where （计算出的值）

本质：在where语句中嵌套一个子查询语句

代码演示：

-- 1. 查询数据库结构-1 的所有考试结果（学号，科目名称，成绩），降序排列-- 方式一：使用连接查询
SELECT studentno,subjectname,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)
SELECT studentno,subjectno,studentresult
FROM result 
WHERE subjectno = (SELECT subjectno FROM `subject` WHERE  subjectname = '数据库结构-1'  -- 查询所有数据库结构-1 的学生学号
)
ORDER BY studentresult DESC-- 2. 分数不小于80分的学生的学号和姓名
SELECT DISTINCT s.studentno,studentname  -- DISTINCT:去重
FROM student s
INNER JOIN result r
ON r.studentno = s.studentno
WHERE studentresult>=80-- 在此基础上增加一个科目
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'
)-- 再改造(由里及外)
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')
)