左邊加入3張桌子？

Question

我必須在3個表格上使用LEFT JOIN：UNITS,ROOMS_CHECK_IN和COMMENTS。基本上我想顯示UNITS和每個房間的入住單位數和評論數。但是當我運行房間檢查和評論計數時，我得到的是相同的4位數字。如果我用單獨的左連接分開2個查詢，它可以正常工作。

下面是查詢：

SELECT u.ID, 
     u.unit_name, 
     count(c.checkin_status) as total_chekin , 
     count(com.ID) as total_comment , 
     h.hospital_name 
FROM HOSPITALS h, UNITS u 
LEFT OUTER JOIN ROOMS_CHECK_IN c ON c.unit_id = u.ID AND c.room_status=0 
LEFT OUTER JOIN COMMENTS com ON com.unit_id = u.ID 
WHERE h.ID = u.hospital_id AND u.hospital_id=3 
GROUP BY u.ID; 

請幫助。

Answer 1

從不在FROM條款中使用逗號。 始終使用明確使用JOIN上下文。

然後，你可能想count(distinct)（或前聚集聯接）：

SELECT u.ID, u.unit_name, 
     count(distinct c.checkin_status) as total_chekin, 
     count(distinct com.ID) as total_comment, 
     h.hospital_name 
FROM HOSPITALS h JOIN 
    UNITS u 
    ON h.ID = u.hospital_id LEFT OUTER JOIN 
    ROOMS_CHECK_IN c 
    ON c.unit_id = u.ID AND c.room_status = 0 LEFT OUTER JOIN 
    COMMENTS com 
    ON com.unit_id = u.ID 
WHERE u.hospital_id = 3 
GROUP BY u.ID, u.unit_name, h.hospital_name;