如何设置主键和外键,实现级联更新、级联删除
主键、外键和索引的区别? 主键 外键 索引 定义: 唯一标识一条记录,不能有重复的,不允许为空 表的外键是另一表的主键, 外键可以有重复的, 可以是空 该字段没有重复,但可以有一个空 作用: 用来保证数据完整性 用来和其他表建立联系用的 是提高查询排序的
主键、外键和索引的区别?
主键 | 外键 | 索引 | |
定义: | 唯一标识一条记录,不能有重复的,不允许为空 | 表的外键是另一表的主键, 外键可以有重复的, 可以是空值 | 该字段没有重复值,但可以有一个空值 |
作用: | 用来保证数据完整性 | 用来和其他表建立联系用的 | 是提高查询排序的速度 |
个数: | 主键只能有一个 | 一个表可以有多个外键 | 一个表可以有多个惟一索引 |
设置索引
若要设置外键,在参照表(pc表) 和被参照表(parts表)中,相对应的两个字段必须都设置索引(index)。
对parts表:
ALTER TABLE parts ADD INDEX idx_model (model);
这句话的意思是,为parts表增加一个索引,索引建立在model字段上,给这个索引起个名字叫idx_model。
对pc表也类似:
ALTER TABLE pc ADD INDEX idx_cpumodel (cpumodel);
定义外键
下面为两张表之间建立前面所述的那种“约束”。因为pc的CPU型号必须参照parts表中的相应型号,所以我们将pc表的cpumodel字段设置为“外键”(FOREIGN KEY),即这个键的参照值来自于其他表。
ALTER TABLE pc ADD CONSTRAINT fk_cpu_model
FOREIGN KEY (cpumodel)
REFERENCES parts(model);
级联操作
级联更新:更新主键时,外键也随之更新。
可以在定义外键的时候,在最后加入这样的关键字:
ON UPDATE CASCADE;
即在主表更新时,子表(们)产生连锁更新动作,似乎有些人喜欢把这个叫“级联”操作。
如果把这语句完整的写出来,就是:
ALTER TABLE pc ADD CONSTRAINT fk_cpu_model
FOREIGN KEY (cpumodel)
REFERENCES parts(model)
ON UPDATE CASCADE;
级联删除:删除主键时,外键也随之删除。
ALTER TABLE pc ADD CONSTRAINT fk_cpu_model
FOREIGN KEY (cpumodel)
REFERENCES parts(model)
ON DELETE CASCADE;
级联更新、删除:
on update cascade on delete cascade

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