* John Cowan:

> 1) The ? character has no meaning in any known SQL dialect other than
> parameterization: it can otherwise appear only in strings, which are
> delimited by required apostrophes, and symbolic names (databases, tables,
> columns, etc.), which are delimited by double quotes if needed or desired.
> See <https://crate.io/docs/sql-99/en/latest/index.html> for a fully
> detailed but readable presentation of SQL-99.

This is not entirely correct.  PostgreSQL uses ? in operator names.
There is even a predefined ? operator for JSON types:

fw=> SELECT '{"a":true}'::jsonb ? 'a';
 ?column?
----------
 t
(1 row)