Mysql連接來自三個表的數據（很多關系）

Question

這是我的mysql數據庫模式：

我想創建查詢，該查詢將通過以下方式返回有關所有培訓的數據：

training。*，[type.type]，[voivodeship.name]如果沒有與給定培訓相關的類型或省份，則應在列值中返回null。

例如：

 {
    "id": 1,
    "name": "Example training 2",
    "description": "This is a description 2",
    "status": "status 2",
    "registerFrom": null,
    "registerTo": null,
    "DateOfInsert": "2018-02-06T12:00:57.000Z",
    "training types": "Example type 1,Example type 3,Example type 2"
    "localizations": "loc 1, loc 3"
 },
...
 {
    "id": 99,
    "name": "Example training 99",
    "description": "This is a description 99",
    "status": "status 2",
    "registerFrom": null,
    "registerTo": null,
    "DateOfInsert": "2018-02-06T12:00:57.000Z",
    "training types": null,
    "localizations": null
 },
 {
    "id": 99,
    "name": "Example training 99",
    "description": "This is a description 99",
    "status": "status 2",
    "registerFrom": null,
    "registerTo": null,
    "DateOfInsert": "2018-02-06T12:00:57.000Z",
    "training types": "Example type 9,Example type 4,Example type 2",
    "localizations": "loc 56, loc 32"
  },

這是我當前的查詢，它返回有關本地化信息的培訓（可悲的是，沒有本地化信息，它不會返回本地化信息），而且我也不知道如何修改它以返回所有類型的信息：

SELECT `training`.*, GROUP_CONCAT(`voivodeship`.`name`) AS `Localizations`
FROM `training_localization` 
INNER JOIN `training` ON (`training_localization`.`training_id` = `training`.`id`) 
INNER JOIN `voivodeship` ON (`training_localization`.`voivodeship_id` = `voivodeship`.`id`) 
GROUP BY `training`.`id`

我對sql不太有經驗。 一個查詢甚至可能嗎？

根據戈登的答案，我提出了新的查詢，看來它正在工作:)：

SELECT t.*, GROUP_CONCAT(vs.name) AS Localizations,  tw.types AS types
FROM training t 
     LEFT JOIN training_localization tl ON tl.training_id = t.id 
     LEFT JOIN voivodeship vs ON tl.voivodeship_id = vs.id
     LEFT JOIN(
        SELECT t.*, GROUP_CONCAT(ty.type) AS Types
        FROM training t 
             LEFT JOIN training_type tt ON tt.training_id = t.id 
             LEFT JOIN type ty ON tt.type_id = ty.id
        GROUP BY t.id
        ) tw ON tw.id = t.id
GROUP BY t.id;

Answer 1

如果您想要所有東西，請考慮“外部加入”。 如果要進行所有培訓，則應該是一系列left join s中的第一個表：

SELECT t.*, GROUP_CONCAT(vs.name) AS Localizations
FROM training t LEFT JOIN
     training_localization tl
     ON tl.training_id = t.id LEFT JOIN 
     voivodeship vs
     ON tl.voivodeship_id = vs.id
GROUP BY t.id;

筆記：

• LEFT JOIN將所有內容保留在第一個表中，並將匹配的行保留在后續表中（除非您使用內部join或where子句將其撤消）。
• 表別名使查詢更易於編寫和閱讀。
• 消除不必要的反引號使查詢更易於編寫和閱讀。