MySQL SQL：如何获取列中每个不同值的计数？
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SQL: How to get the count of each distinct value in a column?
提问by Jeff Gortmaker
I have a SQL table called "posts" that looks like this:
我有一个名为“posts”的 SQL 表，如下所示：
id | category
-----------------------
1 | 3
2 | 1
3 | 4
4 | 2
5 | 1
6 | 1
7 | 2
Each category number corresponds to a category. How would I go about counting the number of times each category appears on a post all in one sql query?
每个类别编号对应一个类别。我将如何计算每个类别在一个 sql 查询中全部出现在帖子中的次数？
As an example, such a query might return a symbolic array such as this: (1:3, 2:2, 3:1, 4:1)
例如，这样的查询可能会返回一个符号数组，如下所示： (1:3, 2:2, 3:1, 4:1)
My current method is to use queries for each possible category, such as: SELECT COUNT(*) AS num FROM posts WHERE category=#, and then combine the return values into a final array. However, I'm looking for a solution that uses only one query.
我目前的方法是对每个可能的类别使用查询，例如: SELECT COUNT(*) AS num FROM posts WHERE category=#，然后将返回值组合成一个最终的数组。但是，我正在寻找一种仅使用一个查询的解决方案。
回答by Dan Grossman
SELECT
category,
COUNT(*) AS `num`
FROM
posts
GROUP BY
category
