By using UNION
you may get several times rows with the same ID. What about using LEFT JOIN
?
If I've understood your question:
SELECT table_zero.id, table_1.varchar_field, table_2.varchar_field
FROM table_zero
LEFT JOIN table_1 ON table_zero.id = table_1.id
LEFT JOIN table_2 ON table_zero.id = table_2.id
WHERE table_1.varchar_field LIKE '%str%'
OR table_2.varchar_field LIKE '%str%'