带有 JOIN 和 WHERE 子句的 SQL SELECT - 从两个表中选择一个记录

Question

我得到了这样的东西：

MySqlCommand sqlcmd = new MySqlCommand(@"SELECT Pupil.*, Instructor.* 
    FROM Pupil
    INNER JOIN Instructor ON Pupil.email = Instructor.email
    WHERE Pupil.email = '" + userEmail.Text + "'
      and Pupil.password = '" + userPassword.Text + "'", sqlconn);

我需要获取隐藏在第一个或第二个表中的记录。我检查并将相同的数据插入到第二个表中。此 SELECT 向我显示了公共记录，但我只想要第一个或第二个表中的唯一记录。

Answer 1

根据您上面的评论，我修改了查询。以下是示例数据的两个示例：

学生

• Adam@abc.com
• Bob@abc.com

教师

• Bob@abc.com（请注意您的数据结构实际上允许这样做）
• Chris@abc.com

查询

/* Adam is a pupil but not an instructor */
select Pupil.*
from Pupil
left join Instructor on Pupil.email = Instructor.email
where Instructor.email is null
  and Pupil.email = 'Adam@abc.com'
union 
select Instructor.*
from Instructor
left join Pupil on Pupil.email = Instructor.email
where Pupil.email is null
  and Instructor.email = 'Adam@abc.com';

/* Chris is an instructor but not a pupil */
select Pupil.*
from Pupil
left join Instructor on Pupil.email = Instructor.email
where Instructor.email is null
  and Pupil.email = 'Chris@abc.com'
union 
select Instructor.*
from Instructor
left join Pupil on Pupil.email = Instructor.email
where Pupil.email is null
  and Instructor.email = 'Chris@abc.com';

您可以在以下位置找到一个工作示例：http://sqlfiddle.com/#!9/c490c7/28