Question

对于这样的表：

tblA  

A,B,C  
1,2,t3a  
1,3,d4g  
1,2,b5e  
1,3,s6u

我想生成一个同时在A和B上选择不同的表，并且仍保留一个C值，如下所示：

tblB  

A,B,C  
1,2,t3a  
1,3,d4g

看起来这很简单，但却找不到我的生活。

DROP TABLE IF EXISTS tblA CASCADE;  
SELECT DISTINCT ON (A,B), C  
INTO tblB  
FROM tblA;

Answer 1

当您使用DISTINCT ON时，您应该ORDER BY：

SELECT DISTINCT ON (A,B), C
INTO tblB
FROM tblA
ORDER BY A, B;

Answer 2

这应该可以解决问题

CREATE TABLE tblB AS (
    SELECT A, B, max(C) AS max_of_C FROM tblA GROUP BY A, B
)

Answer 3

使用视图执行distinct，然后将其连接到原始表以选择一行C列。插入目标留给您了解。哦，你可以从t中获取多个列，而不仅仅是c - 唯一的一点是你的子查询需要找到一种方法将它限制为只有一行。

create table t (a int, b int, c int);

create view tv as select distinct a, b from t;

insert into t (a, b, c) values(1, 2, 10);
insert into t (a, b, c) values(1, 2, 20);
insert into t (a, b, c) values(1, 3, 30);
insert into t (a, b, c) values(1, 3, 40);

CREATE TABLE tblB AS (
select tv.a, tv.b, t.c  from tv, t
where tv.a = t.a and tv.b = t.b
/* pick smallest ctid which is a unique row id built into postgres */
and t.ctid = (select min(ctid) from t s where s.a = t.a and s.b = t.b);
)

同时在多个列上选择distinct，并在PostgreSQL中保留一列

3 个答案: