 1, 1, 1.59, 'f');
insert into discs values (3, 1, 2, 2.99, 'f');
insert into discs values (4, 2, 1, 1.29, 't');
insert into discs values (5, 2, 1, 1.29, 't');
insert into discs values (6, 2, 2, 2.99, 't');
insert into discs values (7, 3, 2, 2.59, 't');
insert into discs values (8, 3, 2, 2.59, 't');
create table customers (
id integer primary key,
name varchar(255),
email varchar(255)
);
insert into customers values (1, 'John', '[email protected]');
insert into customers values (2, 'Jane', '[email protected]');
create table rentals (
id integer primary key,
customer_id integer not null references customers(id),
disc_id integer not null references discs(id),
date_rented date,
date_returned date
);
insert into rentals values (1, 1, 7, '2013-10-01', '2013-10-03');
insert into rentals values (2, 2, 5, '2013-10-05', '2013-10-06');
insert into rentals values (3, 2, 2, '2013-11-02', null);
insert into rentals values (4, 2, 3, '2013-11-02', null);
create table ratings (
customer_id integer not null references customers(id),
movie_id integer not null references movies(id),
rating integer,
primary key (customer_id, movie_id)
);
insert into ratings values (1, 1, 1);
insert into ratings values (1, 2, 4);
insert into ratings values (1, 3, 5);
insert into ratings values (2, 1, 4);
我的逻辑是,我本来是要被插入的收视率表的新值或更新,并使用它们来比较最新的租金表,看是否该客户已经租用了该电影,如果他们这样做了,那么他们可以输入评级。但我不能在这个大声笑中转移那个逻辑。除非有更简单的方法来做到这一点。
你甚至不需要一个数,你可以使用'EXISTS(SELECT * FROM出租r,其中r.uid = new.uid和r.movie_id = new.movie)'在函数中。 (第二个想法,这甚至可能是一个FK的约束?) – wildplasser
@ e4c5即时了解如何计数()如何帮助大声笑 – henryzo
好吧,所以这个函数计数时,新的信息匹配旧的信息权利?所以如果它匹配了1,所以如果租用> 0,那么id在收视率中创建一个新的记录?或者我想念这个大声笑 – henryzo