Why doesn't this SQL run? Firebird stored procedure - sql

Question

I execute the following simple query in IBExpert firebird2.5 and it works fine:
SELECT
pd.NOME_PRODUTO,
es.QTDE_MINIMA
FROM
TBL_ESTOQUE es,
TBL_PRODUTO pd
WHERE
es.qtde_estoque = 0
AND es.produto = pd.id
ORDER BY
pd.NOME_PRODUTO
But, if I create a stored procedure with two output parameters (see below)
begin
SELECT
pd.NOME_PRODUTO,
es.QTDE_MINIMA
FROM
TBL_ESTOQUE es,
TBL_PRODUTO pd
WHERE
es.qtde_estoque = 0
AND es.produto = pd.id
ORDER BY
pd.NOME_PRODUTO
into :nome_produto, :qtde_minima;
suspend;
end
I get a message like this:
multiple rows in singleton select. multiple rows in singleton
select. At proceddure 'SPD_SALDO_PROD_ZERADO_ESTOQUE' line: 7, col:3"
What is this? I don't understand what is happening...

Answer 1

FOR SELECT ...
INTO ...
DO SUSPEND;

Answer 2

rstrelba's answer shows how to fix the problem. If you want to understand it, here's what's going on.
suspend inside a Firebird stored procedure returns a row to the caller. It can be used in various different ways, and can be thought of as similar to Python's yield statement: suspend operation and send back a single value (or row, in this case), then continue when the caller asks for more data. (It has to be done this way for technical reasons, because database drivers don't always pull the entire result set all at once.)
You've got a select query that can return an arbitrary number of values, followed by a single suspend. Firebird is erroring out, telling you that this is probably wrong. What you want to do is put the select in a for loop, which will loop over the result set and suspend once for each row, as rstrelba's answer demonstrates. This makes sure that the caller gets all the results back.