CREATE table student( s_id integer, s_name varchar(100), gender char(2), PIN CHAR(18), UNIQUE (PIN) );#’指定一个候选键’
CREATE table student_BAK20230910( s_id integer, s_name varchar(100), gender char(2), PIN CHAR(18), UNIQUE (PIN,S_ID) );#’指定多个组合候选键’
CREATE table student_score( s_id int, c_id int, score int, primary key(c_id) );#指定主键
CREATE table student_score( s_id int , primary key, c_id int, score int);#指定主键
CREATE table student_score_bak20230910( s_id int, c_id int, score int, primary key(c_id,s_id) );#指定组合主键
CREATE table student_score2( s_id int , c_id int, score int, primary key(c_id), foreign key(s_id) references student(s_id) );#指定外键主键 #被关联表的字段必须是主键才行
alter table student add constraint primary key(s_id); #增加主键
alter table student drop constraint primary key(s_id); #删除主键
