学习mysql

docs/MySQL/mysql是怎样使用的.md

@@ -993,15 +993,72 @@ SELECT CONCAT('学科:', subject), COUNT(student_score.number) AS 人数 FROM st
 ## 11.2 标量子查询
 
 - 标量子查询代表的仅仅是一个值
+- 子查询必须用括号括起来
 
+```sql
+-- 标量子查询 查名字叫狗哥的人的所有学科成绩
+SELECT * FROM student_score WHERE number = (SELECT number FROM student_info WHERE name = '狗哥');
+
+-- 查比狗哥学号大的所有人的信息
+SELECT * FROM student_info WHERE number > (SELECT number FROM student_info WHERE name = '狗哥');
+```
+
+## 11.3 列子查询
+
+```sql
+-- 列子查询 查计算机科学与工程的学生的成绩
+SELECT * FROM student_score WHERE number IN (SELECT number FROM student_info WHERE major = '计算机科学与工程');
+```
+
+## 11.4 行子查询
+
+- 加入不知道子查询会出现几条记录，加上`LIMIT 1`吧
+- 两边均要用括号
+
+```sql
+SELECT * FROM student_score WHERE (number, subject) = (SELECT number, 'MySQL是怎样运行的' FROM student_info LIMIT 1);
+```
 
+## 11.5 表子查询 (多行多列)
 
+```sql
+-- 此版本的MySQL还不支持“LIMIT&IN/ALL/AANY/SOME子查询” MYSQL8.3
+SELECT * FROM student_score WHERE (number, subject) IN (SELECT number, 'MySQL是怎样运行的' FROM student_info WHERE major = '计算机科学与工程');
+```
 
+## 11.6 `EXISTS` 和 `NOT EXISTS`子查询
 
+```sql
+-- 如果存在记录就查
+SELECT * FROM student_info WHERE EXISTS(SELECT * FROM student_info WHERE number = 10086);
+-- 如果不存在记录就查
+SELECT * FROM student_info WHERE NOT EXISTS(SELECT * FROM student_info WHERE number = 10086);
+-- 相等
+SELECT * FROM student_info WHERE (SELECT COUNT(*) FROM student_info WHERE number != 10086) > 1;
+```
 
+## 11.7 不相关子查询和相关子查询
 
+- 像上面的，外层查询和子查询不想管，独立运行叫做不相关查询
+- 相关查询是，外层查询的值需要被子查询应用，外层查询每查到一行，再拿行数据和子查询进行匹配
 
+```sql
+-- 相关子查询 查存在成绩的学生信息
+SELECT * FROM student_info a WHERE EXISTS(SELECT number FROM student_score b WHERE  a.number = b.number);
+-- 显然这个效率很低
+SELECT * FROM student_info a WHERE number = IF((SELECT COUNT(number) FROM student_score b WHERE  a.number = b.number) > 0, a.number, NULL);
+```
+
+## 11.8 对同一个表的子查询
+
+```sql
+-- 查大于平均分的成绩
+SELECT * FROM student_score WHERE score > (SELECT AVG(score) FROM student_score);
+-- 查和狗哥一个学院的人
+SELECT * FROM student_info WHERE department = (SELECT department FROM student_info WHERE name = '狗哥');
+```
 
+# 第十二章 连接查询