Deduplicate table data based on column value in another row

Question

I have a table containing two columns src & dest, the data looks like this in the table:

table data

The requirement here is to deduplicate the data such that only one record exists where the dest column has a value populated as a src column or no value at all in the src column.

For example for the above data, below output should be shown by the sql query:

output data

Answer 1

Not sure. This may get you what you want:

select * from T
where src  not in (select dest from T)
   or dest not in (select src  from T);