SELECT count(DISTINCT CONTENTS) FROM boxes
Find the number of all distinct contents in all the boxes.
CREATE TABLE Warehouses (
   Code INTEGER NOT NULL,
   Location VARCHAR(255) NOT NULL ,
   Capacity INTEGER NOT NULL,
   PRIMARY KEY (Code)
 );
CREATE TABLE Boxes (
    Code CHAR(4) NOT NULL,
    Contents VARCHAR(255) NOT NULL ,
    Value REAL NOT NULL ,
    Warehouse INTEGER NOT NULL,
    PRIMARY KEY (Code),
    FOREIGN KEY (Warehouse) REFERENCES Warehouses(Code)
 );
INSERT INTO Warehouses(Code,Location,Capacity) VALUES(1,'Chicago',3);
INSERT INTO Warehouses(Code,Location,Capacity) VALUES(2,'Chicago',4) 
INSERT INTO Boxes(Code,Contents,Value,Warehouse) VALUES('0MN7','Rocks',180,3)