Informix subqueries with FIRST option
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06-07-2019 - |
Question
What is the best way of transcribing the following Transact-SQL code to Informix Dynamic Server (IDS) 9.40:
Objective: I need the first 50 orders with their respective order lines
select *
from (select top 50 * from orders) a inner join lines b
on a.idOrder = b.idOrder
My problem is with the subselect because Informix does not allow the FIRST option in the subselect.
Any simple idea?.
Solution
The official answer would be 'Please upgrade from IDS 9.40 since it is no longer supported by IBM'. That is, IDS 9.40 is not a current version - and should (ideally) not be used.
Solution for IDS 11.50
Using IDS 11.50, I can write:
SELECT *
FROM (SELECT FIRST 10 * FROM elements) AS e
INNER JOIN compound_component AS a
ON e.symbol = a.element
INNER JOIN compound AS c
ON c.compound_id = a.compound_id
;
This is more or less equivalent to your query. Consequently, if you use a current version of IDS, you can write the query using almost the same notation as in Transact-SQL (using FIRST in place of TOP).
Solution for IDS 9.40
What can you do in IDS 9.40? Excuse me a moment...I have to run up my IDS 9.40.xC7 server (this fix pack was released in 2005; the original release was probably in late 2003)...
First problem - IDS 9.40 does not allow sub-queries in the FROM clause.
Second problem - IDS 9.40 does not allow 'FIRST n' notation in either of these contexts:
SELECT FIRST 10 * FROM elements INTO TEMP e;
INSERT INTO e SELECT FIRST 10 * FROM elements;
Third problem - IDS 9.40 doesn't have a simple ROWNUM.
So, to work around these, we can write (using a temporary table - we'll remove that later):
SELECT e1.*
FROM elements AS e1, elements AS e2
WHERE e1.atomic_number >= e2.atomic_number
GROUP BY e1.atomic_number, e1.symbol, e1.name, e1.atomic_weight, e1.stable
HAVING COUNT(*) <= 10
INTO TEMP e;
SELECT *
FROM e INNER JOIN compound_component AS a
ON e.symbol = a.element
INNER JOIN compound AS c
ON c.compound_id = a.compound_id;
This produces the same answer as the single query in IDS 11.50. Can we avoid the temporary table? Yes, but it is more verbose:
SELECT e1.*, a.*, c.*
FROM elements AS e1, elements AS e2, compound_component AS a,
compound AS c
WHERE e1.atomic_number >= e2.atomic_number
AND e1.symbol = a.element
AND c.compound_id = a.compound_id
GROUP BY e1.atomic_number, e1.symbol, e1.name, e1.atomic_weight,
e1.stable, a.compound_id, a.element, a.seq_num,
a.multiplicity, c.compound_id, c.name
HAVING COUNT(*) <= 10;
Applying that to the original orders plus order lines example is left as an exercise for the reader.
Relevant subset of schema for 'Table of Elements':
-- See: http://www.webelements.com/ for elements.
-- See: http://ie.lbl.gov/education/isotopes.htm for isotopes.
CREATE TABLE elements
(
atomic_number INTEGER NOT NULL UNIQUE CONSTRAINT c1_elements
CHECK (atomic_number > 0 AND atomic_number < 120),
symbol CHAR(3) NOT NULL UNIQUE CONSTRAINT c2_elements,
name CHAR(20) NOT NULL UNIQUE CONSTRAINT c3_elements,
atomic_weight DECIMAL(8,4) NOT NULL,
stable CHAR(1) DEFAULT 'Y' NOT NULL
CHECK (stable IN ('Y', 'N'))
);
CREATE TABLE compound
(
compound_id SERIAL NOT NULL PRIMARY KEY,
name VARCHAR(100) NOT NULL UNIQUE
);
-- The sequence number is used to order the components within a compound.
CREATE TABLE compound_component
(
compound_id INTEGER REFERENCES compound,
element CHAR(3) NOT NULL REFERENCES elements(symbol),
seq_num SMALLINT DEFAULT 1 NOT NULL
CHECK (seq_num > 0 AND seq_num < 20),
multiplicity INTEGER NOT NULL
CHECK (multiplicity > 0 AND multiplicity < 20),
PRIMARY KEY(compound_id, seq_num)
);
Output (on my sample database):
1 H Hydrogen 1.0079 Y 1 H 1 2 1 water
1 H Hydrogen 1.0079 Y 3 H 2 4 3 methane
1 H Hydrogen 1.0079 Y 4 H 2 6 4 ethane
1 H Hydrogen 1.0079 Y 5 H 2 8 5 propane
1 H Hydrogen 1.0079 Y 6 H 2 10 6 butane
1 H Hydrogen 1.0079 Y 11 H 2 5 11 ethanol
1 H Hydrogen 1.0079 Y 11 H 4 1 11 ethanol
6 C Carbon 12.0110 Y 2 C 1 1 2 carbon dioxide
6 C Carbon 12.0110 Y 3 C 1 1 3 methane
6 C Carbon 12.0110 Y 4 C 1 2 4 ethane
6 C Carbon 12.0110 Y 5 C 1 3 5 propane
6 C Carbon 12.0110 Y 6 C 1 4 6 butane
6 C Carbon 12.0110 Y 7 C 1 1 7 carbon monoxide
6 C Carbon 12.0110 Y 9 C 2 1 9 magnesium carbonate
6 C Carbon 12.0110 Y 10 C 2 1 10 sodium bicarbonate
6 C Carbon 12.0110 Y 11 C 1 2 11 ethanol
8 O Oxygen 15.9990 Y 1 O 2 1 1 water
8 O Oxygen 15.9990 Y 2 O 2 2 2 carbon dioxide
8 O Oxygen 15.9990 Y 7 O 2 1 7 carbon monoxide
8 O Oxygen 15.9990 Y 9 O 3 3 9 magnesium carbonate
8 O Oxygen 15.9990 Y 10 O 3 3 10 sodium bicarbonate
8 O Oxygen 15.9990 Y 11 O 3 1 11 ethanol
OTHER TIPS
If I understand your question you are having a problem with "TOP". Try using a TOP-N query.
For example:
select *
from (SELECT *
FROM foo
where foo_id=[number]
order by foo_id desc)
where rownum <= 50
This will get you the top fifty results (because I order by desc in the sub query)