Mysql 显示重复结果的左连接查询问题

我有表

person

和

讲师开设的课程

。我正在执行一个sql查询，但是我得到了所需结果的重复值。我正在从

person

表中获取所有当前正在教授

课程的
讲师角色=1
。可以对查询或表格提出哪些建议

预期结果：

+-----------+-----------------+-------------+------------+
| person_id | instructor_name | course_name | company_id |
+-----------+-----------------+-------------+------------+
|       154 | James Newton    |             |            |
|       154 | James Newton    |             |            |
|       478 | Tina Turner     |             |            |
|       258 | James Fuller    |             |            |
|       717 | Michael Jordan  | Shoe Biz    |          7 |
|       717 | Michael Jordan  | Basketball  |          7 |
|       964 | Steve Johnson   |             |            |
+-----------+-----------------+-------------+------------+

当前：

+-----------+-----------------+-------------+------------+
| person_id | instructor_name | course_name | company_id |
+-----------+-----------------+-------------+------------+
|       154 | James Newton    |             |            |
|       154 | James Newton    |             |            |
|       478 | Tina Turner     |             |            |
|       258 | James Fuller    |             |            |
|       717 | Michael Jordan  | Shoe Biz    |          7 |
|       717 | Michael Jordan  | Basketball  |          7 |
|       717 | Michael Jordan  | Shoe Biz    |          7 |
|       717 | Michael Jordan  | Basketball  |          7 |
|       964 | Steve Johnson   |             |            |
+-----------+-----------------+-------------+------------+

为了得到你的要求：

SELECT * from person
left join (select * from courses_by_instructors group by company_id) as mytable on mytable.person_id = person.person_id
where instructor_role = 1
order by person.person_name

但是，您在初始查询中没有重复项，因为
公司id
将不同
查询正常，但问题出在您的模型中。
我认为person表应该这样创建：

CREATE TABLE person
    (
     company_id int, 
     person_id int, 
     person_name varchar(30),
     instructor_role tinyint,
     staff_role tinyint,
     CONSTRAINT PK_PERSON_ID PRIMARY KEY NONCLUSTERED (person_id), 
    );

CREATE TABLE courses_by_instructors
    (
     company_id int, 
     person_id int, 
     course_id int,
     course_name varchar(30),
     CONSTRAINT FK_PERSON_ID FOREIGN KEY (person_id) 
        REFERENCES person(person_id) 
        ON DELETE CASCADE
        ON UPDATE CASCADE
    );

将个人ID作为主键。“课程由讲师”表的创建方式应如下所示：

CREATE TABLE person
    (
     company_id int, 
     person_id int, 
     person_name varchar(30),
     instructor_role tinyint,
     staff_role tinyint,
     CONSTRAINT PK_PERSON_ID PRIMARY KEY NONCLUSTERED (person_id), 
    );

CREATE TABLE courses_by_instructors
    (
     company_id int, 
     person_id int, 
     course_id int,
     course_name varchar(30),
     CONSTRAINT FK_PERSON_ID FOREIGN KEY (person_id) 
        REFERENCES person(person_id) 
        ON DELETE CASCADE
        ON UPDATE CASCADE
    );

具有引用个人ID的外键

请注意，我使用的是SQL Server，因此您可能需要稍微调整语法。
尝试以下操作：

SELECT p.person_id, p.person_name, p.instructor_role, IFNULL(lj.company_id, 0) as  company_id, lj.person_id, lj.course_name
FROM person as p
LEFT JOIN courses_by_instructors as lj
      ON (lj.person_id = p.person_id AND lj.company_id = 7 )
WHERE instructor_role = 1
GROUP BY p.person_id,lj.course_name
ORDER BY person_name

它会给你你所需要的

请参见
中的输出，它可以通过SELECT语句中的DISTINCT实现

SELECT DISTINCT p.person_id, p.person_name, p.instructor_role, IFNULL(lj.company_id, 0) as  company_id, lj.person_id, lj.course_name
FROM person as p
LEFT JOIN courses_by_instructors as lj
      ON (lj.person_id = p.person_id AND lj.company_id = 7 )
WHERE instructor_role = 1
ORDER BY person_name

下面是代码，请将
p.company\u id as notdupe
添加到您的选择列表中，您可以看到它们不是重复结果这正是我需要的<代码>按p.person\u id，lj.course\u name分组，我想这会有所不同……你能详细说明一下，它是如何解决结果集中出现重复记录的问题的吗？@DipenduPaul:我想问题在于如何定义表。如果您检查该链接，您会注意到person表中的记录具有相同的person ID、讲师角色和员工角色，这在我看来毫无意义……设计未规范化。。但是，公司id不应该是Person表的一部分，如果Person和company之间存在N:N关系，那么公司应该进入其他表，为了维护N:N关系，可以创建另一个表。