问题
The sequence:
table1 ===== id - Description ---------------- |1 |Proj-x |2 |Settlers |3 |Bank |4 |Newiest table2 ===== id table1Id value alternate-value --------------------------------- |1| 1 |12 |null |1| 4 |6 | null |1| null |22 |Desktop |2| 2 |7 |null |2| 3 |11 |null |2| null |2 |Camby Jere |3| 1 |8 |null |3| 4 |6 |null |3| null |7 |Camby Jere
The select instruction must return
|table1.id|Proj-x|Settlers|Bank |Newiest|Desktop|Camby Jere ---------------------------------------------------------- |1 |12 |null |null |null |null |null |1 |null |null |6 |null |null |null |1 |null |null |null |null |22 |null |2 |null |7 |null |null |null |null |2 |null |null |11 |null |null |null |2 |null |null |null |null |null |2 |3 |8 |null |null |null |null |null |3 |null |null |null |6 |null |null |3 |null |null |null |null |null |7
The columns are description from table1 when id exists in table2 or the column "alternate-value" when table1Id is null.
Is it possible? Or do I need construct the query dynamically?
回答1:
Well, yes, it is possible (if done in two steps), but it is a bit complex so I'm not certain whether you should do it. First, you could execute the following select:
with tmp1(MyFieldName) as
(select distinct coalesce(t2.alternate_value, t1.Description)
from table2 t2
left join table1 t1 on t2.Table1ID = t1.id),
tmp2(MyPivotSource) as
(select 'iif(coalesce(t2.alternate_value, t1.Description) = '''||MyFieldName||''', t2.MyValue, 0) as "'||MyFieldName||'"'
from tmp1)
select 'select t2.id as "table1.id", '||list(MyPivotSource)||'from table2 t2
left join table1 t1 on t2.Table1ID = t1.id'
from rdb$database
cross join tmp2
And then you would have to run the result. Note that I used MyValue rather than Value and that the columns may not appear in the order you desire (although that could also be possible).
Pivottables are not something that easily converts to SQL in Firebird and I generally prefer to create Pivot tables in Excel rather than Firebird, but as you can see it is possible.
来源：https://stackoverflow.com/questions/29896351/pivot-rows-into-columns-firebird-2-5