SQL 查询用于选择单个表中 2 个相关列之间的更改

Question

Is there a query that can select the rows where a change occurred between 2 related columns that hold different values?

To illustrate:

I have ONE table where there is an alphanumeric code, and another column to store its encrypted equivalent.

I need to find the rows where these 2 don't match. So if I have the following table "codes"

code | encryption
A1   | jl2
A1   | jl2
A1   | ki4
B2   | jl2

I want a query to select the rows where A1 resulted in ki4 and B2 resulted in jl2, because these 2 did not match to their usual results (A1 should always be jl2 and viceversa)

The codes are just examples of course, so I cannot just write a query that looks for these exact instances.

Answer 1

You can use not exists to get a list of the rows that don't have matches:

select distinct code, encryption
from t
where not exists (select 1
                  from t t2
                  where t2.code = t.code and
                        t2.encryption <> t.encryption
                 );

Answer 2

For those interested in an answer, I think this actually got it done. Basically compare the table to itself once and then select the instances where the other column doesnt match

select a.code,a.encryption,b.encryption
from codes a, codes b
where a.code = b.code
and a.encryption <> b.encryption