ProductCategories
---
productid    categoryid
1
2
3
4


Products
--------
productid    productname
1            Apple
2            Apricot
3            Tomato
4            Watermelon

ProductCategories
---
productid    categoryid
1            1
2            1
3            2
4            3

UPDATE ProductCategories SET categoryid = '1' WHERE product_id IN (SELECT product_id FROM Products WHERE productname LIKE 'Ap%')

UPDATE
    ProductCategories,
    Products
SET
    ProductCategories.categoryid = CASE LEFT(UCASE(LTRIM(Products.productname)), 2)
                                      WHEN 'AP' THEN 1
                                      WHEN 'TO' THEN 2
                                      WHEN 'WA' THEN 3
                                      ELSE ProductCategories.categoryid
                                   END
    WHERE
        ProductCategories.product_id = Products.product_id