那些SQL语句

根据book_id,class_id确定老师uid

select user_id
from lessons
left join book on lessons.lesson_id = book.lesson_id
where lessons.class_id = 2
and book.id = 1
group by user_id;

根据book_id,class_id,semester_id确定老师uid

select user_id
from lessons
left join book on lessons.lesson_id = book.lesson_id
where lessons.class_id = 2
and book.id = 1  and
semester_id = 2
group by user_id;

分析：写sql要脑袋里考虑很多东西，合理性~根据条件，获取什么~从哪里获取~

题目：

user表

姓名  年龄  性别  出身日期

name  age   sex   birth_day

a 20  0
b 21  1
c 21  0
d 23  1
e 20  1

写sql

年龄  男数  女数

SELECT age AS '年龄', username AS '姓名',SUM(CASE  WHEN sex = '' THEN 1 ELSE 0 END) AS '男数',SUM(CASE WHEN sex='' THEN 1 ELSE 0 END) AS '女数' FROM userinfo GROUP BY age