MySQL CASE，按GROUP BY计数

Question

I have these three queries

SELECT COUNT(question_id) AS correct_count, uID FROM mytable  WHERE id
>= 0 AND id <= 1000 AND correct = 1 AND answer_id IS NOT NULL GROUP BY user_id


SELECT COUNT(question_id) AS incorrect_count, uID FROM mytable  WHERE id >= 0 AND id <= 1000 AND correct !=1 AND answer_id IS NOT NULL GROUP BY user_id

SELECT COUNT(question_id) AS null_count, uID FROM mytable  WHERE id >= 0 AND id <= 1000 AND answer_id IS NULL GROUP BY user_id

How can I join this as a single query using CASE or IF ?

Answer 1

SELECT user_id, 
       sum(id >= 0 AND id <= 1000 AND correct = 1 AND answer_id IS NOT NULL) AS correctCnt, 
       sum(id >= 0 AND id <= 1000 AND correct !=1 AND answer_id IS NOT NULL) AS incorrectCnt,
       sum(id >= 0 AND id <= 1000 AND answer_id IS NULL) as nullCnt
FROM mytable  
GROUP BY user_id

Answer 2

count only counts non- null items, so you could convert the where clauses to case expressions that return null when they do not match:

SELECT   user_id,
         COUNT (CASE WHEN id >= 0 AND id <= 1000 AND correct = 1 AND answer_id IS NOT NULL THEN 1 ELSE NULL) AS correct_count, 
         COUNT (CASE WHEN id >= 0 AND id <= 1000 AND correct !=1 AND answer_id IS NOT NULL THEN 1 ELSE NULL) AS incorrect_count,
         COUNT (CASE WHEN WHERE id >= 0 AND id <= 1000 AND answer_id IS NULL) AS null_count
FROM     mytable  
GROUP BY user_id

Note that all these case statements share some conditions, so those could be extracted to a where clause:

SELECT   user_id,
         COUNT (CASE WHEN correct = 1 AND answer_id IS NOT NULL THEN 1 ELSE NULL) AS correct_count, 
         COUNT (CASE WHEN correct !=1 AND answer_id IS NOT NULL THEN 1 ELSE NULL) AS incorrect_count,
         COUNT (CASE WHEN WHERE answer_id IS NULL THEN 1 ELSE NULL) AS null_count
FROM     mytable
WHERE    id >= 0 AND id <= 1000  
GROUP BY user_id