mysql - 如何在同一行获取不同值的计数？

Question

我需要一份报告，其中包含按日期分组的列具有的不同值的计数，每个日期一个输出行。

表格列是日期、位置 ID (var) 和评级 (var)。评级只能有四个值之一，“A”、“B”、“C”或空/空白。使用此选择，我可以在每个日期每个值获取一行，但是如何获取每个日期在同一行上的所有四个值的计数？

SELECT date, rating, count(rating) AS ratings
FROM table
GROUP BY date, rating

Answer 1

与布尔值一起使用SUM，如下所示：

SELECT
    date,
    SUM(rating = 'A') AS count_A,
    SUM(rating = 'B') AS count_B,
    SUM(rating = 'C') AS count_C,
    SUM(rating = '' OR rating IS NULL) AS count_blank
FROM yourtable
GROUP BY date