Question

我有一个这样的表course：

studentName  courseName

 Alen        basketball
 Alen        football
 Alen        tennis
 Bob         tennis
 Dean        football
 Charlie     football
 Charlie     basketball

我想选择同时选择两者篮球和足球的学生。

预期输出：

studentName  courseName

 Alen        basketball
 Alen        football
 Alen        tennis
 Charlie     football
 Charlie     basketball

如何有效地做到这一点？

我当前正在使用此sql。它工作正常，但似乎很慢：

select * from course as ss1 where 
exists 
(select * from course as ss2 where ss2.studentName = ss1.studentName and ss2.courseName = 'basketball')
and exists
(select * from course as ss2 where ss2.studentName = ss1.studentName and ss2.courseName = 'football')
order by ss1.studentName desc

Answer 1

您可以尝试使用EXIST来代替使用JOIN来获取和比较相交的数据。

下面用sql-server编写的示例。

SELECT wholeStudents.*
FROM course wholeStudents
JOIN
  (SELECT DISTINCT StudentName
   FROM course
   WHERE CourseName IN ('basketball', 'football')
   GROUP BY StudentName 
   HAVING COUNT(1) =2) 
   AS disticnctStudents -- this table will only have a list of students selected both sports
ON (disticnctStudents.StudentName = wholeStudents.StudentName)

注意：根据这样的假设，studentName是唯一的，并且学生只能进行一项运动。

Answer 2

select ss1.studentName, ss1.courseName c1, ss2.courseName c2 from course as ss1 
LEFT JOIN course as ss2 on ss1.studentName = ss2.studentName
where c1 != c2 and (c1 ='basketball' and c2 ='football')

选择学生姓名

select ss1.studentName from course as ss1 
LEFT JOIN course as ss2 on ss1.studentName = ss2.studentName
where ss1.courseName != ss2.courseName and (ss1.courseName ='basketball' and 
ss2.courseName ='football')

Answer 3

选择*从课程WHERE中，课程名称=“篮球”或课程名称= “足球”

万岁简单xD

Answer 4

您可以使用2个简单的查询，然后找到两者的交集

SELECT studentName FROM course WHERE courseName = 'basketball'
INTERSECT
SELECT studentName FROM course WHERE courseName = 'football'

有关postgres的信息，请参见 http://www.postgresqltutorial.com/postgresql-intersect/

对于Oracle还是这样 https://www.techonthenet.com/oracle/intersect.php

选择同时选择足球和篮球的学生

4 个答案: