假设student表有以下记录
STUDENT_NO SEX AGE
----------------------------
1 M 20
2 F 30
3 NULL NULL
TEACHER 表有以下记录
STUDENT_NO TEACHER
--------------------------
1 Tim
1 Jack
2 Tim
查询语句select count(*) from student S LEFT JOIN TEACHER T on S.STUDENT_NO=T.STUDENT_NO; 的结果是