SQLite外键的实现
SQLite现在的版本还不支持外键功能，虽然外键约束会被解析，但执行的时候被忽略。但我们可以手动实现外键，实现的原理就是触发器。下面是我的实现方法。主要是针对一个例子：
先看下面两个表。
CREATE TABLE PLU (PluID integer NOT NULL PRIMARY KEY,
Name text NOT NULL,
Property text,
Price double NOT NULL,
Left integer NOT NULL,
Department text,
Other text);
CREATE TABLE PluSuit (SuitID integer NOT NULL PRIMARY KEY,
Price double NOT NULL,
Property text,
Name text NOT NULL,
PluID integer NOT NULL CONSTRAINT fk_plu_id REFERENCES PLU(PluID) ON DELETE
CASCADE,Numbers integer NOT NULL);
这样就为PluSuit表建立对PLU表的外键约束，这样就可以实现CORE2数据需求中的要求，问题是SQLite不执行这个约束，所以这样创建以后，我们还要再创建三个触发器，INSERT,UPDATE,DELETE触发器：
create trigger plusuit_insert_trigger
BEFORE INSERT ON PluSuit
FOR EACH ROW BEGIN
SELECT RAISE(ROLLBACK, 'insert on table "PluSuit" violates foreign key constraint "fk_plu_id"')
WHERE (SELECT PluID FROM PLU WHERE PluID = NEW.PluID) IS NULL;
END;
CREATE TRIGGER plusuit_update_trigger
BEFORE UPDATE ON PluSuit
FOR EACH ROW BEGIN
SELECT RAISE(ROLLBACK, 'update on table "PluSuit" violates foreign key constraint "fk_plu_id"')
WHERE (SELECT PluID FROM PLU WHERE PluID = NEW.PluID) IS NULL;
END;
CREATE TRIGGER fkd_plusuit_pluid
BEFORE DELETE ON PLU
FOR EACH ROW BEGIN
DELETE from PluSuit WHERE PluID = OLD.PluID;
END;
下面我们分别来作三个实验：
一、插入实验
首先我们在PLU里面插入一个数据（一双anta运动鞋的信息）：
insert into PLU values(1,'anta','sport',299,100,'sales','ok');
insert into PLU values(3,'nike','sport',699,200,'sales','ok');
然后我们开始在PluSuit里面插入一个数据（两双一起打折卖）：
insert into PluSuit values(100,350,'old','anta',1,2);成功了
insert into PluSuit values(100,350,'old','anta',2,2);失败，得到正确的错误信息
更新实验
update PluSuit set PluID=2 where SuitID=100;失败，得到正确的错误信息
update PluSuit set PluID=3 where SuitID=100;成功
删除实验
delete from PLU where PluID=1;
查看PluSuit中数据被正确删除。
实验结果，触发器的实现完全正确。