Postgres Pro
Downloads
Home   >   mailing lists

Thread: How to select based on the condition of a column exists

How to select based on the condition of a column exists

From
Shaozhong SHI
Date:
The following does not work.
select count(test) from table where exists (select test from table)


How to  select based on the condition of a column exists?   It column test is not exists, a message need to be returned.

Regards,

David

Re: How to select based on the condition of a column exists

From
"David G. Johnston"
Date:
On Fri, Aug 26, 2022 at 7:01 AM Shaozhong SHI <shishaozhong@gmail.com> wrote:
The following does not work.
select count(test) from table where exists (select test from table)


How to  select based on the condition of a column exists?   It column test is not exists, a message need to be returned.


You have to query the system catalog (or information schema) for the absence of an expected row.  Otherwise the only message you will end up returning is "column not found" since you cannot write and execute an SQL query without knowing and defining its entire structure.

Alternatively, you write a function with an exception block.

David J.