Tente isto:
UPDATE tableA
SET Mashup = IF(Keyword = Title, Keyword, CONCAT(Keyword, ' ', Title));
Verifique esta SQL FIDDLE DEMO
SAÍDA
| KEYWORD | TITLE | MASHUP |
|------------|------------|------------------|
| Green | Green | Green |
| Green | Watermelon | Green Watermelon |
| Watermelon | Watermelon | Watermelon |
