Database schema:

CREATE TABLE "Ingredient" (
	ingredient_id INTEGER -- example: ['1', '6'],
	category TEXT -- example: ['dairy', 'Mexican products'],
	name TEXT -- example: ['1% lowfat cottage cheese', '1% lowfat milk'],
	plural TEXT -- example: ['#NAME?', 's'],
	PRIMARY KEY (ingredient_id)
)


CREATE TABLE "Nutrition" (
	recipe_id INTEGER -- example: ['214', '215'],
	protein REAL -- example: ['5.7', '5.47'],
	carbo REAL -- example: ['41.29', '23.75'],
	alcohol REAL -- example: ['0.0', '1.93'],
	total_fat REAL -- example: ['1.08', '11.53'],
	sat_fat REAL -- example: ['2.21', '0.58'],
	cholestrl REAL -- example: ['3.48', '1.39'],
	sodium REAL -- example: ['46.17', '260.78'],
	iron REAL -- example: ['0.57', '0.81'],
	vitamin_c REAL -- example: ['13.02', '8.89'],
	vitamin_a REAL -- example: ['586.2', '2738.24'],
	fiber REAL -- example: ['0.62', '0.87'],
	pcnt_cal_carb REAL -- example: ['67.38', '56.8'],
	pcnt_cal_fat REAL -- example: ['35.68', '6.89'],
	pcnt_cal_prot REAL -- example: ['16.17', '7.53'],
	calories REAL -- example: ['141.01', '290.79'],
	PRIMARY KEY (recipe_id), 
	CONSTRAINT fk_nutrition_recipe FOREIGN KEY(recipe_id) REFERENCES "Recipe" (recipe_id)
)


CREATE TABLE "Quantity" (
	quantity_id INTEGER -- example: ['2', '1'],
	recipe_id INTEGER -- example: ['214', '215'],
	ingredient_id INTEGER -- example: ['3334', '1613'],
	max_qty REAL -- example: ['0.25', '2.0'],
	min_qty REAL -- example: ['0.25', '2.0'],
	unit TEXT -- example: ['cup(s)', 'teaspoon(s)'],
	preparation TEXT -- example: ['or water', 'melted'],
	optional TEXT -- example: ['FALSE', 'TRUE'],
	PRIMARY KEY (quantity_id), 
	CONSTRAINT fk_quantity_recipe FOREIGN KEY(recipe_id) REFERENCES "Recipe" (recipe_id),
	CONSTRAINT fk_quantity_ingredient FOREIGN KEY(ingredient_id) REFERENCES "Ingredient" (ingredient_id),
	CONSTRAINT fk_quantity_nutrition FOREIGN KEY(recipe_id) REFERENCES "Nutrition" (recipe_id)
)


CREATE TABLE "Recipe" (
	recipe_id INTEGER -- example: ['214', '215'],
	title TEXT -- example: ['Raspberry Chiffon Pie', 'Apricot Yogurt Parfaits'],
	subtitle TEXT -- example: ['with Banana Cream Whip', 'a La Creme'],
	servings INTEGER -- example: ['10', '4'],
	yield_unit TEXT -- example: ['8 tarts', '1 pie'],
	prep_min INTEGER -- example: ['5', '20'],
	cook_min INTEGER -- example: ['2', '8'],
	stnd_min INTEGER -- example: ['305', '65'],
	source TEXT -- example: ['The California Tree Fruit Agreement', 'Produce for Better Health Foundation and 5 a Day'],
	intro TEXT -- example: ['For a quick, low-cal dessert, serve this on other ', 'Serve in stemmed glasses and top with sliced apric'],
	directions TEXT -- example: ['For crust, preheat oven to 375 degrees F.\nIn light', 'Drain canned apricots, pour 1/4 cup of the juice i'],
	PRIMARY KEY (recipe_id)
)

Question: What is the average vitamin C amount of all cakes?
Evidence: average vitamin C refers to AVG(vitamin_c); all cakes refers to title LIKE '%cake%'

SQL:
SELECT AVG(T1.vitamin_c) FROM Nutrition AS T1 INNER JOIN Recipe AS T2 ON T2.recipe_id = T1.recipe_id WHERE T2.title LIKE '%cake%'