12.MySQL

4.6子查询

where(这个值是计算出来的)
本质：在where语句中嵌套一个子查询语句
where(select*from)
  
  
– 1、查询 数据库结构-1 得所有考试结果（学号，科目编号，成绩），降序排列
– 方式一：连接查询
SELECT studentno,r.subjectno,studentresult
FROM result r
INNER JOIN subject sub
ON r.subjectno = sub.subjectno
WHERE subjectname = ‘数据库结构-1’
ORDER BY studentresult DESC

– 方式二：使用子查询（由里及外的）
SELECT studentno,subjectno,studentresult
FROM result
WHERE subjectno = (
SELECT subjectno FROM subject
WHERE subjectname = ‘数据库结构-1’
)

  
  
– 练习 2、查询高等数学-1的分数不小于80的学生的学号和姓名

– 方式一：连接查询
SELECT DISTINCT s.studentno,studentname
FROM student s
INNER JOIN result r
ON s.studentno = r.studentno
INNER JOIN subject sub
ON sub.subjectno = r.subjectno
WHERE subjectname = ‘高等数学-1’ AND studentresult>=80

– 方式二：子查询
SELECT DISTINCT s.studentno,studentname – distinct去重
FROM student s
INNER JOIN result r
ON r.studentno = s.studentno
WHERE studentresult>=80 AND subjectno = (
SELECT subjectno FROM subject
WHERE subjectname = ‘高等数学-1’
)

– 方式三：嵌套查询
SELECT studentno,studentname FROM student WHERE studentno IN (
SELECT studentno FROM result WHERE studentresult>=80 AND subjectno = (
SELECT subjectno FROM subject WHERE subjectname = ‘高等数学-1’
)
)

  
  
遇到的问题：

• 报错信息翻译:

子查询返回超过1行

- 分析与解决方法:

1. 在重复写入时会出现这种问题, 可通过去掉重复数据解决

- 通过在写入时加逻辑判断或者外键防止数据重复写入