INSERT INTO TableC
SELECT
t.word,
SUM(COALESCE(a.countA, 0)) AS CountA,
SUM(COALESCE(b.countB, 0)) AS countB
FROM
(
SELECT wordA AS word FROM tableA
UNION
SELECT wordB FROM tableB
) AS t
LEFT JOIN tableA AS a on t.word = a.wordA
LEFT JOIN tableB AS b on t.word = b.wordb
GROUP BY t.word
Demostración de SQL Fiddle
Esto le dará:
| WORD | COUNTA | COUNTB |
|--------|--------|--------|
| ab | 0 | 10 |
| abc | 25 | 40 |
| abcd | 29 | 0 |
| abcde | 45 | 90 |
| abcdef | 0 | 55 |
