MYSQL adds constraints, deletes constraints, adds columns, modifies columns, and deletes columns.

MYSQL adds constraints, deletes constraints to add columns, modifies columns, and deletes columns to add primary key constraints: alter table name add constraint primary key (for example, PK _ table name) primary key table name (primary key field); add foreign key constraint: alter table add constraint foreign key from the table (for example, FK _ slave table _ master table) foreign key: From the table (foreign key field) references main table (primary key field); Delete the primary key constraint: alter table Name drop primary key; Delete the foreign key constraint: alter table Name drop foreign key (case sensitive); add column: alter table name add column name varchar (30); delete column: alter table Name drop column name; modify the column name MySQL: alter table bbb change nnnnn hh int; modify the column attribute: alter table t_book modify name varchar (22 );