（Oracle）SQL DDL - 完整性约束

Question

My professor wanted us to fill in some blanks in this code for our Database class. I think I'm doing it right but I'm unsure if I am as we have just started on the topic. I have no idea where to start with the fifth constraint (IC5) if someone could give me some direction or a helpful source.

SPOOL ddl.out 
SET ECHO ON 
-- 
-- Author:
-- 
-- IMPORTANT: use the names IC-1, IC-2, etc. as given below. 
-- -------------------------------------------------------------------- 
DROP TABLE Orders CASCADE CONSTRAINTS; 
DROP TABLE OrderLine CASCADE CONSTRAINTS; 
-- 
CREATE TABLE Orders 
( 
orderNum INTEGER PRIMARY KEY, 
priority CHAR(10) NOT NULL, 
cost INTEGER NOT NULL, 
/* 
IC1: The priority is one of: high, medium, or low 
*/ 
CHECK priority=('high' OR 'medium' OR 'low'),
/* 
IC2: The cost of a high priority order is above 2000. 
*/ 
CHECK priority='high' AND cost>2000,
/* 
IC3: The cost of a medium priority order is between 800 and 2200 (inclusive). 
*/ 
CHECK priority='medium' AND cost BETWEEN 800 AND 2200,
/* 
IC4: The cost of a low priority order is less than 1000. 
*/ 
CHECK priority='low' AND cost<1000,
); 
-- 
-- 
CREATE TABLE OrderLine 
( 
orderNum INTEGER, 
lineNum INTEGER, 
item CHAR (10) NOT NULL, 
quantity INTEGER, 
PRIMARY KEY (orderNum, lineNum), 
/* 
IC5: Every order line must belong to an order in the Order table. 
Also: if an order is deleted then all its order lines must be deleted. 
IMPORTANT: DO NOT declare this IC as DEFERRABLE. 
*/ 
<<< YOUR SQL CODE GOES HERE >>> 
); 
-- 
-- ---------------------------------------------------------------- 
-- TESTING THE SCHEMA 
-- ---------------------------------------------------------------- 
INSERT INTO Orders VALUES (10, 'high', 2400); 
INSERT INTO Orders VALUES (20, 'high', 1900); 
INSERT INTO Orders VALUES (30, 'high', 2100); 
INSERT INTO Orders VALUES (40, 'medium', 700); 
INSERT INTO Orders VALUES (50, 'low', 1100); 
INSERT INTO Orders VALUES (60, 'low', 900); 
SELECT * from Orders; 

Answer 1

你需要什么是外键约束。

Answer 2

IC5：

FOREIGN KEY (orderNum) REFERENCES Orders (orderNum) ON DELETE CASCADE