NULL value causes blank row in SELECT results for text concatenation

Use COALESCE function to return the current value or an empty string.

SELECT '  (' || obj_id || ', ''' || coalesce(obj_type, '') || '''),' 
FROM   objects;

| ?column?        |
| :-------------- |
|   (1, 'val 1'), |
|   (2, ''),      |
|   (3, 'val 3'), |
|   (4, 'val 4'), |

db<>fiddle here

Why is the result of the first SELECT giving me a blank row?

Because concatenating NULL with any character type (or most other types, array types being a notable exception) results in NULL. Related:

How can the concatenatation of a text and a NULL value return a non-null result?

The representation of NULL depends on your client. Some spell out NULL, some (incl. psql) put nothing instead. Often that's configurable.

Even casting obj_type::text still gave me a blank row.

Casting NULL to (almost) any type still returns NULL - of another data type.

I have a query for producing some text for creating some VALUES text to put in a .sql file for inserting some rows.

Have you considered COPY or the psql equivalent \copy?

If your example isn't simplified, you might just select whole ROW values:

SELECT o  -- whole row
FROM   il2.objects o
WHERE  obj_id < 11
ORDER  BY obj_id;

If you need that specific format, use format() to make it simple. Works with NULL values out of the box:

SELECT format('(%s, %L),', obj_id, obj_type)
FROM   objects;

You get NULL (unquoted) in place of NULL values (which is distinct from '' and may have to be distinguishable.)

db<>fiddle here (added to the existing fiddle of McNets, cudos)

Related