ORA-00909: invalid number of arguments when using NVL - sql

Question

I have a column that gets the specific amount which has a condition like below.
I tried this one to get his specific value but I getting a multiple rows which it should be single row.
select distinct
(case
when aila.line_type_lookup_code = 'ITEM' and aila.tax_classification_code = 'VAT12 SERVICES' then to_char(aila.assessable_value) else '0'
end) as taxable_lines
from
ap_invoice_lines_all aila
where
aila.invoice_id = '31004'
then I tried this one to replace a null values.
select distinct
(case
when aila.line_type_lookup_code = 'ITEM' and aila.tax_classification_code = 'VAT12 SERVICES' then nvl(to_char(aila.assessable_value,0)) end) as taxable_lines
from
ap_invoice_lines_all aila
where
aila.invoice_id = '31004'
For example I have a table named ap_invoice_lines_all that has a columns name
line_type_lookup_code string
tax_classification_code string
assessable_value double
then the expected output I want based on the query I tried above is
taxable lines
1300
but the one I get is
taxable lines
0
1300
How do I remove the 0 in the result?
Thanks!

Answer 1

First off, you are using to_char wrong. This function should only have a single argument in this context:
nvl(to_char(aila.assessable_value,0))
should be
nvl(to_char(aila.assessable_value),0)
I would recommend against using a case expression like this in such a simple query to improve readability. Case expressions are great tools, but typically decrease the query readability. And in this particular instance, you don't really need an IF..THEN..ELSE function (which is the reason to use case expressions).
Secondly, are you sure there is only record where aila.invoice_id = '31004'? The query will only return two rows if there are actually two rows in the table where that clause is true. In this case it finds one row where assessable_value is null and one where it isn't.
In any case, to remove the zero (or in this case a null value) from the resultset, you can simply do this:
select distinct aila.assessable_value as taxable_lines
from ap_invoice_lines_all aila
where aila.invoice_id = '31004'
and aila.line_type_lookup_code = 'ITEM' -- Originally a condition in the case statement
and aila.tax_classification_code = 'VAT12 SERVICES' -- Originally a condition in the case statement
and aila.assessable_value is not null; -- Removes any null values from the result
Or if you want null values to be replaced by a zero;
select distinct nvl(aila.assessable_value, 0) as taxable_lines
from ap_invoice_lines_all aila
where aila.invoice_id = '31004'
and aila.line_type_lookup_code = 'ITEM' -- Originally a condition in the case statement
and aila.tax_classification_code = 'VAT12 SERVICES' -- Originally a condition in the case statement
Note that the distinct clause will cause all rows where assessable_value to be grouped into a single row if multiple rows are a possibility. Remove the distinct clause if you want all rows where assessable_value is null to show in the result as 0.
Lastly, you might want to think about implementing a primary key (if the table doesn't have one) or unique index on invoice_id, if there shouldn't ever be duplicate ids. And it's simply good practice to do so for ID columns which are used often in queries and should be unique.