如果数据没有值，则在查询中显示零 [重复]

Question

我想在我的数据库中显示诊所名称和我的总收入，条件是年份和月份内的日期。

这是我得到的：

SELECT tbl_clinics.clinic_name, COALESCE(SUM(tbl_bill.bill_amt),0) AS clinic_tot FROM tbl_clinics
LEFT OUTER JOIN tbl_check_up ON tbl_clinics.clinic_id = tbl_check_up.clinic_id 
LEFT OUTER JOIN tbl_bill ON tbl_check_up.bill_id = tbl_bill.bill_id 
WHERE MONTH(tbl_bill.date_billed) = "01" 
    AND YEAR(tbl_bill.date_billed) = "2018" 
    AND tbl_clinics.user_id = "2-2017" 
GROUP BY tbl_clinics.clinic_name

现在，只有当我有从我的数据库中获取的数据时，我的查询才会返回正确。现在我想要这样的东西：例如我更改日期YEAR(tbl_bill.date_billed) = "2019" 然后我想像这样输出：

_________________________________
|   Clinic_name |  clinic_tot   |
---------------------------------
|  Clinic 1     |     0         |
---------------------------------

Answer 1

您的WHERE 子句正在将外连接变为内连接。您需要将除第一个表之外的所有表的条件移动到 ON 子句中：

SELECT c.clinic_name, COALESCE(SUM(b.bill_amt), 0) AS clinic_tot 
FROM tbl_clinics c LEFT OUTER JOIN
     tbl_check_up cu
     ON c.clinic_id = cu.clinic_id LEFT OUTER JOIN
     tbl_bill b
     ON cu.bill_id = b.bill_id AND
        MONTH(b.date_billed) = 1 AND
        YEAR(b.date_billed) = 2018 
WHERE c.user_id = '2-2017'
GROUP BY c.clinic_name