同时在多个列上选择不重复，并在PostgreSQL中保留一列

Question

For a table such as this:

tblA  

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

I want to produce a table that selects distinct on both A and B simultaneously, and still keep one value of C, like so:

tblB  

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

Seems like this would be simple, but not finding it for the life of me.

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

Answer 1

When you use DISTINCT ON you should have ORDER BY :

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

Answer 2

This should do the trick

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

Answer 3

Use a view to do the distinct and then join it to the original table to pick one row of column C. Inserting into the target is left for you to figure out. Oh, and you could pick up multiple columns from t, not just c - the only thing is that your subquery needs to find a way to limit it to only one row.

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);
)