Обсуждение: How to allow null as an option when using regexp_matches?

Am Wed, Dec 08, 2021 at 12:07:13PM +0000 schrieb Shaozhong SHI:

> We can do this:
> select count(*) from regexp_matches('Great London', 'Great
> London|Information Centre|Department for Transport', 'g');
>
> Is it possible to allow null as an option?  something like this
> select count(*) from regexp_matches('Great London', 'null|Great
> London|Information Centre|Department for Transport', 'g');

You seem to want to apply coalesce() judiciously.

Best,
Karsten
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On Wed, 8 Dec 2021 17:07:27 +0000
Shaozhong SHI <shishaozhong@gmail.com> wrote:
>
> Any good example?
>

Plenty of them in the fine documentation :

https://www.postgresql.org/search/?q=coalesce




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On 12/8/2021 4:07 AM, Shaozhong SHI wrote:
> We can do this:
> select count(*) from regexp_matches('Great London', 'Great London|Information Centre|Department for Transport',
'g');
>
> Is it possible to allow null as an option?  something like this
> select count(*) from regexp_matches('Great London', 'null|Great London|Information Centre|Department for Transport',
'g');
>
> Regards,
>
> David
>
Hi David,

I'm assuming that 'Great London' is coming from some column value.
Given that NULL is a state, not a value, regexp really cannot "find" or not "find"  it.
you could use COALESCE the source of 'Great London' to a predictable value that you CAN match on.

or you could possibly construct your query something like this:

select CASE WHEN 'Great London' IS NULL THEN 1 ELSE 0 END + (SELECT count(*) from regexp_matches('Great London', 'Great
London|InformationCentre|Department for Transport', 'g'))
 

select CASE WHEN NULL IS NULL THEN 1 ELSE 0 END + (SELECT count(*) from regexp_matches(NULL, 'Great London|Information
Centre|Departmentfor Transport', 'g'))
 

Interestingly to me,  the following returns 2 - possibly because an empty string matches anything?

select count(*) from regexp_matches('Great London', 'Great London||Information Centre|Department for Transport', 'g');

Roxanne