Using a Loop in Firebird SQL - firebird

Question

I want my Firebird SQL to loop through part of the code WHILE a condition is meet.
Initially I didn't even think it was possible. However I have done some reading and now believe that I can use WHILE loop.
I understand a FOR loop is not what I want as it applies to the whole code, not just part of it.
I am using this in Excel and could use some VBA code to do what I want, but it would be better if I can do it all via Firebird SQL as then I can apply it elsewhere.
SELECT
'1' as "Qty",
'of ' || ALP3.PROPERTYVALUE AS "Total Qty"
FROM ASSEMBLYLINES
LEFT JOIN ASSEMBLYLINEPROPS ALP1 ON ALP1.HEADERSYSUNIQUEID = ASSEMBLYLINES.SYSUNIQUEID AND ALP1.PROPERTYNAME = 'Process2'
LEFT JOIN ASSEMBLYLINEPROPS ALP2 ON ALP2.HEADERSYSUNIQUEID = ASSEMBLYLINES.SYSUNIQUEID AND ALP2.PROPERTYNAME = 'Process3'
LEFT JOIN ASSEMBLYLINEPROPS ALP3 ON ALP3.HEADERSYSUNIQUEID = ASSEMBLYLINES.SYSUNIQUEID AND ALP3.PROPERTYNAME = 'Job Quantity'
LEFT JOIN ASSEMBLYLINEPROPS ALP4 ON ALP4.HEADERSYSUNIQUEID = ASSEMBLYLINES.SYSUNIQUEID AND ALP4.PROPERTYNAME = 'Drawing No'
WHERE ASSEMBLYLINES.ORDERNUMBER='16708R01'
AND ASSEMBLYLINES.LINECODE='FABPART'
AND ASSEMBLYLINES.SYSUSERCREATED <> 'EXTERNAL USER'
ORDER BY ALP4.PROPERTYVALUE
My results using the code above is:
Qty Total Qty
1 4
However, what I want is:
My results using the code above is:
Qty Total Qty
1 4
2 4
3 4
4 4
I understand the While loop would be something like:
While Qty <= ALP3.PROPERTYVALUE Do
<<output>>
Loop

Answer 1

Qty Total Qty
1 4
2 4
3 4
4 4
I understand the While loop would be something like:
While Qty <= ALP3.PROPERTYVALUE Do
<<output>>
Loop
So, your "quantity" column is not actually a quantity of some real data (like quantity of containers in cargo ship), but a row number in some your output report/grid.
And then what you want is limiting the output "rowset" - matrix, table, grid - to some N first rows.
Well, that is exactly how it is done, asking for the first rows only.
Select FIRST(4) column1, column2, column3
From table 1
Where condition1 and condition2 or condition3
See the "first" clause in documentation: https://www.firebirdsql.org/file/documentation/reference_manuals/fblangref25-en/html/fblangref25-dml-select.html
Also see "Limiting result rows" chapyer in Wikipedia: https://en.wikipedia.org/wiki/Select_%28SQL%29#Limiting_result_rows
You can also use "window functions" starting with Firebird version 3, but they are somewhat overkill for the simple task of "only give me first N rows".
Now, there is one more method that provides for embedding a totally voluntary condition, but that is from "ugly hacks" toolsets and does not work in a typical situation when several simultaneous connections form different client programs are running. You can use a "generator" as part of the WHILE clause:
Select .....
Where (GEN_ID(cancel_generator_name, 0) = 0) AND ( ...you normal conditions...)
You set the generator value to 0 before the query, and your client evaluates some conditions of your choice while reading the data, and when it wants to - from some another SQL command library object it issues the generator change command, which would immediately skip the rest of the query. However while sometimes this is a useful technique, but only in very specific rare situations.
Since Mark seems to be better guessing than me, then some outlines for future guesswork.
SP is a standard abbreviation for SQL Stored Procedure. Firebird's Execute Block is essentially an anonymous non-persistent SP.
So, we start with a persistent and named SP.
create or alter procedure SEQ (
FROM_1_TO integer not null)
returns (
COUNTER integer)
as
begin
counter = 1;
while ( counter <= from_1_to ) do begin
suspend;
counter = counter + 1;
end
end
Select 1, s.counter from rdb$database, seq(5) s
CONSTANT COUNTER
1 1
1 2
1 3
1 4
1 5
The next question would be how to
join the table with SP (stored procedure) dependent upon specific table row values
avoid SP being executed with NULL parameter values
The answer is - LEFT JOIN, as shown in the FAQ: http://www.firebirdfaq.org/faq143/
CREATE TABLE T2 (
ID INTEGER NOT NULL PRIMARY KEY,
TITLE VARCHAR(10) NOT NULL,
QTY INTEGER NOT NULL
);
INSERT INTO T2 (ID, TITLE, QTY) VALUES (1, 'aaaa', 2);
INSERT INTO T2 (ID, TITLE, QTY) VALUES (2, 'bbbb', 5);
INSERT INTO T2 (ID, TITLE, QTY) VALUES (3, 'ccccc', 4);
Select * from t2 t
left join seq(t.qty) s on 1=1
ID TITLE QTY COUNTER
1 aaaa 2 1
1 aaaa 2 2
2 bbbb 5 1
2 bbbb 5 2
2 bbbb 5 3
2 bbbb 5 4
2 bbbb 5 5
3 ccccc 4 1
3 ccccc 4 2
3 ccccc 4 3
3 ccccc 4 4
If you would have many different queries on different tables/fields that would require this rows-cloning added then having a dedicated counter-generating SP makes sense.
However if you only need this rather exotic rows cloning once, then maybe polluting a global namespace with an SP you would never need again would be less of a good idea.
It seems one can not select from an EB, though: Select from execute block?
So you would have to make a specific ad-hoc EB exactly for your select statement. Which, arguably, might be the very reason d'etre for anonymous non-persistent EB.
execute block
returns (ID INTEGER, TITLE VARCHAR(10), QTY INTEGER, COUNTER INTEGER)
as
begin
for select
id, title, qty from t2
into :id, :title, :qty
do begin
counter = 1;
while
(counter <= qty)
do begin
suspend;
counter = counter + 1;
end
end
end
However the data access library your application uses to connect to Firebird should understand then that while this query is not SELECT-query it still returns the "rowset". Usually they do, but who knows.