I managed to run the query without getting the error but it still seems like there is some strange behaviour going on. I pass all the tests except for the last random one (Find Inventory Changes (Randomized Data) validates the query against multiple random date combinations.

In general, I think you were on the right track with the nulls - returning a NULL generates the initial error.

Another possibility is that you return no rows (i.e. an empty result set) when there should be some.

Sounds good and thanks for the quick response. As far as I can tell, nulls are not being returned but I'll give it another look in a little bit. To add some info, this what comes back from the backtrace:
./spec/query_spec.rb:168:in map' ./spec/query_spec.rb:168:in format_output'
./spec/query_spec.rb:158:in block (3 levels) in <top (required)>' ./spec/query_spec.rb:127:in times'
./spec/query_spec.rb:127:in block (2 levels) in <top (required)>' ./spec/query_spec.rb:6:in block (3 levels) in <top (required)>'
/usr/local/bundle/gems/sequel-5.47.0/lib/sequel/database/transactions.rb:251:in _transaction' /usr/local/bundle/gems/sequel-5.47.0/lib/sequel/database/transactions.rb:233:in block in transaction'
/usr/local/bundle/gems/sequel-5.47.0/lib/sequel/connection_pool/threaded.rb:92:in hold' /usr/local/bundle/gems/sequel-5.47.0/lib/sequel/database/connecting.rb:269:in synchronize'
/usr/local/bundle/gems/sequel-5.47.0/lib/sequel/database/transactions.rb:195:in transaction' ./spec/query_spec.rb:5:in block (2 levels) in <top (required)>'

One reason for this error could be that you returned NULL in a column which expects a value. Make sure your query returns no NULLs and try again.

Nonetheless, the error sohuld be fixed, because tests should not crash like that on unexpected values.

I can't complete as I'm getting this error: ./spec/query_spec.rb:169:in block in format_output': undefined method ljust' for nil:NilClass (NoMethodError)
Any idea what is going on?