MySQL CASE那么空案例值

Question

    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:

这个数据集没有任何年龄<18岁的记录.所以ageband"<18"没有显示出来.我怎样才能使它显示并返回值0 ??

Answer 1

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