Question

我收到此错误：不是唯一的表/别名：'邮政编码'

"SELECT sub.city AS city, sub.postalcode AS postalcode FROM 
(SELECT postcode.city AS city,  customers.postalcode AS postalcode, COUNT(customers.postalcode) AS postcode_numbers
FROM orders, postcode 
INNER JOIN customers ON orders.userID = customers.ID 
INNER JOIN postcode ON customers.postalcode = postcode.city 
GROUP BY customers.postalcode) sub 
WHERE sub.postcode_numbers > 3";

Answer 1

Remove postcode  table in the from statement   
 SELECT sub.city AS city, sub.postalcode AS postalcode FROM 
    (SELECT postcode.city AS city,  customers.postalcode AS postalcode, COUNT(customers.postalcode) AS postcode_numbers
    FROM orders
    INNER JOIN customers ON orders.userID = customers.ID 
    INNER JOIN postcode ON customers.postalcode = postcode.city 
    GROUP BY customers.postalcode) sub 
    WHERE sub.postcode_numbers > 3

Answer 2

您的postcode子句和第二个from子句中都有join。从from子句中将其删除，您应该可以：

SELECT sub.city AS city, sub.postalcode AS postalcode FROM 
(SELECT postcode.city AS city,  customers.postalcode AS postalcode, COUNT(customers.postalcode) AS postcode_numbers
FROM orders -- Removed postcode from here 
INNER JOIN customers ON orders.userID = customers.ID 
INNER JOIN postcode ON customers.postalcode = postcode.city 
GROUP BY customers.postalcode) sub 
WHERE sub.postcode_numbers > 3

Answer 3

您在FROM子句和INNER JOIN子句中都使用了邮政编码表，这两个表都没有别名，当您进行INNER JOIN时，您不需要在FROM子句中使用该表，以及其他方式当然。

如果要在FROM子句中联接表，您将执行以下操作：

SELECT
    sub.city AS city, sub.postalcode AS postalcode 
FROM 
    (SELECT 
         postcode.city AS city,  customers.postalcode AS postalcode, 
         COUNT(customers.postalcode) AS postcode_numbers
     FROM 
         orders, postcode 
     INNER JOIN 
         customers ON orders.userID = customers.ID 
     --INNER JOIN postcode ON customers.postalcode = postcode.city
     WHERE 
         postcode.city = customers.postalcode
     GROUP BY 
         customers.postalcode) sub 
WHERE 
    sub.postcode_numbers > 3;

所以基本上，您将失去INNER JOIN邮政编码...行

另一种方法是仅使用INNER JOIN子句，这是我的首选方法。为此，您只需丢失FROM子句中的邮政编码，就像这样：

SELECT 
    sub.city AS city, sub.postalcode AS postalcode 
FROM 
    (SELECT 
         postcode.city AS city,  customers.postalcode AS postalcode, 
         COUNT(customers.postalcode) AS postcode_numbers
     FROM 
         orders 
     INNER JOIN 
         customers ON orders.userID = customers.ID 
     INNER JOIN 
         postcode ON customers.postalcode = postcode.city
     GROUP BY 
         customers.postalcode) sub 
WHERE 
    sub.postcode_numbers > 3;

希望这会有所帮助！

Answer 4

您可以这样写：

SELECT p.city,  c.postalcode , COUNT(*) AS num_orders
FROM orders o JOIN
     customers c
     ON o.userID = c.ID JOIN
     postcode p
     ON c.postalcode = p.city 
GROUP BY p.city, c.postalcode
HAVING COUNT(*) > 3;

尚不清楚是要每个city还是每个city / postalcode一行。假定后者。如果要每个city排一行，请将postalcode和SELECT中的GROUP BY从中删除。

注意：

从不在FROM子句中使用逗号。始终使用正确的，明确的，标准 JOIN语法。那是你的具体问题。
表别名使查询更易于编写和阅读。
SELECT中未聚合的列应与GROUP BY键匹配。
不需要子查询。

不是唯一的表格/别名？

4 个答案: