treat main as the default schema

[mbostock]

Per https://sqlite.org/lang_attach.html

The schema-names 'main' and 'temp' refer to the main database and the database used for temporary tables. The main and temp databases cannot be attached or detached. … If the name of the table is unique across all attached databases and the main and temp databases, then the schema-name prefix is not required. If two or more tables in different databases have the same name and the schema-name prefix is not used on a table reference, then the table chosen is the one in the database that was least recently attached.

[annie]

ooh NULLIF is nice