MySQL CASE那么空案例值

    SELECT CASE WHEN age IS NULL THEN 'Unspecified' 
                WHEN age < 18 THEN '<18' 
                WHEN age >= 18 AND age <= 24 THEN '18-24' 
                WHEN age >= 25 AND age <= 30 THEN '25-30' 
                WHEN age >= 31 AND age <= 40 THEN '31-40' 
                WHEN age > 40 THEN '>40' 
            END AS ageband,COUNT(*) 
       FROM (SELECT age 
               FROM table) t 
   GROUP BY ageband

这是我的查询.这些是结果：

但是,如果table.age在一个类别中没有至少1个年龄,那么它将在结果中忽略该情况.像这样：

该数据集没有任何年龄的记录<因此,年龄段“< 18”没有出现.我怎样才能使它显示并返回值0 ??

最佳答案

您需要一个agebands表来填充没有匹配行的条目的结果.这可以通过实际表来完成,或者使用如下子查询动态生成：

SELECT a.ageband,IFNULL(t.agecount,0)
FROM (
  -- ORIGINAL QUERY
  SELECT
    CASE
      WHEN age IS NULL THEN 'Unspecified'
      WHEN age < 18 THEN '<18'
      WHEN age >= 18 AND age <= 24 THEN '18-24'
      WHEN age >= 25 AND age <= 30 THEN '25-30'
      WHEN age >= 31 AND age <= 40 THEN '31-40'
      WHEN age > 40 THEN '>40'
    END AS ageband,COUNT(*) as agecount
  FROM (SELECT age FROM Table1) t
  GROUP BY ageband
) t
right join (
  -- TABLE OF POSSIBLE AGEBANDS
  SELECT 'Unspecified' as ageband union
  SELECT '<18' union
  SELECT '18-24' union
  SELECT '25-30' union
  SELECT '31-40' union
  SELECT '>40'
) a on t.ageband = a.ageband

演示：http://www.sqlfiddle.com/#!2/7e2a9/10