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2207/MySQL练习题/查询练习.md

@@ -407,16 +407,45 @@ FROM (
 )s3;
 
 #19.查询每门课程被选修的学生数
-
+SELECT c.`name`, COUNT(*)
+FROM courses c
+LEFT JOIN scores sc ON sc.course_id = c.id
+GROUP BY c.id;
 #20.查询出只选修两门课程的学生学号和姓名
+SELECT st.id, st.`name`
+FROM students st
+WHERE (
+	SELECT COUNT(*)
+	FROM scores sc
+	WHERE sc.student_id = st.id
+)=2;
 
 #21.查询男生、女生人数
+SELECT st.gender, COUNT(*)
+FROM students st
+GROUP BY st.gender;
+
+SELECT COUNT(IF(st.gender='男', TRUE, NULL)) '男生人数',
+COUNT(IF(st.gender='女', TRUE, NULL)) '女生人数'
+FROM students st;
 
 #22.查询名字中含有「风」字的学生信息
+SELECT *
+FROM students st
+WHERE st.`name` LIKE '%风%';
+
 
 #23.查询同名同性学生名单，并统计同名人数
+SELECT s.`name`, s.gender, COUNT(*)
+FROM students s
+GROUP BY s.`name`, s.gender
+HAVING COUNT(*) > 1;
+
 
 #24.查询 1990 年出生的学生名单
+SELECT *
+FROM students s
+WHERE YEAR(s.birth) = 1990;
 
 #25.查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列