I have a case when select data from PostgreSQL the table have more than 3 columns; 2 columns are a numeric type with a default value of 0 for both of them. While inserting data into Column A, it is filled with some numeric value.
So now I wanna select with the COALESCE function such that if Column B is still 0, it will return the value from Column A.
How can I do that?
Option 1: use nullif
coalesce(nullif("B", 0), "A")
Option 2: use CASE
CASE WHEN "B" = 0 THEN "A" ELSE "B" END
I'd go for option 2, even though it doesn't use coalesce as you requested.
Related
I have a database table like this:
idx[PK]
a[numeric]
b[numeric]
1
1
1
2
2
2
3
3
3
4
4
4
...
...
...
In pgadmin4, I tried to update this table with some null values, and I noticed the following queries failed:
UPDATE test as t SET
a = e.a,b = e.b
FROM (VALUES (1,NULL,NULL),(2,NULL,NULL),(3,NULL,NULL))
AS e(idx, a, b)
WHERE t.idx = e.idx
UPDATE test as t SET
a = e.a,b = e.b
FROM (VALUES (1,NULL,1),(2,NULL,2),(3,NULL,NULL))
AS e(idx, a, b)
WHERE t.idx = e.idx
The error message is like this:
ERROR: column "a" is of type numeric but expression is of type text
LINE 2: a = e.a,b = e.b
^
HINT: You will need to rewrite or cast the expression.
SQL state: 42804
Character: 43
However, this will be successful:
UPDATE test as t SET
a = e.a,b = e.b
FROM (VALUES (1,NULL,1),(2,2,NULL),(3,NULL,NULL))
AS e(idx, a, b)
WHERE t.idx = e.idx
It seems like if the new values for one of the columns I am updating are all NULL, then the query fails. However, as long as there is at least one value is numeric but NOT NULL, the query would be successful. Why is this?
I did simplify my real world case here as my actual table has millions of rows and more than 10 columns. Using Python and psycopg2, when I tried to update 50,000 rows in one query, even though there is a value in a column is NOT NULL, the previous error could still show up. I guess that is because the system scans a certain number of rows to decide if the type is correct or not instead of all 50,000 rows.
Therefore, how to avoid this failure in my real world situation? Is there a better query to use instead of UPDATE?
Thank you very much!
UPDATE
Per comments from #Marth and #Gordon Linoff, and as I am using psycopg2, I did the following in my code:
from psycopg2.extras import execute_values
sql = """UPDATE test as t SET
a = (e.a::numeric),
b = (e.b::numeric)
FROM (VALUES %s)
AS e(idx, a, b)
WHERE t.idx = e.idx"""
execute_values(cursor, sql, data)
cursor is from the database connection. data is a list of tuples in the form (idx, a, b) of my values.
This is due to the default behavior of how NULL works in these situations. NULL is generally an unknown type, which is then treated as whatever type is necessary.
In a values() statement, Postgres tries to decipher the types. It treats the individual records as it would with a union. But if all are NULL . . . well, then there is no information. And Postgres decides on using text as the universal default.
It is also important to understand that this fails with the same error:
UPDATE test t
SET a = ''
WHERE t.id = 1;
The issue is that Postgres does not convert empty strings to numbers (unlike some other databases).
In any case, this is easily fixed by casting the NULL to an appropriate type:
UPDATE test t
SET a = e.a,b = e.b
FROM (VALUES (1, NULL::numeric, NULL::numeric),
(2, NULL, NULL),
(3, NULL, NULL)
) e(idx, a, b)
WHERE t.idx = e.idx ;
You can be explicit for all occurrences of NULL, but that is not necessary.
Here is a db<>fiddle that illustrates some of this.
I am trying to bulk update some rows in postgres. Now not all of the rows need to update the same column values. For example, row 1 needs to update column 1 and 3 whereas row 2 needs to update column 2 and 4. so row 1 column 2 and 4 should not change and row 2 column 1 and 3 should not change.
I have tried using CASEs to conditionally SET the correct column values but it doesn't work with multiple rows. It DOES work if I only try to update 1 row at a time.
update topic as tmp set
"from" = (CASE WHEN tmp2."from2"::text = 'OLD_VALUE' THEN tmp."from"::int2 ELSE tmp2."from2"::int2 end),
"text_search" = (CASE WHEN tmp2."text_search2"::text = 'OLD_VALUE' THEN tmp."text_search"::text ELSE tmp2."text_search2"::text end),
"weight" = (CASE WHEN tmp2."weight2"::text = 'OLD_VALUE' THEN tmp."weight"::numeric ELSE tmp2."weight2"::numeric end)
from (values
(1051,1,'Electronic Devices',3),
(1052,'OLD_VALUE','OLD_VALUE',100)
) as tmp2("id2","from2","text_search2","weight2")
where tmp2."id2" = tmp."id"
This is the error message i get
SQL Error [22P02]: ERROR: invalid input syntax for type integer: "OLD_VALUE"
When I try with only 1 FROM value
from (values (1051,1,'Electronic Devices',3))
or
from (values (1052,'OLD_VALUE','OLD_VALUE',100))
it works correctly.
It even works correctly if the same columns need to be updated eg.
from (values
(1051,1,'Electronic Devices',3),
(1052,2,'Topic 2',100)
)
Why is it not working correctly when I need to update different columns for each row?
When you provide the list of values as values (1051,1,'Electronic Devices',3),(1052,'OLD_VALUE','OLD_VALUE',100)), the first set of values is interpreted as the "template" of data types, and in this case (1051,1,'Electronic Devices',3), it's int, int, text, int. Then, any subsequent values provided, will be expected to have the same data type signature. When it parses (1052,'OLD_VALUE','OLD_VALUE',100), it sees int,text,text,int, which doesn't match the data type signature it expects, so it reports an error.
When you omit the first value and provide only (1052,'OLD_VALUE','OLD_VALUE',100), then it identifies int,text,text,int as the "template" data type signature, and it proceeds without complaint.
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 have this following code:
BEGIN
x := split_part(text, ',', 1);
UPDATE albumphoto SET order = 1 WHERE idtable = 1 AND idx = x;
END
But my column table named idx is a numeric type, and the split_part returns a character type to the variable x. I've tried using CAST, but I don't know how to use it properly.
Any ideas?
Like this:
UPDATE albumphoto SET order = 1 WHERE idtable = 1 AND idx = CAST (x AS INTEGER);
(Use appropriate numeric type instead of INTEGER).
Or simpler:
UPDATE albumphoto
SET "order" = 1
WHERE idtable = 1
AND idx = split_part(text, ',', 1)::int -- cast to actual type (if not a string type)
AND "order" IS DISTINCT FROM 1;
expression::type is the simple (non-SQL-standard) Postgres way to cast. Details in the manual in the chapter Type Casts.
More about data types in PostgreSQL.
The last predicate I added is useful if "order" could already be 1, in which case the update wouldn't change anything - at full cost. Rather do nothing instead. Related (see last paragraph):
How do I (or can I) SELECT DISTINCT on multiple columns?
And you don't need a variable.