On 11/08/2017 16:57, Jonathan Moules wrote:
Hi List,
I have a simple table:
CREATE TABLE my_table
(
id integer NOT NULL UNIQUE,
cat integer
);
insert into my_table (1, 2);
insert into my_table (2, 2);
insert into my_table (3, 3);
insert into my_table (4, 3);
insert into my_table (5, 2);
I want do a very basic query:
select id from my_table where cat = 3
|id|
|3|
|4|
This will of course return the two rows with that category. But I also want to be able to run the query with a non-existent cat and get a result of "null" for the id.
select id from my_table where cat = 500
would return
|id|
|NULL|
now I can do that with a union all:
select id from my_table where cat = 500
union all
select
NULL as id
|id|
|NULL|
But if I then run that query using a cat value of "3", it will not only return the results, but a third result, of NULL, which I don't want.
|id|
|3|
|4|
|NULL|
I would like to always get a result, either of NULL, or if there are actual results, or those actual results without a NULL if they exist.
I don't see it being possible with any of the coalesce/ifnull/case features, as they only action based on a single row and this can return multiple rows. Maybe something with the window functions or a CTE, but they're both new to me.
Do smth like along the lines (might want to add additional code for ordering, this just happens to run correctly) :
select * from my_table where cat = 500 UNION select null,null LIMIT CASE WHEN (select count(*) from my_table where cat=500)>0 THEN (select count(*) from my_table where cat=500) ELSE 1 END ;
Of course you can write a function to do that for you, but what made you want this in the first place? Maybe this is bad design ?
Is this possible?
Thanks
--
Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt
