子查詢指將一個查詢語句嵌套在另一個查詢語句中。子查詢可以在 SELECT、UPDATE 和 DELETE 語句中使用，而且可以進行多層嵌套。在實際開發時，子查詢經常出現在 WHERE 子句中。子查詢在 WHERE 中的語法格式如下：
WHERE <表達式> <操作符> (子查詢)
其中，操作符可以是比較運算符和 IN、NOT IN、EXISTS、NOT EXISTS 等關鍵字。
要求：查出指定范圍的最大值，同時顯示與之相關的其它信息
實例：查詢全班最高分，查詢全級最高分，查詢全校最高分，同時顯示最高分者的姓名，所在班級以及各科成績情況。
1、查詢全班最高分
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 | 陳煒濤 | 88 | 100 | 96 | 284 | 94.7 | |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
1 row in set (0.00 sec)
2、查詢全級最高分
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 | 賴嘉欣 | 84 | 98 | 96 | 278 | 92.7 | |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
1 row in set (0.00 sec)
3、查詢全校最高分
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 | 陳資彬 | 89.5 | 98 | 98.5 | 286 | 95.3 | |
+------+---------------+--------------+-----------+----------------+-------------+----------------+--------------+----------------+---------+
1 row in set (0.00 sec)
第一行*是選所有列進行查詢，全顯示，也可根據需要指定列名查詢，第二行中操作符前面的"="可用in代替，效果一樣。