MySQL的加入對單個資源ID 3個表

Question

我有三個表

• resources_connection
  1. RESOURCE_ID
  2. resource_tag_id
• resources_flags
  1. USER_ID
  2. RESOURCE_ID
• resources_votes
  1. USER_ID
  2. RESOURCE_ID

每個是一個2列的表，INT（11），用於既設計成允許我查詢的標籤的數量，標誌和投票基於單個'資源ID'

我目前使用此查詢嘗試和ge標籤TA計數（resources_connection），旗（resources_flags）和投票（resources_votes）：

SELECT COUNT(DISTINCT t1.resource_id) as votes, 
     COUNT(DISTINCT t2.resource_id) as flags, 
     COUNT(DISTINCT t3.resource_tag_id) as tags 
FROM ecruit_demo.resources_votes t1 
LEFT JOIN ecruit_demo.resources_flags t2 
    ON (t1.resource_id = t2.resource_id) 
JOIN ecruit_demo.resources_connection t3 
    ON (t1.resource_id = t3.resource_id) WHERE t1.resource_id = 4 

的問題是，該查詢返回正確的結果resource_id = 1但是當我設置resource_id到（針對其存在一個標籤）它返回全零。什麼是適當的查詢結構，以確保此查詢始終返回給定resource_id的標籤，標誌和投票的正確數量？

我還要補充一點，唯一的地方RESOURCE_ID = 4發生在數據庫處於resources_connection，其他兩個表沒有這個值

Answer 1

SELECT resource_ID, 
     MAX(CASE WHEN types = 'votes' THEN totals ELSE NULL END) votes, 
     MAX(CASE WHEN types = 'flags' THEN totals ELSE NULL END) flags, 
     MAX(CASE WHEN types = 'tags' THEN totals ELSE NULL END) tags 
FROM  
    (
     SELECT resource_ID, 'votes' types, 
       COUNT(DISTINCT resource_ID) totals 
     FROM resources_votes 
     GROUP BY resource_ID 
     UNION 
     SELECT resource_ID, 'flags' types, 
       COUNT(DISTINCT resource_ID) totals 
     FROM resources_flags 
     GROUP BY resource_ID 
     UNION 
     SELECT resource_ID, 'tags' types, 
       COUNT(DISTINCT resource_tag_id) totals 
     FROM resources_connection 
     GROUP BY resource_ID 
    ) s 
-- WHERE resource_ID = 1 
GROUP BY resource_ID 

Answer 2

我不是在MySQL確實不錯，但也許你可以試試：

將JOIN改爲LEFT JOIN，這樣查詢就會是這樣。

SELECT COUNT(DISTINCT t1.resource_id) as votes, 
      COUNT(DISTINCT t2.resource_id) as flags, 
      COUNT(DISTINCT t3.resource_tag_id) as tags 
    FROM ecruit_demo.resources_votes t1 
    LEFT JOIN ecruit_demo.resources_flags t2 
     ON (t1.resource_id = t2.resource_id) 
    LEFT JOIN ecruit_demo.resources_connection t3 
     ON (t1.resource_id = t3.resource_id) WHERE t1.resource_id = 4