根据条件 SQL 排除行

Question

I am looking for some help in figuring out how to remove all rows where column 1 = a if column 2 also = a.

It is quite difficult to describe, so I will use an example below.

1    2
a    b
a    a
a    null
b    c
b    e
c    c
d    f
d    a
d    e

The table above is what currently exists, I am looking to write a query which gives the following result

1    2
b    c
b    e
d    f
d    a
d    e

If any of them match, ie column 1 = a column 2 = a, then all rows where column 1 = a should be removed. Is there any way to do this?

TIA

Answer 1

You seem to want rows where col1 has not rows where col1 = col2 . If that is a correct interpretation, you can use a subquery:

select t.*
from t
where not exists (select 1
                  from t t2
                  where t2.col1 = t.col1 and
                        t2.col1 = t2.col2
                 );

Answer 2

You can try the following query using <> (not) operator.

Select * from Yourtable where [Col1] <> [Col2] and Col1 <> 'a'

Here is the live db<>fiddle demo.