For Exercise 1.2.1 in the QGIS Training Manual Training Manual Link I attempted to extract data from multipolygon layer using the query;
building != 'NULL'
I had double checked if there were features with no information in their 'building' field in my data and there were many. Unfortunately the query failed with message shown in image below.
I have tried to exclude spaces, replacing != with NOT, different data, and using " " instead of "NULL"
As I am a bit of a rookie with coding, I have been having no luck.
No luck with Kazuhito Answer, "building" != 'NULL'. This query isn't selecting any features, please see photos for reference.