Question

假设有一个表格有两列，名字和眼睛颜色。

在此表中，您可能有：

Name - Green Eyes
Name - Brown Eyes

我正在尝试为绿眼睛的名字选择所有行，但如果有一个没有绿眼睛的名字，那么我想选择棕色眼睛的名字。我永远不想为给定的名称返回两行。有关如何做到这一点的任何想法？任何帮助将不胜感激！

Answer 1

SELECT COALESCE(g.name, b.name) name,
    COALESCE(g.eye_color, b.eye_color) eye_color

FROM (
  SELECT DISTINCT name, eye_color
  FROM eye_colors
  WHERE eye_color = 'green'
) g

FULL OUTER JOIN (
  SELECT DISTINCT name, eye_color
  FROM eye_colors
  WHERE eye_color = 'brown'
) b
ON b.name = g.name

Answer 2

IF EXISTS(SELECT * FROM Table WHERE Color='Green')
    SELECT * FROM Table WHERE Color = 'Green'
ELSE
    SELECT * FROM Table WHERE Color='Brown'

Answer 3

我愿意

select * 
from person 
where person.eyecolor = 'green' 
   or (     person.eyecolor = 'brown' 
        and not exists(select null from person where person.eyecolor = 'green')
      )

Answer 4

我会在表格的（名称，颜色）上有一个索引，然后加入到自身，但先取“绿色”，然后再找棕色

select
      justName.Name,
      coalesce( Green.Eye_Color, Brown.Eye_Color ) as Eye_Color
   from
      ( select distinct name 
           from eye_colors ) justName
         left join eye_colors as Green
            on justName.Name = Green.Name
           and Green.Eye_Color = 'Green'
         left join eye_colors as Brown
            on justName.Name = Brown.Name
           and Brown.Eye_Color = 'Brown'

预查询只获得不同的名称，然后通过左连接到表两次，一次为绿色，第二次为棕色。如果绿色记录不匹配，则会获得棕色实例。

Answer 5

select *
from person p
inner join (
    select decode(count(name), 1, 'Green Eyes', 'Brown Eyes') as color
    from person
    where color = 'Green Eyes'
    and rownum < 2
) t
on p.color = t.color

如果至少有一个人带有“ Green Eyes”，则子查询应返回“ Green Eyes”，否则将返回“ Brown Eyes”。代替内部联接，我们可以使用where子句进行过滤。

SQL查询 - 如果存在，则选择一个其他，从不两者

5 个答案: