Run any of these queries:
To get this error:
SQL Error  : Operand should contain 1 column(s)
This query works just fine:
It seems like the operands that map to IN or NOT IN are implemented, but not the others. If the correct implementation of the feature is not a high priority, I would at least suggest a fix in the error message, to hint at a missing implementation, rather than a syntax error.
For example this query here:
Offers a much more helpful error message:
SQL Error  : (conn=5) This version of MariaDB doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'