[sql] Select multiple rows with the same value(s)

I have a table, sort of like this:

ID  |  Chromosome | Locus | Symbol | Dominance |
===============================================
1   |      10     |   2   |   A    |   Full    |
2   |      10     |   2   |   a    |   Rec.    |
3   |      10     |   3   |   B    |   Full    |
4   |      10     |   3   |   b    |   Rec.    |

I'd like to select all rows with the same locus and chromosome. For example, rows 3 and 4. There may be more than 2 at a time and they may not be in order.

I tried this:

SELECT *
FROM Genes
GROUP BY Locus
HAVING Locus='3' AND Chromosome='10'

But it always returns row 3, never row 4, even when repeated. I think I'm missing something obvious and simple, but I'm at a loss.

Can someone help?

The problem is GROUP BY - if you group results by Locus, you only get one result per locus.

Try:

SELECT * FROM Genes WHERE Locus = '3' AND Chromosome = '10';

If you prefer using HAVING syntax, then GROUP BY id or something that is not repeating in the result set.

One way of doing this is via an exists clause:

select * from genes g
where exists
(select null from genes g1
 where g.locus = g1.locus and g.chromosome = g1.chromosome and g.id <> g1.id)

Alternatively, in MySQL you can get a summary of all matching ids with a single table access, using group_concat:

select group_concat(id) matching_ids, chromosome, locus 
from genes
group by chromosome, locus
having count(*) > 1

Assuming that you want all rows for which there is another row with the exact same Chromosome and Locus:

You can achieve this by joining the table to itself, but only returning the columns from one "side" of the join.

The trick is to set the join condition to "the same locus and chromosome":

select left.*
from Genes left
inner join Genes right
on left.Locus = right.Locus and 
  left.Chromosome = right.Chromosome and left.ID != right.ID

You can also easily extend this by adding a filter in a where-clause.

This may work for you:

select t1.*
from table t1
join (select t2.Chromosome, t2.Locus
    from table2
    group by t2.Chromosome, t2.Locus
    having count(*) > 1) u on u.Chromosome = t1.Chromosome and u.Locus = t1.Locus