请教一条SQL语句 - Java / Java EE

create table student
(
 s_id int, 
 s_name varchar(20),
 s_age int
)

create table teacher
(
t_id int,
t_name varchar(20),
t_age int
)

create table si
(
t_id int,
s_id int
)
查询出所有老师代课的名字，和数量，条件是老师的年龄大于30，学生的年龄是大于12

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貌视条件不够·
create table si
(
t_id int,
s_id int
)
这个表代表什么具体意思？

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引用

貌视条件不够·
create table si
(
t_id int,
s_id int
)
这个表代表什么具体意思？

这是teacher和student的外键表

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select t.t_name from si join teacher t on si.t_id = t.t_id join student s on si.s_id = s.s_id where t.t_age > 30 and s.s_age > 12

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条件不够吧？要查出代课的名字和数量，怎么看不到有关课程的table呢？

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引用

select t.t_name from si join teacher t on si.t_id = t.t_id join student s on si.s_id = s.s_id where t.t_age > 30 and s.s_age > 12

up！

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这是我自己写的
insert into students values(1,'aa',22);
insert into student values(2,'bb',21);
insert into student values(3,'cc',24);
insert into student values(4,'dd',26);


insert into teacher values(1,'a1',43);
insert into teacher values(2,'b2',51);
insert into teacher values(3,'