text: # Q3
  # 查询在SC表存在成绩的学生信息
  # 以下分别是子查询和联结，虽然子查询看上去比较简单，但是大部分其实DBMS对联结有查询优化
  select *
  from Student as s
  where s.SId in (select distinct SId from SC);
  # 或
  select DISTINCT s.*
  from Student s,
       SC sc
  where s.SId = sc.SId;
  
  # +---+-----+-------------------+----+
  # |SId|Sname|Sage               |Ssex|
  # +---+-----+-------------------+----+
  # |01 |赵雷   |1990-01-01 00:00:00|男   |
  # |02 |钱电   |1990-12-21 00:00:00|男   |
  # |03 |孙风   |1990-12-20 00:00:00|男   |
  # |04 |李云   |1990-12-06 00:00:00|男   |
  # |05 |周梅   |1991-12-01 00:00:00|女   |
  # |06 |吴兰   |1992-01-01 00:00:00|女   |
  # |07 |郑竹   |1989-01-01 00:00:00|女   |
  # +---+-----+-------------------+----+
  
  