Database schema:

CREATE TABLE generalinfo (
	id_restaurant INTEGER NOT NULL -- example: ['2', '1'],
	label TEXT -- example: ['kabul afghan cuisine', "sparky's diner"],
	food_type TEXT -- example: ['afghani', '24 hour diner'],
	city TEXT -- example: ['san francisco', 'san carlos'],
	review REAL -- example: ['2.3', '3.8'],
	PRIMARY KEY (id_restaurant), 
	CONSTRAINT fk_generalinfo_geographic FOREIGN KEY(city) REFERENCES geographic (city)
)


CREATE TABLE geographic (
	city TEXT NOT NULL -- example: ['alamo', 'alameda'],
	county TEXT -- example: ['alameda county', 'contra costa county'],
	region TEXT -- example: ['napa valley', 'bay area'],
	PRIMARY KEY (city)
)


CREATE TABLE location (
	id_restaurant INTEGER NOT NULL -- example: ['2', '1'],
	street_num INTEGER -- example: ['242', '135'],
	street_name TEXT -- example: ['church st', 'el camino real'],
	city TEXT -- example: ['san francisco', 'san carlos'],
	PRIMARY KEY (id_restaurant), 
	CONSTRAINT fk_location_geographic FOREIGN KEY(city) REFERENCES geographic (city),
	CONSTRAINT fk_location_generalinfo FOREIGN KEY(id_restaurant) REFERENCES generalinfo (id_restaurant)
)

Question: Which county in northern California has the highest number of cities?
Evidence: northern California refers to region = 'northern california'; the highest number of cities refers to max(count(city))

SQL:
SELECT county FROM geographic WHERE region = 'northern california' GROUP BY county ORDER BY COUNT(city) DESC LIMIT 1