SQL/DB2 替代扩展 CASE 语句

Question

我有一个项目，其中单/多列中的数据需要按组分类。 我的程序使用 CASE 语句来创建更通用的类别。 由于我的代码很快变得笨拙，我认为我没有一个很好的方法。

我怎样才能做得更好？

我的 Db2 查询使用“CASE STATEMENT”生成“分类”列：

SELECT id
,plant
,CASE
     
when plant like 'rose'
     
 then 'flower'

when plant like 'wildflower'
 then 'flower'
when plant like 'spruce'
 then 'tree'
when plant like 'willow'
 then 'tree'
when plant like 'peony'
 then 'flower'
when plant like 'oak'
 then 'tree'
else 'unknown'
     
end as classification
FROM green_table

+----+------------+----------------+
| id | plant      | classification |
+----+------------+----------------+
| 1  | rose       | flower         |
+----+------------+----------------+
| 2  | wildflower | flower         |
+----+------------+----------------+
| 3  | spruce     | tree           |
+----+------------+----------------+
| 4  | willow     | tree           |
+----+------------+----------------+
| 5  | peony      | flower         |
+----+------------+----------------+
| 6  | oak        | tree           |
+----+------------+----------------+

Answer 1

对派生表使用join ：

select g.*, coalesce(c.classification, 'unknown') as classification
from green_table g left join
     (select 'rose' as plant, 'flower' as classification from sysibm.sysdummy1 union all
      select 'wildflower' as plant, 'flower' as classification from sysibm.sysdummy1 union all
      . . .
     ) c
     on g.plant = c.plant;

我实际上建议你有一个名为plants的表，每株植物有一行和分类 - 以及一个用于连接名称的数字主键。

注意：这使用字符串相等而不是like 。 Equality 似乎就足够了， like将对join的性能产生很大影响。

Answer 2

您可以使用in operator列出相同的类别

SELECT id,plant,
CASE   
  when plant in ('rose','wildflower','peony') then 'flower'
  when plant in ('spruce','willow','oak') then 'tree'
else 'unknown' end as classification
FROM green_table

Answer 3

您可以使用DECODE() 。 它甚至包括一个“其他”案例。 例如：

SELECT 
  id,
  plant,
  decode(plant, 'rose', 'flower', 'wildflower', 'flower', 'spruce', 'tree',
                'willow', 'tree', 'peony', 'flower', 'oak', 'tree',
                'unknown') as classification
FROM green_table