使用UNION在多个表中进行选择

Question

我返工我的数据库从一个用户表中多个用户表(每个角色分): ,,tblStudentstblTeacherstblAdmin

登录时,我不想运行三个查询来检查用户是否存在于我的数据库中的某个位置.所以我所做的是将以下查询放在一起union

select s.id as id, s.email as email, s.password as password, s.role as role from tblStudents s
union
select a.id as id, a.email as email, a.password as password, a.role as role from tblAdmin a
union
select t.id as id, t.email as email, t.password as password, t.role as role from tblTeachers t

这将选择所有表中相同的字段,并为我输出结果.

所以,我决定试试这个,出于某种原因,我的登录表单不起作用.对于我的登录表单,我添加了一个where检查电子邮件地址的子句.我在我的数据库应用程序中运行查询,令人惊讶的是,当我这样做时where email = "admin@admin.be"(此电子邮件存在于我的数据库中tblAdmin),它还从我的学生表中选择一条记录.

使用where子句:

select s.id as id, s.email as email, s.password as password, s.role as role from tblStudents s
union
select a.id as id, a.email as email, a.password as password, a.role as role from tblAdmin a
union
select t.id as id, t.email as email, t.password as password, t.role as role from tblTeachers t
where email = "admin@admin.be"

记录两者have id = 1但我不明白为什么当我在管理员电子邮件地址上过滤时会选择学生记录.为什么是这样？有人可以解释并为我提供更好的解决方案吗？我基本上有一个登录表单,需要跨多个表选择以检查用户是否存在于我的数据库中.

Answer 1

感谢您更新查询; 现在我们可以看到WHERE条件仅应用于最后一个 UNIONed查询.你需要要么添加WHERE子句每个查询,或者把它包装成一个子查询和WHERE子句应用到这一点.

select s.id as id, s.email as email, s.password as password, s.role as role from tblStudents s
where email = "admin@admin.be"
union
select a.id as id, a.email as email, a.password as password, a.role as role from tblAdmin a
where email = "admin@admin.be"
union
select t.id as id, t.email as email, t.password as password, t.role as role from tblTeachers t
where email = "admin@admin.be"

要么

SELECT * FROM (
select s.id as id, s.email as email, s.password as password, s.role as role from tblStudents s
union
select a.id as id, a.email as email, a.password as password, a.role as role from tblAdmin a
union
select t.id as id, t.email as email, t.password as password, t.role as role from tblTeachers t
) foo where email = "admin@admin.be"