如何獲得同一行上不同值的計數？

Question

我需要一個報告，列中有不同值的計數，按日期分組，每個日期有一個輸出行。

表格列是日期，位置ID（var）和評分（var）。評分只能有四個值中的一個，'A'，'B'，'C'或空/空白。我可以用此選擇每個日期的每個值獲得一行，但是如何獲得同一行上所有四個值的計數，對於每個日期？

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

Answer 1

使用SUM用布爾如下：

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