It seems that in PostgreSQL, empty_field != 1 (or some other value) is FALSE. If this is true, can somebody tell me how to compare with empty fields?
I have following query, which translates to "select all posts in users group for which one hasn't voted yet:
SELECT p.id, p.body, p.author_id, p.created_at
FROM posts p
LEFT OUTER JOIN votes v ON v.post_id = p.id
WHERE p.group_id = 1
AND v.user_id != 1
and it outputs nothing, even though votes table is empty. Maybe there is something wrong with my query and not with the logic above?
Edit: it seems that changing v.user_id != 1, to v.user_id IS DISTINCT FROM 1, did the job.
From PostgreSQL docs:
For non-null inputs, IS DISTINCT FROM
is the same as the <> operator.
However, when both inputs are null it
will return false, and when just one
input is null it will return true.
If you want to return rows where v.user_id is NULL then you need to handle that specially. One way you can fix it is to write:
AND COALESCE(v.user_id, 0) != 1
Another option is:
AND (v.user_id != 1 OR v.user_id IS NULL)
Edit: spacemonkey is correct that in PostgreSQL you should use IS DISTINCT FROM here.
NULL is a unknown value so it can never equal something. Look into using the COALESCE function.
Related
In my Postgres database, I'm checking user answers for correctness by checking if two IDs, "user_answered_id" and "expected_answer_id", are equivalent. If the user doesn't provide a "user_answered_id", then we still mark their answer as incorrect.
In Postgres, the following queries
select case when 1 != null then TRUE else FALSE end as test;
select case when 1 = null then TRUE else FALSE end as test;
both result in FALSE. This is true for any number check (e.g., when 2 != null, when 3 != null, ..., etc.
Why doesn't CASE WHEN show TRUE for 1 != null?
Must I put in the check "or is null"? E.g.,
CASE WHEN
user_answered_id != expected_answer_id
OR user_answered_id IS NULL
THEN TRUE
ELSE FALSE
END as user_incorrect_tally
What you are looking for is: IS DISTINCT FROM
select 2 is distinct from null;
?column?
----------
t
select 2 is distinct from 1;
?column?
----------
t
From the docs:
datatype IS DISTINCT FROM datatype → boolean
Not equal, treating null as a comparable value.
1 IS DISTINCT FROM NULL → t (rather than NULL)
NULL IS DISTINCT FROM NULL → f (rather than NULL)
SQL uses three-valued logic: true, false, and null. Null is not false. Null can be thought of as "no value".
Operations on null almost always yield null. So 1 != null is null. 1 = null is null. null = null is null. 5 < null is null. Etc.
To check for null, use is null and is not null.
Back to your query. is not distinct from and is distinct from are like = and != which treat null as a comparable value. So null is distinct from 1 will be true.
select
user_answered_id is distinct from expected_answer_id as user_incorrect
If you need to convert a null into a different value such as 0 or an empty string, use coalesce.
select
coalesce(user_answered_text, 'No Answer')
Your column is named "tally", but a tally means a count. If you intend to count a user's true and false answers use count with a filter.
select
count(user_answered_id) filter (
where user_answered_id = expected_answer_id
) as user_correct_tally,
-- count ignores null, this will only be the questions they tried to answer
count(user_answered_id) as user_answered_tally,
count(user_answered_id) filter (
where user_answered_is is distinct from expected_answer_id
) as user_incorrect_tally
Yes, You should check NULL value with is null, And last query you wrote is correct.
I suggest you to read below documents:
https://www.postgresql.org/docs/current/functions-comparison.html
original query looks like this :
UPDATE reponse_question_finale t1, reponse_question_finale t2 SET
t1.nb_question_repondu = (9-(ISNULL(t1.valeur_question_4)+ISNULL(t1.valeur_question_6)+ISNULL(t1.valeur_question_7)+ISNULL(t1.valeur_question_9))) WHERE t1.APPLICATION = t2.APPLICATION;
I know you cannot update 2 tables in a single query so i tried this :
UPDATE reponse_question_finale t1
SET nb_question_repondu = (9-(COALESCE(t1.valeur_question_4,'')::int+COALESCE(t1.valeur_question_6,'')::int+COALESCE(t1.valeur_question_7)::int+COALESCE(t1.valeur_question_9,'')::int))
WHERE t1.APPLICATION = t1.APPLICATION;
But this query gaves me an error : invalid input syntax for integer: ""
I saw that the Postgres equivalent to MySQL is COALESCE() so i think i'm on the good way here.
I also know you cannot add varchar to varchar so i tried to cast it to integer to do that. I'm not sure if i casted it correctly with parenthesis at the good place and regarding to error maybe i cannot cast to int with coalesce.
Last thing, i can certainly do a co-related sub-select to update my two tables but i'm a little lost at this point.
The output must be an integer matching the number of questions answered to a backup survey.
Any thoughts?
