MySQL CASE WHEN THEN 空大小写值

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，它会直接忽略结果中的这种情况。像这样:

此数据集没有任何年龄 < 18 的记录。因此年龄段“<18”不会显示。我怎样才能使它显示并返回值 0？？

Answer 1

您需要一个年龄段表来填充没有匹配行的条目的结果。这可以通过一个实际的表来完成，或者使用像这样的子查询动态生成:

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