Question

I’m attempting to set a row value to 1 when it is the only row in a group, with a single query.

Current Table (CATEGORY_BRAND):

|categoryId | brandId | featured |
|-----------+---------+----------|
| 1         | 100     | -1       |
| 1         | 101     | -1       |
| 1         | 102     | -1       |
| 2         | 100     | -1       |
| 3         | 110     | -1       |
| 3         | 111     | -1       |
| 4         | 102     | -1       |
| 5         | 101     | -1       |
| 6         | 100     | -1       |
| 6         | 110     | -1       |
| ...       | ...     | ...      |

Hoped for Result (CATEGORY_BRAND):

|categoryId | brandId | featured |
|-----------+---------+----------|
| 1         | 100     | -1       |
| 1         | 101     | -1       |
| 1         | 102     | -1       |
| 2         | 100     | 1        |
| 3         | 110     | -1       |
| 3         | 111     | -1       |
| 4         | 102     | 1        |
| 5         | 101     | 1        |
| 6         | 100     | -1       |
| 6         | 110     | -1       |
| ...       | ...     | ...      |

(My sincerest apologies if this is a duplicate, I couldn’t find anyone else doing an UPDATE like this.)

Answer 1

This may not be the most performant, but should work (although you should test it first).

UPDATE category_brand JOIN (
    SELECT categoryId
    FROM category_brand
    GROUP BY categoryId
    HAVING COUNT(categoryId) = 1)
  AS single_cbs USING(categoryId) SET category_brand.featured = 1;