CREATE TABLE table_23537091_1 (copper_age VARCHAR, ubaid_period_in_mesopotamia VARCHAR)

When middle assyrian empire is the ubaid period in mesopotamia what is the copper age?
SELECT copper_age FROM table_23537091_1 WHERE ubaid_period_in_mesopotamia = "Middle Assyrian Empire"
SELECT "copper_age" FROM "table_23537091_1" WHERE "Middle Assyrian Empire" = "ubaid_period_in_mesopotamia"