删除重复的子串

试试这个:

UPDATE tableA 
SET Mashup = IF(Keyword = Title, Keyword, CONCAT(Keyword, ' ', Title));

检查这个SQL FIDDLE DEMO

OUTPUT

|    KEYWORD |      TITLE |           MASHUP |
|------------|------------|------------------|
|      Green |      Green |            Green |
|      Green | Watermelon | Green Watermelon |
| Watermelon | Watermelon |       Watermelon |