SQL 查询 - 将 count() 结果拆分为一行 GROUP BY 结果

Question

I want to use GROUP BY to get a row per month-year. Additionally, I have another field which, for this example, is colour and can have values of Blue or Red.

I want to see a table with columns for Blue, Red, month and year = I want to count the number of each colour per month-year. Can I do this with an SQL query?

I know that count(colour) will give me the total number of rows for each month-year.

It's good if it's possible to do this but some solutions may involve coding in "Red" and "Blue" = just 2 values. Is it possible to run a query to execute a count which will split out the "answers" from the count into each line rather than a line each?

Example data:

Year	Month	Day	Colour
2021	3	6	Blue
2021	3	7	Blue
2021	3	8	Blue
2021	3	9	Red
2021	4	5	Blue
2021	4	6	Red
2021	4	7	Blue
2021	4	8	Red
2021	4	9	Red

to give result

Year	Month	Blue	Red
2021	3	3	1
2021	4	2	3

I'm doing this in mysql and also in javascript using alasql but a suggestion for any version of SQL will probably be helpful here...

Answer 1

You can use SUM(CASE WHEN... to do this

SELECT Year, Month, SUM(CASE WHEN Colour = 'Blue' THEN 1 ELSE 0 END) AS Blue, etc
FROM table
GROUP BY Year, Month

Answer 2

I will suggest fixing the design. You could store date in one column only, in date format. Be aware of the MySQL reserved word such as YEAR , MONTH , DATE , you should use backticks for those columns.

If you only have blue and red color you could do an easy solution:

SELECT Year, 
       Month, 
       SUM(Colour='Blue') as Blue, 
       SUM(Colour='Red') as Red   
FROM test_tbl    
GROUP BY `Year`, `Month`;

Result:

 Year Month Blue Red 2021 3 3 1 2021 4 2 3

Demo

Answer 3

You could do the following:

  GROUP BY Year(yourdate),
     Month(yourdate),
     Day(yourdate),
     Colour

Group By is not limited to acting on a single column (or expression).

Answer 4

You can write a query like this

SELECT Year, 
       Month, 
       COUNT(CASE WHEN Colour = 'Blue' THEN 1 END) AS Blue, 
       COUNT(CASE WHEN Colour = 'Red' THEN 1 END) AS Red    
FROM table    
GROUP BY Year, Month;