[SQL]586(輸出某特性衆數的對應ID)+585(兩個臨時表的差)

586. 訂單最多的客戶

題目

在表 orders 中找到訂單數最多客戶對應的 customer_number 。

數據保證訂單數最多的顧客剛好只有一位。

SELECT customer_number
FROM orders
GROUP BY customer_number
ORDER BY COUNT(customer_number) DESC
LIMIT 1 OFFSET 0;   --取1行數據，從第0行開始

585. 2016年的投資

思路

分步驟：

1. 找出2015年投資相同的人，臨時表記爲A

(SELECT DISTINCT a.*
      FROM Insurance a, Insurance b 
      WHERE a.TIV_2015 = b.TIV_2015
      AND a.PID != b.PID) A

2. 找出全部座標同樣的人，臨時表記爲B

(SELECT DISTINCT b.PID
           FROM Insurance a, Insurance b
           WHERE a.LAT = b.LAT
           AND a.LON = b.LON
           AND a.PID != b.PID) B

3. 採用左聯結，保留全部A表中的信息，此時篩選出A表有而B表沒有的用戶，把他們的TIV_2016用SUM()加一下

最後代碼

SELECT SUM(A.TIV_2016) AS TIV_2016
FROM (SELECT DISTINCT a.*
      FROM Insurance a, Insurance b 
      WHERE a.TIV_2015 = b.TIV_2015
      AND a.PID != b.PID) A
LEFT JOIN (SELECT DISTINCT b.PID
           FROM Insurance a, Insurance b
           WHERE a.LAT = b.LAT
           AND a.LON = b.LON
           AND a.PID != b.PID) B 
ON (A.PID = B.PID)
WHERE B.PID IS NULL;