在SQL上计算类别中的不同项目

Question

I need to compose a SQL statement as follows: 我需要编写一个SQL语句，如下所示：

I have a table with many items, each item having a category. 我有一个包含许多项目的表格，每个项目都有一个类别。 In total there are 3 categories. 总共有3个类别。

I need to select the DISTINCT categories and then order them by number of items within each category. 我需要选择DISTINCT类别，然后按每个类别中的项目数量排序。

Would this be a good way? 这会是一个好方法吗？ Or too slow? 还是太慢了？

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 : 由于您使用GROUP BY category因此不需要DISTINCT ：

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. GROUP BY正在做你想要的。 DISTINCT is redundant. DISTINCT是多余的。

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。