select query from one table and replace all values from another table

I need to write a select query for a network diagram. There are two tables;

select * from node;
 num | node
-----+------
   1 | a
   2 | b
   3 | c
   4 | d

select * from edge;
 e1 | e2 
----+----
 a  | b
 c  | d
 a  | c

I need a select statement which would give me the edge values in the form of numbers like this;

1-开发者_如何学Python2
3-4
1-3

I am using postgresql 8.4 but is should not have any relevance to my question.

Thank you.

A couple of JOINs should do the trick:

SELECT
    n1.num AS num1,
    n2.num AS num2
FROM node n1
JOIN edge e ON n1.node = e.e1
JOIN node n2 ON n2.node = e.e2

If you want the output as a string exactly as in your question then use this instead:

SELECT n1.num || '-' || n2.num AS nums
FROM ...