Thanks.
coalesce() returns the first non-null value from the list supplied. So, if the column value is null the expression COALESCE(t1.valeur_question_4,'') returns an empty string and that's why you get the error.
But it seems you want something completely different: you want check if the column is null (or empty) and then subtract a value if it is to count the number of non-null columns.
To return 1 if a value is not null or 0 if it isn't you can use:
(nullif(valeur_question_4, '') is null)::int
nullif returns null if the first value equals the second. The IS NULL condition returns a boolean (something that MySQL doesn't have) and that can be cast to an integer (where false will be cast to 0 and true to 1)
So the whole expression should be:
nb_question_repondu = 9 - (
(nullif(t1.valeur_question_4,'') is null)::int
+ (nullif(t1.valeur_question_6,'') is null)::int
+ (nullif(t1.valeur_question_7,'') is null)::int
+ (nullif(t1.valeur_question_9,'') is null)::int
)
Another option is to unpivot the columns and do a select on them in a sub-select:
update reponse_question_finale
set nb_question_repondu = (select count(*)
from (
values
(valeur_question_4),
(valeur_question_6),
(valeur_question_7),
(valeur_question_9)
) as t(q)
where nullif(trim(q),'') is not null);
Adding more columns to be considered is quite easy then, as you just need to add a single line to the values() clause
I'm trying to run a multi-column in clause that matches null values as well. Right now I'm using coalesce like so:
select * from table
where (coalesce(foo, ''), coalesce(bar, '')) in (('foo_val', 'bar_val'), ('foo_val', ''));
For integer columns though, this throws 'invalid input syntax for integer: ""' on the coalesce. I could coalesce to -1 instead of an empty string, but was wondering if there was a more elegant solution.
Sample input/output:
Table data:
{{foo: 1, bar: 2}, {foo: 1, bar: NULL}}
User input: [(1, 2), (1, nil)]
Expected output: both rows.
EDIT: I'll try to clarify what I'm trying to do: I want to match DB rows by multiple column value combinations given by the user. That is, if the user inputs [(1,2), (3,4)], I'd want to return rows where column_A == 1 AND column_B == 2, OR column_A == 3 AND column_B == 4. Where I run into trouble is allowing for the user to input NULL and have it matched like any other value. So if the user inputs [(1,NULL)], I'd want to return rows where column_A == 1 AND column_B == NULL (but note I wouldn't want to return them for the previous query, since the user didn't specify he wanted rows with NULL values in column_B).
This code will not just match nulls, it will also match empty strings. If that's what you want, then fine.
Using a hard-coded list in an IN clause, where you wish to also return nulls, the simplest solution is to coalesce against the first value in your IN list. That way you don't have to worry about matching non-null values (like empty strings or -1) which aren't in the list.
select *
from table
where (coalesce(foo, 'foo_val'), coalesce(bar, 'foo_val'))
in (('foo_val', 'bar_val'), ('foo_val', ''));
I'm writing some triggers to check if an arrtibute of a newly create tuple is present in another table. What is the best way to check this?
IF (SELECT * FROM TABLEB where NEW.Attribute = Attribute) = NULL THEN
return NULL
END IF
Is there a better way? And does a Select that returns nothing = NULL or empty set?
Use IF EXISTS (SELECT * FROM TABLEB where NEW.Attribute = Attribute).
An empty rowset isn't the same as NULL, which represents an empty cell of a row. Your syntax would only be valid if the query only selected one row, in which case PostgreSQL will allow use of comparison operators against the top cell returned, which counts as NULL if none are returned.
I have a large database, that I want to do some logic to update new fields.
The primary key is id for the table harvard_assignees
The LOGIC GOES LIKE THIS
Select all of the records based on id
For each record (WHILE), if (state is NOT NULL && country is NULL), update country_out = "US" ELSE update country_out=country
I see step 1 as a PostgreSQL query and step 2 as a function. Just trying to figure out the easiest way to implement natively with the exact syntax.
====
The second function is a little more interesting, requiring (I believe) DISTINCT:
Find all DISTINCT foreign_keys (a bivariate key of pat_type,patent)
Count Records that contain that value (e.g., n=3 records have fkey "D","388585")
Update those 3 records to identify percent as 1/n (e.g., UPDATE 3 records, set percent = 1/3)
For the first one:
UPDATE
harvard_assignees
SET
country_out = (CASE
WHEN (state is NOT NULL AND country is NULL) THEN 'US'
ELSE country
END);
At first it had condition "id = ..." but I removed that because I believe you actually want to update all records.
And for the second one:
UPDATE
example_table
SET
percent = (SELECT 1/cnt FROM (SELECT count(*) AS cnt FROM example_table AS x WHERE x.fn_key_1 = example_table.fn_key_1 AND x.fn_key_2 = example_table.fn_key_2) AS tmp WHERE cnt > 0)
That one will be kind of slow though.
I'm thinking on a solution based on window functions, you may want to explore those too.