从3列中选择不同的值作为1

Question

I'm currently working with a (MySQL) database wich has a table with 3 columns containing INT values. Each column may have NULL and repeated values, as such:

I'd like to know if it's posssible to, in 1 query, select all distinct values from each column and merge them into 1 resulting column; in this example something like:

col
-----
 1
 30
 40
 60

Thanks,

Answer 1

You can use union

select * from (
select col1 as col from table
union 
select col2 as col from table
union
select col3 as col from table
) t where col is not null
order by col

If you have indexed your columns then you can use separate where clauses for each 3 queries in order to best utilize your index