ruby-on-rails - Postgres 在一列上选择重复但在另一列上选择不同

Question

我有一个包含两列的数据库:

author_id, message

条目如下:

123, "message!"
123, "message!"
123, "different message"
124, "message!"

我想做一个允许我选择的查询:

123, “消息!”

或

124, “消息!”

本质上，message 相同但 author_id 不同的条目。

然后我想删除其中一个条目。 (哪一个不重要，我只能选择其中一个)。

This question让我接近，但它适用于跨两列的重复项。

Answer 1

还有一个替代示例:

-- Test table
CREATE TABLE dummy_data (
    author_id   int,
    message     text
);

-- Test data
INSERT INTO dummy_data ( author_id, message )
VALUES
( 123, '"message!"' ),
( 123, '"message!"' ),
( 123, '"different message"' ),
( 124, '"message!"' ),
( 124, '"message!"' ),
( 125, '"message!"' );

-- Delete query
DELETE FROM dummy_data
WHERE   ctid NOT IN (
            SELECT  max( ctid )
            FROM    dummy_data
            GROUP BY message     -- this is important to specify
        )
 -- just for test returning deleted records,
 -- you may ignore it, if don't want
RETURNING *;

-- Confirming result:
SELECT * FROM dummy_data ;
 author_id |       message
-----------+---------------------
       123 | "different message"
       125 | "message!"
(2 rows)

查看有关系统列的更多信息:https://www.postgresql.org/docs/current/static/ddl-system-columns.html

编辑:
请求的附加示例通过 ID (author_id) 限制范围。

纯查询:

DELETE FROM dummy_data
USING   ( SELECT ARRAY[ 123, 124] ) v(id)
WHERE   author_id = ANY ( v.id )
AND     ctid NOT IN (
            SELECT  max( ctid )
            FROM    dummy_data
            WHERE   author_id = ANY ( v.id )
            GROUP BY message
        );

带有评论的相同查询:

DELETE FROM dummy_data
-- Add your 'author_id' values into array here.
-- Reason we list it here with USING statement is
-- because we need to compare values in two places
-- and if list is too big it would be annoyance to
-- write it 2 times :)
USING   ( SELECT ARRAY[ 123, 124] ) v(id)
-- First we get all the authors in the batch by ID
WHERE   author_id = ANY ( v.id )
-- Secondly we get max CTID to ignore using same
-- authors range in batch scope
AND     ctid NOT IN (
            SELECT  max( ctid )
            FROM    dummy_data
            WHERE   author_id = ANY ( v.id )
            GROUP BY message
        );

-- This will delete following rows:
 author_id |  message
-----------+------------
       123 | "message!"
       123 | "message!"
       124 | "message!"
(3 rows)

-- Leaving the state to table:
 author_id |       message
-----------+---------------------
       123 | "different message"
       124 | "message!"
       125 | "message!"
(3 rows)