【Teradata SQL】一個字段為空即取另外一個字段（連續取4個字段）-case when

目標：如果col1為空則取col2的值，如果col2也為空則取col3的值，如果col3還為則取col4的值，如果四個字段均為空則取默認值

1.數據准備

create multiset table test1 
(
col_type varchar(20),
col1 varchar(10),
col2 varchar(20),
col3 varchar(30),
col4 varchar(10)
)no primary index;

insert into test1 values ('fetch_col1','col1','col2','col3','clo4'); --字段均有值
insert into test1 values ('fetch_col1','col1','','col3','clo4');--col2無值
insert into test1 values ('fetch_col1','col1','col2','','clo4');--col3無值
insert into test1 values ('fetch_col1','col1','col2','col3','');--col4無值

insert into test1 values ('fetch_col2','','col2','col3','clo4');--col1無值
insert into test1 values ('fetch_col2','','col2','','clo4');--col3無值
insert into test1 values ('fetch_col2','','col2','col3','');--col4無值
insert into test1 values ('fetch_col2','','col2','','');--僅col2有值，其余均無值

insert into test1 values ('fetch_col3','','','col3','clo4');--col1和col2無值
insert into test1 values ('fetch_col3','','','col3','');--僅col3有值，其余均無值

insert into test1 values ('fetch_col4','','','','clo4');--僅col4有值，其余均無值

insert into test1 values ('fetch_default','','','','');--全部無值

2.查詢結果

sel 
col_type,
case 
   when col1 <>'' then col1 
   when col2 <> '' then col2
   when col3 <>'' then col3
   when col4 <>'' then col4
   else 'default'
end fetch_value
,col1,col2,col3,col4
from test1 order by 3 desc,4 desc,5 desc,6 desc;