如何获得同一行上不同值的计数？

Question

我需要一个报告，列中有不同值的计数，按日期分组，每个日期有一个输出行。

表格列是日期，位置ID（var）和评分（var）。评分只能有四个值中的一个，'A'，'B'，'C'或空/空白。我可以用此选择每个日期的每个值获得一行，但是如何获得同一行上所有四个值的计数，对于每个日期？

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

Answer 1

使用SUM用布尔如下：

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