子查询指将一个查询语句嵌套在另一个查询语句中。子查询可以在 SELECT、UPDATE 和 DELETE 语句中使用，而且可以进行多层嵌套。在实际开发时，子查询经常出现在 WHERE 子句中。子查询在 WHERE 中的语法格式如下：
WHERE <表达式> <操作符> (子查询)
其中，操作符可以是比较运算符和 IN、NOT IN、EXISTS、NOT EXISTS 等关键字。
要求：查出指定范围的最大值，同时显示与之相关的其它信息
实例：查询全班最高分，查询全级最高分，查询全校最高分，同时显示最高分者的姓名，所在班级以及各科成绩情况。
1、查询全班最高分
mysql> select * from studscoreinfo
-> where total_scores = (select max(total_scores) from studscoreinfo where grade_classes =201);
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
| Id | Grade_Classes | Seat_Numbers | Names | Chinese_Scores | Math_Scores | English_Scores | Total_Scores | Score_Averages | Remarks |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
| 22 | 201 | 5 | 陈炜涛 | 88 | 100 | 96 | 284 | 94.7 | |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
1 row in set (0.00 sec)
2、查询全级最高分
mysql> select * from studscoreinfo
-> where total_scores = (select max(total_scores) from studscoreinfo where grade_classes between 101 and 102);
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
| Id | Grade_Classes | Seat_Numbers | Names | Chinese_Scores | Math_Scores | English_Scores | Total_Scores | Score_Averages | Remarks |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
| 9 | 101 | 9 | 赖嘉欣 | 84 | 98 | 96 | 278 | 92.7 | |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
1 row in set (0.00 sec)
3、查询全校最高分
mysql> select * from studscoreinfo
-> where total_scores = (select max(total_scores) from studscoreinfo where grade_classes between 301 and 302);
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
| Id | Grade_Classes | Seat_Numbers | Names | Chinese_Scores | Math_Scores | English_Scores | Total_Scores | Score_Averages | Remarks |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
| 38 | 302 | 2 | 陈资彬 | 89.5 | 98 | 98.5 | 286 | 95.3 | |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
1 row in set (0.00 sec)
第一行*是选所有列进行查询，全显示，也可根据需要指定列名查询，第二行中操作符前面的"="可用in代替，效果一样。