sqlite3 UPDATE generating nulls - sql

Question

I'm trying to transition from MySQL to SQLIte3 and running into an update problem. I'm using SQLite 3.6.20 on redhat.
My first line of code behaves normally
update atv_covar set noncomp= 2;
All values for noncomp (in the rightmost column) are appropriately set to 2.
select * from atv_covar;
A5202|S182|2
A5202|S183|2
A5202|S184|2
It is the second line of code that gives me problems:
update atv_covar
set noncomp= (select 1 from f4003 where
atv_covar.study = f4003.study and
atv_covar.rpid = f4003.rpid and
(rsoffrx="81" or rsoffrx="77"));
It runs without generating errors and appropriately sets atv_covar.noncomp to 1 where it matches the SELECT statement. The problem is that it changes atv_covar.noncomp for the non-matching rows to null, where I want it to keep them as 2.
select * from atv_covar;
A5202|S182|
A5202|S183|1
A5202|S184|
Any help would be welcome.

Answer 1

#Dan, the problem with your query is not specific to SQLite; you are updating all rows of atv_covar, but not all of them have correspondence in f4003, so these default to NULL. You should filter the update or provide a default value.
The following statement sets 1 only to the rows that macth the filtering condition:
UPDATE atv_covar
SET noncomp = 1
WHERE EXISTS (
SELECT 'x'
FROM f4003
WHERE atv_covar.study = f4003.study
AND atv_covar.rpid = f4003.rpid
AND (rsoffrx="81" or rsoffrx="77")
);
The following statement sets 1 or 2 for all rows of noncomp, depending on the filtering match (use this instead of two updates):
UPDATE atv_covar
SET noncomp = COALESCE((
SELECT 1
FROM f4003
WHERE atv_covar.study = f4003.study
AND atv_covar.rpid = f4003.rpid
AND (rsoffrx="81" or rsoffrx="77")
), 2);