只需将您的查询用作 AVG 的子查询，并且不要将列命名为 COUNT保留字.

Just use your query as subquery for the AVG, and dont name a column COUNT that is a reserved word.

SQL Fiddle 演示

SELECT AVG(ITEM_COUNT)
FROM (
        SELECT Count(*) AS ITEM_COUNT
        FROM store_sales 
        GROUP BY ItemSize, ItemColor
     ) T