Select multiple rows with the same value(s)

Question

I have a table, sort of like this:

ID  |  Chromosome | Locus | Symbol | Dominance |
===============================================
1   |      10     |   2           


        

Answer 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.