mysql给一张表新增字段，并设置该字段为外键
首先给usercategory表新增libraryid字段：
alter table usercategory add libraryid varchar(50)
修改picturelibrary表的libraryid字段为表usercategory的外键
alter table usercategory add constraint libraryid foreign key(libraryid) references picturelibrary(libraryid)