Excerpt from the complete ddl: http://sqlfiddle.com/#!9/2047e0

CREATE TABLE food
( 
name VARCHAR(12) NOT NULL,  quantity INT NOT NULL,  PRIMARY KEY (name)
);

CREATE TABLE species
( 
name VARCHAR(12) NOT NULL,  location VARCHAR(12) NOT NULL, diet VARCHAR(12) NOT NULL,  PRIMARY KEY (name),  FOREIGN KEY (diet) REFERENCES food(name)
);

CREATE TABLE pets
( 
name VARCHAR(12) NOT NULL,  sex VARCHAR(1) NOT NULL,  species VARCHAR(12) NOT NULL,  color VARCHAR(8) NOT NULL,  PRIMARY KEY (name),  FOREIGN KEY (species) REFERENCES species(name));