MySQL：帶有LEFT JOIN的GROUP_CONCAT

Question

我遇到了MySQL的“GROUP_CONCAT”功能問題。 我將使用一個簡單的幫助台數據庫來說明我的問題：

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.');

請注意，此幫助台數據庫有兩個票證，每個票證都有兩個解決方案條目。 我的目標是使用SELECT語句創建數據庫中所有票證的列表及其相應的解決方案條目。 這是我正在使用的SELECT語句：

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

上面的SELECT語句的問題是它只返回一行：

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.

請注意，它將使用故障單1和故障單2的解決方案條目返回故障單1的信息。

我究竟做錯了什么？ 謝謝！

Answer 1

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

備用：

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

Answer 2

我認為@Dylan Valade的評論是最簡單的答案，所以我將其作為另一個答案發布：只需在OP的SELECT中添加GROUP BY Tickets.id即可解決問題。 它解決了我自己的問題。

但是，對於不小的數據庫，接受的答案，特別是如果Tickets.id上有任何謂詞似乎不涉及總表掃描，所以雖然前一段返回正確的結果，但在我的情況下似乎效率低得多。

Answer 3

您只需要添加GROUP_BY：

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