在select語句內的Select語句中找不到列

Question

嘗試一次從此查詢中選擇一行（例如，rank = 1）。 但是，它不起作用，因為“ unknown column'rank'。如果我將其更改為“ WHERE id = 1”，那么它就可以了，但是由於某種原因，它甚至不知道它是什么等級

    SET @rownum=0;

  SELECT @rownum := @rownum + 1 AS rank, id, client, date, time, pickupCity, pickupState
  FROM (
        SELECT r.id, CONCAT(c.fname, ' ', c.lname) as client, r.date,
    LOWER(TIME_FORMAT(r.time, '%l:%i%p')) as time, r.pickupCity,               r.pickupState 

    FROM request r  
    INNER JOIN client c  ON r.client = c.id 
    INNER JOIN pickup p ON r.id = p.request
    INNER JOIN driver d ON d.id = p.driver

    WHERE date = '2018-04-18' AND d.id = 1
    GROUP BY time

  ) AS tab
    HAVING rank = 1;

Answer 1

在MySQL中，您可以使用HAVING來做到這一點：

SELECT @rownum := @rownum + 1 AS rank, id, client, date, time, pickupCity, pickupState
FROM (SELECT r.id, CONCAT(c.fname, ' ', c.lname) as client, r.date, 
             LOWER(TIME_FORMAT(r.time, '%l:%i%p')) as time, r.pickupCity, 
             r.pickupState
      FROM request r JOIN
           client c
           ON r.client = c.id JOIN
           driver d
           ON ?
           pickup p
           ON d.id = p.driver
      WHERE date = '2018-04-18' AND d.id = 1
      GROUP BY time
     ) t CROSS JOIN
     (SELECT @rank := 0) params
HAVING rank = 1;

筆記：

• ?? 用於缺少的JOIN條件。
• 我經過努力來修復您的JOIN語法。 您應該努力學習正確，顯式，標准的JOIN語法。
• 您可以在查詢中設置@rank ； 您不需要單獨的聲明。
• GROUP BY沒有意義，因為SELECT有許多未聚合的列。

如果我不得不推測，問題的根本原因是缺少JOIN條件，而您為此付出了很多努力。