Database schema:

CREATE TABLE "City" (
	"ID" INTEGER NOT NULL -- example: ['2', '1'],
	"Name" TEXT DEFAULT '' NOT NULL -- example: ['Qandahar', 'Kabul'],
	"CountryCode" TEXT DEFAULT '' NOT NULL -- example: ['AFG', 'NLD'],
	"District" TEXT DEFAULT '' NOT NULL -- example: ['Qandahar', 'Kabol'],
	"Population" INTEGER DEFAULT 0 NOT NULL -- example: ['237500', '1780000'],
	PRIMARY KEY ("ID"), 
	CONSTRAINT fk_city_country FOREIGN KEY("CountryCode") REFERENCES "Country" ("Code")
)


CREATE TABLE "Country" (
	"Code" TEXT DEFAULT '' NOT NULL -- example: ['AFG', 'ABW'],
	"Name" TEXT DEFAULT '' NOT NULL -- example: ['Afghanistan', 'Aruba'],
	"Continent" TEXT DEFAULT 'Asia' NOT NULL -- example: ['Asia', 'North America'],
	"Region" TEXT DEFAULT '' NOT NULL -- example: ['Southern and Central Asia', 'Caribbean'],
	"SurfaceArea" REAL DEFAULT (0.00) NOT NULL -- example: ['193.0', '652090.0'],
	"IndepYear" INTEGER DEFAULT NULL -- example: ['1919', '1975'],
	"Population" INTEGER DEFAULT 0 NOT NULL -- example: ['22720000', '103000'],
	"LifeExpectancy" REAL DEFAULT NULL -- example: ['45.9', '78.4'],
	"GNP" REAL DEFAULT NULL -- example: ['828.0', '5976.0'],
	"GNPOld" REAL DEFAULT NULL -- example: ['793.0', '7984.0'],
	"LocalName" TEXT DEFAULT '' NOT NULL -- example: ['Afganistan/Afqanestan', 'Aruba'],
	"GovernmentForm" TEXT DEFAULT '' NOT NULL -- example: ['Nonmetropolitan Territory of The Netherlands', 'Islamic Emirate'],
	"HeadOfState" TEXT DEFAULT NULL -- example: ['Mohammad Omar', 'Beatrix'],
	"Capital" INTEGER DEFAULT NULL -- example: ['129', '1'],
	"Code2" TEXT DEFAULT '' NOT NULL -- example: ['AF', 'AW'],
	PRIMARY KEY ("Code")
)


CREATE TABLE "CountryLanguage" (
	"CountryCode" TEXT DEFAULT '' NOT NULL -- example: ['AFG', 'ABW'],
	"Language" TEXT DEFAULT '' NOT NULL -- example: ['English', 'Dutch'],
	"IsOfficial" TEXT DEFAULT 'F' NOT NULL -- example: ['F', 'T'],
	"Percentage" REAL DEFAULT (0.0) NOT NULL -- example: ['5.3', '9.5'],
	PRIMARY KEY ("CountryCode", "Language"), 
	CONSTRAINT fk_countrylanguage_country FOREIGN KEY("CountryCode") REFERENCES "Country" ("Code")
)

Question: What is the average surface area of all countries?
Evidence: average surface area = AVG(SurfaceArea);

SQL:
SELECT AVG(SurfaceArea) FROM Country