mysql - 从`MySQL`中删除重复项并根据订单保留单个记录

Question

我需要删除所有重复项，只保留一个数量最多的。也许我应该做一些 JOIN 操作，但我对 . 我有这个查询：

SELECT * 
FROM invoices 
GROUP BY user 
ORDER BY  amount DESC

它查询所有行，按数量对它们进行排序，并在按用户分组时“删除”重复项，但显然不会删除重复项。任何帮助表示赞赏。为了清楚起见，必须永久删除重复项。架构：

 user varchar(125), amount int 

Answer 1

如果您这样做SELECT *，即使使用GROUP BY.

SELECT user, MAX(amount) amount
FROM  invoices
GROUP BY user
ORDER BY amount DESC

Answer 2

请注意，由于您使用GROUP BY. MySQL扩展了它的功能。您希望始终指定在 中选择的列GROUP BY：

SELECT col1, col2, AGGREGATE(col3)
FROM table
GROUP BY col1, col2

我需要选择所有行找到重复项

要找到MAX每个的金额user：

SELECT user, 
       Max(amount) AS amount 
FROM   invoices 
GROUP  BY user 

并只保留金额最高的行

选项1

使用LEFT JOIN（感谢 JW）：

DELETE    invoices
FROM      invoices
LEFT JOIN 
     (SELECT user, MAX(amount) AS amount
     FROM invoices
     GROUP BY user) j 
          ON j.user = invoices.user 
              AND j.amount = invoices.amount
WHERE j.amount IS NULL

http://sqlfiddle.com/#!2/ce2f8/1

选项 2

创建临时表：

CREATE TABLE invoices (
  user int,
  amount decimal(5,2));

INSERT INTO invoices VALUES
(1, 100.00),
(1, 200.00),
(1, 300.00);

CREATE TABLE invoicesStg (
  user int,
  amount decimal(5,2));

INSERT INTO invoicesStg
(SELECT user, MAX(amount) AS amount
FROM invoices
GROUP BY user);

TRUNCATE invoices;

INSERT INTO invoices
SELECT user, amount
FROM invoicesStg;

DROP TABLE invoicesStg;

http://sqlfiddle.com/#!2/0381e/1

Answer 3

为了查找重复项，您可以尝试：

SELECT  id, COUNT(amount) AS cnt, MAX(amount) AS mx 
FROM    invoices 
GROUP   BY user HAVING cnt > 1 
ORDER   BY amount DESC

从那里，您可以继续删除这些记录。

Answer 4

如果您想要金额最高的行，请尝试以下操作：

select *
from invoices
order by amount desc
limit 1

我不确定您所说的“删除”是什么意思。您真的要删除除数量最多的行之外的所有行吗？