### Database Schema:
CREATE TABLE race (
	id	INTEGER	NOT NULL	PRIMARY KEY,
	race	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE attribute (
	id	INTEGER	NOT NULL	PRIMARY KEY,
	attribute_name	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE alignment (
	id	INTEGER	NOT NULL	PRIMARY KEY,
	alignment	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE hero_power (
	hero_id	INTEGER	NULL	DEFAULT NULL,
	power_id	INTEGER	NULL	DEFAULT NULL,
	foreign key (hero_id) references superhero (id),
	foreign key (power_id) references superpower (id)
);

CREATE TABLE colour (
	id	INTEGER	NOT NULL	PRIMARY KEY,
	colour	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE gender (
	id	INTEGER	NOT NULL	PRIMARY KEY,
	gender	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE hero_attribute (
	hero_id	INTEGER	NULL	DEFAULT NULL,
	attribute_id	INTEGER	NULL	DEFAULT NULL,
	attribute_value	INTEGER	NULL	DEFAULT NULL,
	foreign key (hero_id) references superhero (id),
	foreign key (attribute_id) references attribute (id)
);

CREATE TABLE publisher (
	id	INTEGER	NOT NULL	PRIMARY KEY,
	publisher_name	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE superpower (
	id	INTEGER	NOT NULL	PRIMARY KEY,
	power_name	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE superhero (
	id	INTEGER	NOT NULL	PRIMARY KEY,
	superhero_name	TEXT	NULL	DEFAULT NULL,
	full_name	TEXT	NULL	DEFAULT NULL,
	gender_id	INTEGER	NULL	DEFAULT NULL,
	eye_colour_id	INTEGER	NULL	DEFAULT NULL,
	hair_colour_id	INTEGER	NULL	DEFAULT NULL,
	skin_colour_id	INTEGER	NULL	DEFAULT NULL,
	race_id	INTEGER	NULL	DEFAULT NULL,
	publisher_id	INTEGER	NULL	DEFAULT NULL,
	alignment_id	INTEGER	NULL	DEFAULT NULL,
	height_cm	INTEGER	NULL	DEFAULT NULL,
	weight_kg	INTEGER	NULL	DEFAULT NULL,
	foreign key (gender_id) references gender (id),
	foreign key (hair_colour_id) references colour (id),
	foreign key (publisher_id) references publisher (id),
	foreign key (race_id) references race (id),
	foreign key (eye_colour_id) references colour (id),
	foreign key (alignment_id) references alignment (id),
	foreign key (skin_colour_id) references colour (id)
);

### User's Query:
Calculate the average height of all neutral superheroes.

### Extra Knowledge:

SELECT AVG(T1.height_cm) FROM superhero AS T1 INNER JOIN alignment AS T2 ON T1.alignment_id = T2.id WHERE T2.alignment = 'Neutral'