PostgreSQL 删除重复项

Question

我正在处理 postgres 查询以从表中删除重复项。下表是动态生成的，我想编写一个选择查询，如果第一行有重复值，它将删除记录。

表格看起来像这样

Ist col  2nd col
 4        62
 6        34
 5        26
 5        12

我想编写一个删除第 3 行或第 4 行的选择查询。

Answer 1

             select count(first) as cnt, first, second 
             from df1 
             group by first
             having(count(first) = 1)

如果你想保留其中的一行（抱歉，如果你想要的话，我最初错过了它）：

             select first, min(second) 
             from df1 
             group by first

表名称为df1，列名称为first 和second。

如果你愿意，你实际上可以不用count(first) as cnt。

冒着明显的风险，一旦您知道如何选择您想要（或不想要）的数据，那么删除记录的十几种方法中的任何一种都很简单。

如果你想更换表或创建一个新表，你可以使用create table as进行删除：

             create table tmp as 
             select count(first) as cnt, first, second 
             from df1 
             group by first
             having(count(first) = 1);

             drop table df1;

             create table df1 as select * from tmp;

或使用DELETE FROM:

DELETE FROM df1 WHERE first NOT IN (SELECT first FROM tmp);

您也可以使用select into等。

Answer 2

所以基本上我是这样做的

 create temp t1 as 
 select first, min (second) as second
 from df1 
 group by first

 select * from df1 
 inner join t1 on t1.first = df1.first and t1.second = df1.second

这是一个令人满意的答案。感谢您的帮助@Hack-R

Answer 3

不需要中间表：

delete from df1
where ctid not in (select min(ctid)
                   from df1
                   group by first_column);

如果您要从大表中删除许多行，使用中间表的方法可能更快。

---

如果您只想获取一列的唯一值，可以使用：

select distinct on (first_column) *
from the_table
order by the_table;

或者干脆

select first_column, min(second_column)
from the_table
group by first_column;

Answer 4

• 如果您想SELECT 唯一行：

---

SELECT * FROM ztable u
WHERE NOT EXISTS (      -- There is no other record
    SELECT * FROM ztable x
    WHERE x.id = u.id   -- with the same id
    AND x.ctid < u.ctid -- , but with a different(lower) "internal" rowid
    );                  -- so u.* must be unique

---

• 如果您想SELECT 上一个查询中被禁止的其他行：

---

SELECT * FROM ztable nu
WHERE EXISTS (           -- another record exists
    SELECT * FROM ztable x
    WHERE x.id = nu.id   -- with the same id
    AND x.ctid < nu.ctid -- , but with a different(lower) "internal" rowid
    );

---

• 如果您想DELETE 记录，使表唯一（但每个 id 保留一条记录）：

---

DELETE FROM ztable d
WHERE EXISTS (          -- another record exists
    SELECT * FROM ztable x
    WHERE x.id = d.id   -- with the same id
    AND x.ctid < d.ctid -- , but with a different(lower) "internal" rowid
    );