MySQL将两行视为一

Question

I need to count matches in a database.

Input:

id_to    id_from
1        2  
2        1  
1        3
3        1
1        4
5        1

the 5th and 6th row has only one direction so doesn't count

Sample Output:

id_match
1       
2
3

So, for 1 (implicit), 2 and 3 there is a reverse match but for 4 and 5 there aren't.

---- EDITED ----

Supposing the table name is "example" and I want to get all matches of id=1 then the SQL query will be:

SELECT count(*) FROM
(SELECT id_to FROM example WHERE id_from = 1) as t1,
(SELECT id_from FROM example WHERE id_to = 1) as t2
WHERE t1.id_to = t2.id_from

but maybe there is a better way to do it

Answer 1

You could try

SELECT DISTINCT id_from AS matched_id 
FROM   your_table AS data1 
WHERE  EXISTS (SELECT 1 
               FROM   your_table AS data2 
               WHERE  data1.id_from = data2.id_to 
                      AND data1.id_to = data2.id_from) 

I've created a demo here