Add rows from another table, if random select came in short - sql

Question

I got the following problem and struggle trying to figure out a solution using SQL:
Lets say I got a table full of all kinds of (german) words like this:
WORD
"lieb"
"Gefahr"
"Spagetti"
"hallo"
"danach"
"schiebt"
"ganzem"
"lässt"
"beginnen"
"Schiff"
...
From this table I want to get a random amount of words that fit a special criteria, for example only words that got "mm" in them:
WORD
"Sammlung"
"stimmen"
"Stämme"
"immerzu"
To get the random words containing "mm" I use this query:
SELECT wo_id AS id, wo_word AS word
FROM word
LEFT JOIN wo_wo_ca_rel USING (wo_id)
LEFT JOIN word_category USING (wo_ca_id) -- all word categories like "mm", "aa" ... are stored here
-- word_category stores wo_ca_id : SERIAL and wo_ca_category : TEXT ("mm")
WHERE wo_ca_id = (SELECT wo_ca_id FROM word_category ORDER BY RANDOM() LIMIT 1) -- this gets a random category, but lets pretend it gets us "mm"
ORDER BY RANDOM() LIMIT 5
I figured out how to both of those things. The thing I'm interested now is the following:
Let's say I want to get 5 random words with "mm" in them. If there are not enough in my word table (lets pretend there are only 4, like in my example), then the result of the query should be filled with random words from the word table until it returns 5 words in total.
So instead of getting only:
WORD
"Sammlung"
"stimmen"
"Stämme"
"immerzu"
it should give me something like:
WORD
"Sammlung"
"stimmen"
"Stämme"
"immerzu"
"hallo"
I know I could simply get the result of the "mm" query, check for its count/length in my application and send another query to get the missing words, but I'm curious if this can be done with querying my database once.

Answer 1

-- DDL
CREATE TABLE words (
val TEXT
);
-- sample data
INSERT INTO words VALUES
('Sammlung'),
('stimmen'),
('Stämme'),
('immerzu'),
('aaaaaaa'),
('bbbbbbb');
-- n -> number of records to return;
-- value -> pattern to look for
CREATE OR REPLACE FUNCTION get_random_words(n integer,value text) RETURNS SETOF words AS $$
DECLARE
word_count integer;
pattern text;
extra_row_count integer;
BEGIN
pattern := '%' || value || '%';
--count number of results for given pattern
SELECT count(w.val) INTO word_count
FROM words w WHERE w.val LIKE pattern;
--check all possible conditions
IF word_count = n THEN
RETURN QUERY SELECT val FROM words WHERE val LIKE pattern ORDER BY RANDOM();
ELSEIF word_count > n THEN
RETURN QUERY SELECT val FROM words WHERE val LIKE pattern ORDER BY RANDOM() LIMIT n;
ELSE
extra_row_count := n - word_count;
--return words matching criteria
RETURN QUERY SELECT val FROM words WHERE val LIKE pattern ORDER BY RANDOM();
--return additional number of random words
RETURN QUERY SELECT val FROM words ORDER BY RANDOM() LIMIT extra_row_count;
END IF;
END;
$$ LANGUAGE plpgsql;
-- Sample run
SELECT * FROM get_random_words(5,'mm');

Answer 2

Here's one option using limit with conditional ordering (versus where criteria):
select word
from yourtable
order by word like '%aa%' desc
limit 2
SQL Fiddle Demo
Given your edits, this should work the same way:
SELECT wo_id AS id, wo_word AS word
FROM word
LEFT JOIN wo_wo_ca_rel USING (wo_id)
LEFT JOIN word_category USING (wo_ca_id)
ORDER BY wo_ca_id = (SELECT wo_ca_id FROM word_category ORDER BY RANDOM() LIMIT 1) DESC,
RANDOM()
LIMIT 5