答：Where查询条件，on内外连接时候用,as作为别名,in查询某值是否在某条件里
创建2个表：student，score
student：
score：
where
SELECT * FROM student WHERE s_sex='男'
例如：on
SELECT * FROM student LEFT JOIN score on student.s_id=score.s_id;
on和where组合：
SELECT * FROM student LEFT JOIN score on student.s_id=score.s_id WHERE s_name='赵雷'
例如：in
SELECT * FROM score WHERE s_id in (SELECT s_id FROM student WHERE s_name='赵雷')
as
select * from score as a LEFT JOIN student as b on a.s_id=b.s_id where s_name='赵雷'
