Counting distinct items within a category on SQL

Question

I need to compose a SQL statement as follows:

I have a table with many items, each item having a category. In total there are 3 categories.

I need to select the DISTINCT categories and then order them by number of items within each category.

Would this be a good way? Or too slow?

SELECT DISTINCT category, count(*) AS counter
FROM item_descr
GROUP BY category
ORDER BY counter DESC

Answer 1

The DISTINCT is not needed since you are using GROUP BY category :

SELECT category, count(*) AS counter
FROM item_descr
GROUP BY category
ORDER BY counter DESC

Answer 2

The GROUP BY is doing what you want. DISTINCT is redundant.

Answer 3

如果要获得良好的性能（尤其是在较大的表上），则在类别上建立索引非常重要。