MySQL creates a primary key, a foreign key, and a compound primary KEY statement _mysql

1. Create PRIMARY KEY syntax

ALTER TABLE table_name ADD CONSTRAINT pk_name PRIMARY KEY (column name);

2. Create FOREIGN KEY syntax

ALTER Table news_info[child table name] ADD CONSTRAINT fk_news_info_news_type[constraint name] FOREIGN KEY (info_id) [Child table column] REFERENCES news_type[ primary table name] (ID) [primary table column];

3. Use combination primary key

If a column cannot uniquely distinguish a record from a table, consider the uniqueness of multiple columns to distinguish between tables, in the form

① when created: Create TABLE SC (
Studentno int,
CourseID int,
Score int,
Primary KEY (Studentno,courseid));
② modified: ALTER TABLE Tb_name add primary key (Field 1, Field 2, Field 3);