If my search term is not found add to array - python-3.x

Question

Looping through a list of terms and searching database for each term. Is there a way to return a list of search terms that didn't find results in the database?
This is my SELECT query:
SELECT x_tbl.x_str
FROM x_tbl
LEFT JOIN p_tbl ON p_tbl.p_id = x_tbl.p_id
LEFT JOIN t_tbl ON t_tbl.t_id = x_tbl.t_id
WHERE t_tbl.t = %(uw)s
OR t_tbl.ta = %(uw)s
OR %(uw)s = ANY (t_tbl.tb)
OR t_tbl.tc = %(uw)s
OR p_tbl.pa = %(uw)s
OR %(uw)s = ANY (p_tbl.pb)
I want to return uw if it does not find any results in the database. Is this possible?

Answer 1

I want to return uw if it does not find any results in the database.
For one search term
The two LEFT [OUTER] JOIN between x_tbl, p_tbl and t_tbl do not eliminate any rows by themselves. Two times NOT EXISTS returns a list of search terms, that cannot find anything:
SELECT %(uw)s -- old-style Python placeholder
WHERE NOT EXISTS (
SELECT FROM p_tbl
WHERE pa = %(uw)s
OR %(uw)s = ANY (p.pb)
)
AND NOT EXISTS (
SELECT FROM t_tbl
WHERE %(uw)s IN (t, ta, tc)
OR %(uw)s = ANY (tb)
);
If there can be orphans in t_tbl and/or p_tbl (not linked to any row in x_tbl), the set may be bigger, and the query gets more expensive:
SELECT %(uw)s -- old-style Python placeholder
WHERE NOT EXISTS NOT EXISTS (
SELECT FROM x_tbl JOIN p_tbl p USING (p_id)
WHERE p.pa = %(uw)s
OR %(uw)s = ANY (p.pb)
)
AND (
SELECT FROM x_tbl JOIN t_tbl t USING (t_id)
WHERE %(uw)s IN (t.t, t.ta, t.tc)
OR %(uw)s = ANY (t.tb)
);
This is dealing with one search term at a time, like your original query. You mentioned a list. Running a single query for all of them might be (much) cheaper ...
One query to rule them all
Pass the list as array (Postgres array literal) - which may require an explicit cast (::text[]) - unnest() and attach the same WHERE conditions as above:
SELECT uw
FROM unnest(%(my_list_as_array)s::text[]) q(uw)
WHERE NOT EXISTS (
SELECT FROM p_tbl
WHERE pa = q.uw
OR q.uw = ANY (p.pb)
)
AND NOT EXISTS (
SELECT FROM t_tbl
WHERE q.uw IN (t, ta, tc)
OR q.uw = ANY (tb)
);
Or, including the join to tbl_x, same as above:
SELECT uw
FROM unnest(%(my_list_as_array)s::text[]) q(uw)
WHERE NOT EXISTS (
SELECT FROM x_tbl JOIN p_tbl p USING (p_id)
WHERE p.pa = q.uw
OR q.uw = ANY (p.pb)
)
AND NOT EXISTS (
SELECT FROM x_tbl JOIN t_tbl t USING (t_id)
WHERE q.uw IN (t.t, t.ta, t.tc)
OR q.uw = ANY (t.tb)
);
Basics:
Select rows which are not present in other table
You may want to keep array elements in original order, or even attach an ordinal position. See:
PostgreSQL unnest() with element number
Aside, your original query can multiply rows - if there can be more than one row on the right side of each join. See:
Two SQL LEFT JOINS produce incorrect result