無法使用列成子查詢 - MySQL數據庫

Question

我無法使用此「enquiries.enquiry_id」一欄爲子查詢。我也嘗試過使用表的別名，但沒有成功進入此 錯誤代碼：1054.'where子句'中的未知列'enquiries.enquiry_id'

我們可以使用其他方式嗎？

Tables 
------------------------------ 
Enquiries 
------------------------------ 
enquiry_id | name 
1   A 
2   B 
3   C 
4   D 
------------------------------ 
Bookings 
------------------------------ 
booking_id | name |enquiry_id 
1   A  1 
2   B  2 
3   C  2 
4   D  3 
------------------------------ 


SELECT 
    (SELECT 
      COUNT(*) 
     FROM 
      (SELECT 
       booking_id 
      FROM 
       bookings 
      WHERE 
       enquiry_id = enquiries.enquiry_id) AS t) AS total 
FROM 
    enquiries 
WHERE 
    enquiry_id = 2 

Answer 1

我覺得你的查詢可以簡化像

SELECT COUNT(e.booking_id) as total 
FROM enquiries e 
WHERE e.enquiry_id = 2 

或者如果你需要從booking

SELECT COUNT(b.booking_id) as total 
FROM bookings b 
JOIN enquiries e 
    ON b.enquiry_id = e.enquiry_id 
WHERE e.enquiry_id = 2 

別的東西，你如何解決您的查詢。

SELECT 
     (SELECT COUNT(*) 
     FROM bookings b 
     WHERE b.enquiry_id = e.enquiry_id) AS total    
FROM 
    enquiries e 
WHERE 
    enquiry_id = 2