MySQL 创建主键,外键和复合主键的语句

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MySQL 创建主键，外键和复合主键的方法，需要的朋友可以参考下。

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1.创建主键语法 

ALTER TABLE table_name ADD CONSTRAINT pk_name PRIMARY KEY(列名); 

2.创建外键语法 

ALTER TABLE news_info[子表名] ADD CONSTRAINT FK_news_info_news_type[约束名] FOREIGN KEY (info_id)[子表列] REFERENCES news_type[主表名] (id)[主表列] ; 

3.使用组合主键 

如果一列不能唯一区分一个表里的记录时，可以考虑多个列组合起来达到区分表记录的唯一性，形式 

①创建时：create table sc ( 
studentno int, 
courseid int, 
score int, 
primary key (studentno,courseid) ); 
②修改时：alter table tb_name add primary key (字段1,字段2,字段3);