Select distinct on multiple columns with where statement - php

Question

I want to select each available entry for each column once. This problem was solved with
SELECT DISTINCT a from my_table
UNION
SELECT DISTINCT b from my_table
UNION
SELECT DISTINCT c from my_table
UNION
SELECT DISTINCT d from my_table
in this question: MySQL SELECT DISTINCT multiple columns
I want to go further and use the same WHERE statements on each subquery. Is there any way without defining the WHERE each time? My current query would look like this:
SELECT DISTINCT a from my_table WHERE a='a' AND b=1 AND c='.' AND d='ab'
UNION
SELECT DISTINCT b from my_table WHERE a='a' AND b=1 AND c='.' AND d='ab'
UNION
SELECT DISTINCT c from my_table WHERE a='a' AND b=1 AND c='.' AND d='ab'
UNION
SELECT DISTINCT d from my_table WHERE a='a' AND b=1 AND c='.' AND d='ab'
All parameters don't have to be given, I just want to show the maximum that has to be possible. Is there any way to write this shorter?
I use PHP with doctrine, if that is any help.
Thanks in advance!
Example:
my_table:
a | b | c | d
-----+-----+-----+-----
a | 0 | . | ab
b | 0 | - | ag
a | 1 | . | cfd
c | 1 | . | b
a | 1 | - | ab
c | 1 | - | cfd
should give this result (without where statement):
a | b | c | d
-----+-----+-----+-----
a | 0 | . | ab
b | 1 | - | ag
c | | | cfd
| | | b
And with WHERE b=0 statement:
a | b | c | d
-----+-----+-----+-----
a | 0 | . | ab
b | | - | ag
EDIT: changed subqueries to UNION and made the data types fit to the example

Answer 1

UPDATE: Well, I originally wrote up a generic SQL solution for this problem, not realizing that MySQL apparently doesn't allow for it.
So if you can create a view, that may be the lightest-weight solution. (The view's defining query would be the same as the select in my original solution's with clause.)
Alternately you could create a temporary table. Maybe a little more resource-intensive if there's much data, but less likely that anyone would restrict the required permissions.
For the record, original solution was as follows:
with my_filtered as (
select *
from my_table
where a = 1 and b = 2 -- and ...
)
-- carry on with your query, using my_filtered instead of my_table