mysql - 如何组合这些按同一字段分组的查询？

Question

我有以下三个疑问

SELECT COUNT(*) as cached_type_a, DATE(datetime) as datetime
    FROM requests
    WHERE cached = 'a'
    GROUP BY DAY(datetime);

SELECT COUNT(*) as cached_type_b, DATE(datetime) as datetime
    FROM requests
    WHERE cached = 'b'
    GROUP BY DAY(datetime);

SELECT COUNT(*) as cached_type_c, DATE(datetime) as datetime
    FROM requests
    WHERE cached = 'c'
    GROUP BY DAY(datetime);

是否可以将它们组合成一个包含 4 列的查询

cached_type_a、cached_type_b、cached_type_c、日期时间？

Answer 1

如果cache只有三个可能的值，则可以使用这个，

SELECT DATE(datetime) as datetime,
        SUM(CASE WHEN cached = 'a' THEN 1 ELSE 0 END) cached_a,
        SUM(CASE WHEN cached = 'b' THEN 1 ELSE 0 END) cached_b,
        SUM(CASE WHEN cached = 'c' THEN 1 ELSE 0 END) cached_c
FROM requests
GROUP BY DAY(datetime)

否则，如果您有多个缓存，则可以使用Prepared Statement

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'SUM(CASE WHEN cached =  ''',
      cached,
      ''' then 1 ELSE 0 end) AS ',
      CONCAT('cached_',cached)
    )
  ) INTO @sql
FROM requests;

SET @sql = CONCAT('SELECT DATE(datetime) as datetime, ', @sql, ' 
                   FROM requests 
                   GROUP BY DAY(datetime)');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;