Question

我必须联接2个表，而我要联接的第一个表包括：

Physicians (ID, FirstName, LastName, PracticeID, SpecialtyID, Email)

第二张表是：

PhysicianSpecialties( SpecialtyID, SpecialtyName)

我编写了此查询以将表连接在一起

Select *
from physicians
right join PhysicianSpecialities
on PhysicianSpecialities.SpecialtyID = Physicians.SpecialtyID

当我离开“加入他们”表格时，现在是

(ID, FirstName, LastName, PracticeID, SpecialtyID, Email, SpecialtyID, SpecialtyName)

我该如何重写它，以便只有一个“ SpecialtyID”列？

Answer 1

您没有指定正在使用的DBMS产品，但是：在标准SQL中，如果两个表中的连接列名称相同，则可以使用USING运算符进行连接。

在这种情况下，“重复”列将自动从结果中删除。

Select *
from physicians
  right join PhysicianSpecialities using (SpecialtyID)

尽管并非所有DBMS产品都支持。

Answer 2

您需要指定两个表的列名，而不是（*）

Select a.*,b.SpecialtyName
from physicians a
right join PhysicianSpecialities b
on b.SpecialtyID = a.SpecialtyID

Answer 3

使用

SELECT 
    physicians.ID, physicians.FirstName, physicians.LastName, 
    physicians.PracticeID, physicians.SpecialtyID, physicians.Email, 
    PhysicianSpecialities.SpecialtyName

而不是SELECT *，因此您的查询如下：

SELECT 
    physicians.ID, physicians.FirstName, physicians.LastName, 
    physicians.PracticeID, physicians.SpecialtyID, physicians.Email, 
    PhysicianSpecialities.SpecialtyName
FROM
    physicians
LEFT JOIN 
    PhysicianSpecialities ON Physicians.SpecialtyID = PhysicianSpecialities.SpecialtyID;

我希望它将返回期望的结果。

如何做没有重复列的左联接？

3 个答案: