sql - 取消展平 table

Question

我有一个采用以下形式的表格:

person, date, food1, food2, food3

我想使用这张表并得到如下所示的结果:

person, date, foodId

例如，如果我的原始表中有一行看起来像这样:

Andy, 1/1/2012,false,true,true

我会得到如下所示的结果

Andy, 1/1/2012, food2
Andy, 1/1/2012, food3

到目前为止我已经

SELECT person, date, 
(
    case 
        when food1 = true then 'food1'
        when food2 = true then 'food2'
        when food3 = true then 'food3'
    end
) as foodId

但是，当每个源行最多有 3 个结果时，这只会获取每个源行一个结果。有什么办法可以修复我的查询，从我的示例中获取 2 行而不是仅仅 1 行吗？

Answer 1

您可以取消数据透视:Using PIVOT and UNPIVOT

设置:

create table diet(person varchar(10), entry datetime, food1 varchar(10), food2 varchar(10), food3 varchar(10))
insert into diet values('John', CURRENT_TIMESTAMP, 'Apple','Pizza','Cake');

逆透视查询:

SELECT person, entry, meal, food
FROM 
   (SELECT person, entry, food1, food2, food3
   FROM diet) p
UNPIVOT
   (food FOR meal IN 
      (food1, food2, food3)
      )AS unpvt;

输出:

person  entry   meal    food
John    2012-02-08 14:42:22.940 food1   Apple
John    2012-02-08 14:42:22.940 food2   Pizza
John    2012-02-08 14:42:22.940 food3   Cake