SQL連接3個表（基於2條規定 - ？）

Question

我有3個表設置這樣的（有點簡化）：

time_tracking：ID，tr_proj_id，tr_min，tr_type
time_projects：ID，PROJECT_NAME
time_tasks ：ID，TASK_NAME

基本上，我想要檢索基於tr_type要麼PROJECT_NAME或TASK_NAME它可以是有價值的 「項目」 或 「任務」

個

一個例子

time_tracking

+----+------------+--------+---------+ 
| id | tr_proj_id | tr_min | tr_type | 
+----+------------+--------+---------+ 
| 1 |   3 |  60 | project | 
| 2 |   3 | 360 | task | 
| 3 |   1 | 120 | project | 
| 4 |   2 |  30 | project | 
| 5 |   2 |  30 | task | 
| 6 |   1 |  90 | task | 
+----+------------+--------+---------+ 

time_projects

+----+------------------------+ 
| id |  project_name  | 
+----+------------------------+ 
| 1 | Make someone happy  | 
| 2 | Start a project  | 
| 3 | Jump out of the window | 
+----+------------------------+ 

time_tasks

+----+---------------------+ 
| id |  task_name  | 
+----+---------------------+ 
| 1 | drink a beer  | 
| 2 | drink a second beer | 
| 3 | drink more   | 
+----+---------------------+ 

所需的輸出

+----+------------------------+------------+--------+---------+ 
| id |   name   | tr_proj_id | tr_min | tr_type | 
+----+------------------------+------------+--------+---------+ 
| 1 | Jump out of the window |   3 |  60 | project | 
| 2 | drink more    |   3 | 360 | task | 
| 3 | Make someone happy  |   1 | 120 | project | 
| 4 | Start a project  |   2 |  30 | project | 
| 5 | drink a second beer |   2 |  30 | task | 
| 6 | drink a beer   |   1 |  90 | task | 
+----+------------------------+------------+--------+---------+ 

而處於全真不好JOIN的事情，這是我想出來的，到目前爲止的唯一的事情（不工作..）：

SELECT tt.tr_proj_id, tt.tr_type, tt.tr_min, pp.project_name, pp.id, ta.task_name, ta.id 
FROM time_tracking as tt, time_projects as pp, time_tasks as ta 
WHERE ((tt.tr_type = 'project' AND pp.id = tt.tr_proj_id) OR (tt.tr_type = 'task' AND ta.id = tt.tr_proj_id)) 
AND tt.tr_min > 0 
ORDER BY tt.tr_proj_id DESC 

如果任何人有關於如何做到這一點的想法，請隨時分享！
謝謝！

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更新：貌似我忘了指定我使用的是訪問數據庫。這顯然不接受的東西，如CASE或​​3210 ..顯然有IIF()，但我不太清楚如何在這種情況下使用它。

Answer 1

試試這個：

SELECT 
    tt.id, 
    CASE WHEN tt.tr_type = 'project' THEN pp.project_name 
     WHEN tt.tr_type = 'task' THEN ta.task_name END as name, 
    tt.tr_proj_id, 
    tt.tr_type, 
    tt.tr_min, 
FROM time_tracking as tt 
    left join time_projects as pp on pp.id = tt.tr_proj_id 
    left join time_tasks as ta on ta.id = tt.tr_proj_id 
WHERE tt.tr_min > 0 
ORDER BY tt.tr_proj_id DESC 

Answer 2

使用join子句和WHERE子句中的移動你的加入條件爲on條款：

SELECT 
    tt.tr_proj_id, 
    tt.tr_type, 
    tt.tr_min, 
    pp.project_name, 
    pp.id, 
    ta.task_name, 
    ta.id 
FROM time_tracking as tt 
left join time_projects as pp on tt.tr_type = 'project' AND pp.id = tt.tr_proj_id 
left join time_tasks as ta on tt.tr_type = 'task' AND ta.id = tt.tr_proj_id 
WHERE tt.tr_min > 0 
ORDER BY tt.tr_proj_id DESC,tt.tr_day ASC 

我用left join，它給你從主表中的一行，即使不存在對於參加（你所加入的表列從空，如果沒有加入）

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的一個關鍵點在這裏，許多SQL程序員沒有意識到，就是ON子句可以包含任何康迪即使不是來自聯結表（如本例中）。許多程序員認爲條件只能是與正式的外鍵關係相關的條件。

Answer 3

兩個執行工會加入：

select tt.id, tp.project_name name, tt.tr_proj_id, tt.tr_min, tt.tr_type 
    from time_tracking tt 
     inner join time_projects tp on tp.id = tt.tr_proj_id 
where tt.tr_type = 'project' 
union all 
select tt.id, tp.project_name name, tt.tr_proj_id, tt.tr_min, tt.tr_type 
    from time_tracking tt 
     inner join time_tasks tk on tk.id = tt.tr_proj_id 
where tt.tr_type = 'task' 

這將會給你確切的結果見表你想

Answer 4

SELECT 
    time_tracking.id, 
    time_tracking.tr_min, 
    time_tracking.tr_type, 
    coalesce(time_projects.project_name, time_tasks.task_name) as name 
FROM time_tracking 
LEFT OUTER JOIN time_projects on time_projects.id = time_tracking.tr_proj_id AND time_tracking.tr_type = 'project' 
LEFT OUTER JOIN time_tasks on time_tasks.id = time_tracking.tr_proj_id AND time_tracking.tr_type = 'task' 
WHERE time_tracking.tr_min > 0 
ORDER BY time_tracking.id DESC -- ... 

​​3210是MSSQL，有相當於ISNULL和這樣的其他數據庫技術

這個想法是你加入到表中，如果加入失敗，你會得到NULL連接失敗。然後您使用COALESCE來挑選出成功的連接值。