MySQL:GROUP_CONCAT 与 LEFT JOIN

2021-11-20 00:00:00 join sql mysql aggregate-functions group-concat

我在使用 MySQL 的GROUP_CONCAT"函数时遇到问题.我将使用一个简单的帮助台数据库来说明我的问题:

I'm experiencing a problem with MySQL's "GROUP_CONCAT" function. I will illustrate my problem using a simple help desk database:

CREATE TABLE Tickets (
 id INTEGER NOT NULL PRIMARY KEY,
 requester_name VARCHAR(255) NOT NULL,
 description TEXT NOT NULL);

CREATE TABLE Solutions (
 id INTEGER NOT NULL PRIMARY KEY,
 ticket_id INTEGER NOT NULL,
 technician_name VARCHAR(255) NOT NULL,
 solution TEXT NOT NULL,
 FOREIGN KEY (ticket_id) REFERENCES Tickets.id);

INSERT INTO Tickets VALUES(1, 'John Doe', 'My computer is not booting.');
INSERT INTO Tickets VALUES(2, 'Jane Doe', 'My browser keeps crashing.');
INSERT INTO Solutions VALUES(1, 1, 'Technician A', 'I tried to solve this but was unable to. I will pass this on to Technician B since he is more experienced than I am.');
INSERT INTO Solutions VALUES(2, 1, 'Technician B', 'I reseated the RAM and that fixed the problem.');
INSERT INTO Solutions VALUES(3, 2, 'Technician A', 'I was unable to figure this out. I will again pass this on to Technician B.');
INSERT INTO Solutions VALUES(4, 2, 'Technician B', 'I re-installed the browser and that fixed the problem.');

请注意，此帮助台数据库有两张票，每张票都有两个解决方案条目.我的目标是使用 SELECT 语句创建数据库中所有票证及其对应解决方案条目的列表.这是我正在使用的 SELECT 语句:

Notice that this help desk database has two tickets, each with two solution entries. My goal is to use a SELECT statement to create a list of all of the tickets in the database with their corrosponding solution entries. This is the SELECT statement I'm using:

SELECT Tickets.*, GROUP_CONCAT(Solutions.solution) AS CombinedSolutions
FROM Tickets
LEFT JOIN Solutions ON Tickets.id = Solutions.ticket_id
ORDER BY Tickets.id;

上述 SELECT 语句的问题是它只返回一行:

The problem with the above SELECT statement is it's returning only one row:

id: 1
requester_name: John Doe
description: My computer is not booting.
CombinedSolutions: I tried to solve this but was unable to. I will pass this on to Technician B since he is more experienced than I am.,I reseated the RAM and that fixed the problem.,I was unable to figure this out. I will again pass this on to Technician B.,I re-installed the browser and that fixed the problem.

请注意，它返回了票证 1 的信息以及票证 1 和票证 2 的解决方案条目.

Notice that it's returning ticket 1's information with both ticket 1's and ticket 2's solution entries.

我做错了什么?谢谢！

推荐答案

使用:

   SELECT t.*,
          x.combinedsolutions
     FROM TICKETS t
LEFT JOIN (SELECT s.ticket_id,
                  GROUP_CONCAT(s.soution) AS combinedsolutions
             FROM SOLUTIONS s 
         GROUP BY s.ticket_id) x ON x.ticket_id = t.ticket_id

替代:

   SELECT t.*,
          (SELECT GROUP_CONCAT(s.soution)
             FROM SOLUTIONS s 
            WHERE s.ticket_id = t.ticket_id) AS combinedsolutions
     FROM TICKETS t

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