总是忘记的mysql语句
设置外键alter table table_name_1 add constraint fk_外键名字 foreign key (column_name_1) references table_name_2(column_name_2)
设置联合主键
alter table table_name add constraint pk_主键名字 primary key(column_name_1,column_name_2) 删除列
alter table table_name drop column column_name 添加列
alter table table_name add column column_name column_type 删除表中所有数据
truncate table table_name;
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