Difficulty with filtering and aggregating multiple columns in MySQL

Question

I have a set of MySQL data in the below format:

acode       bcode   unique_code
BA1100018    SE     OX120013
BE1100001    CS     CS140005
BE1100001    SE     SE140079
CS1400000    CS     CS140006
CS1400000    CS     CS140011
CS1400000    CS     CS140009
CS1400000    CS     CS140013
OX1100021    OX     OX110010
OX1100021    SE     OX110013
OX1100021    OX     OX980141

I am trying to identify which acode id's have more than one different bcode id. I would like to return a set of data which would filter the non-relevant data out. In this case the data would be returned as follows:

acode       bcode   unique_code
BE1100001    CS     CS140005
BE1100001    SE     SE140079
OX1100021    OX     OX110010
OX1100021    SE     OX110013
OX1100021    OX     OX980141

My initial attempt was:

select count(bcode),acode from mydataset group by acode having 
count(bcode)>1

I realised that this wasn't going to return me the bcodes, so I tried various permutations of that. Not to much success. I have been trying to get this done as efficiently as possible, as I am dealing with a large dataset, but I am having difficulty with the aggregation techniques while maintaining the detail I need. Is anybody able to help? Thanks

Answer 1

It's better if you tell us what have you tried ...

SELECT acode,bcode,count(bcode)
FROM mydataset
GROUP BY acode,bcode

and if you want only the ones that have more than 1

SELECT acode,bcode,count(bcode) as num
FROM mydataset
GROUP BY acode,bcode
HAVING num>1

Answer 2

SELECT DISTINCT x.*
           FROM my_table x 
           JOIN my_table y 
             ON y.acode = x.acode 
            AND y.bcode <> x.bcode;

Answer 3

This will work for you

SELECT main.bcode, main.acode FROM mydataset main 
INNER JOIN (SELECT acode, count(DISTINCT bcode) AS cnt FROM mydataset GROUP BY acode HAVING cnt>1) AS sub
ON main.acode=sub.acode