Pentaho parametrized datasource - datasource

Question

I am trying to build some parametrized data source (sql query over jndi).
query of my data source is:
SELECT ${param_interval}(dim_date.date), count(docs_fact.id) as docs_count
FROM rel_docs_dates
left join docs_fact on rel_docs_dates.doc_id = docs_fact.id
left join dim_date on rel_docs_dates.date_id = dim_date.id
Parametr ${param_interval} can get two values: MONTH and DAY, and as i checked it got the correct values.
But when i am trying to make preview of my dashboard i get warning "error processing component".
Notice that this query (see bellow) works ok.
SELECT MONTH(dim_date.date), count(docs_fact.id) as docs_count, ${param_interval} as tmp_fiel
FROM rel_docs_dates
left join docs_fact on rel_docs_dates.doc_id = docs_fact.id
left join dim_date on rel_docs_dates.date_id = dim_date.id
Can somebody tell me where is mistake? Or (may be) this way to use parameters in data source is not supported?

Answer 1

finaly i found decision. it isnt what i would like to have but it works and it the most important thing.
i rewrite my query with 'case' constraction and, it is important, i changed type of my parametr from string to numeric (string doesnt work :( ). now my query looks like this:
SELECT
case ${param_interval}
when 1 then MONTH(dim_date.date)
when 2 then DAY(dim_date.date)
end
,count(docs_fact.id) as fact_count
FROM rel_docs_dates
left join docs_fact on rel_docs_dates.doc_id = docs_fact.id
left join dim_date on rel_docs_dates.date_id = dim_date.id
where dim_date.date > LAST_DAY(DATE_SUB(CURDATE(), INTERVAL ${param_period} MONTH))
AND dim_date.date < LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 0 MONTH))
group by
case ${param_interval}
when 1 then MONTH(dim_date.date)
when 2 then DAY(dim_date.date)
end
order by YEAR(dim_date.date), MONTH(dim_date.date)
may be it will help somebody else.