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：

 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;

 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。 

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