SQL Server Trigger functioning backwards - sql-server

Question

I have a little strange issue with my SQL script and I was hoping someone could help me out with it.
I have a Database being created by using
IF EXISTS (SELECT name
FROM sysdatabases
WHERE name = 'travel')
DROP DATABASE travel
GO
CREATE DATABASE travel
GO
USE travel
GO
I then create 3 tables as shown below
CREATE TABLE customer
(
customerID INT,
lastname VARCHAR(70) NOT NULL,
firstname VARCHAR(70) NOT NULL,
phone VARCHAR(10) CONSTRAINT phoneCheck CHECK ((phone LIKE '[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]')),
category VARCHAR(7) NOT NULL CONSTRAINT categoryDefault DEFAULT 'A',
CONSTRAINT categoryCheck CHECK (category IN ('A', 'B', 'C')),
CONSTRAINT customerPK
PRIMARY KEY (customerID)
)
CREATE TABLE package /*Still need to do the Zero Padding*/
(
packageCode VARCHAR(6),
destination VARCHAR(70),
CONSTRAINT packageCodeCheck CHECK (packageCode LIKE ('YFK%')),
price MONEY NOT NULL CONSTRAINT priceCheck CHECK ((price BETWEEN 1000 AND 10000)),
passportRequired VARCHAR(7) NOT NULL CONSTRAINT passportRequiredDefault DEFAULT 'Y',
CONSTRAINT passportCheck CHECK (passportRequired IN ('Y', 'N')),
CONSTRAINT packagePK
PRIMARY KEY (packageCode)
)
CREATE TABLE booking /*Still need to do the Customer and Package delete*/
(
customerID VARCHAR(6),
bookingDate VARCHAR(70) NOT NULL DEFAULT GETDATE(),
amountPaid MONEY CONSTRAINT amountPaidDefault DEFAULT 0.00,
CONSTRAINT bookingPK
PRIMARY KEY (customerID)
)
Now heres the issue, I create a trigger as shown below
GO
CREATE TRIGGER customerDelete ON customer AFTER DELETE
AS
DELETE booking
FROM customer
WHERE customer.customerID = booking.customerID
GO
Which to my understanding it will delete all records in booking... that have the matching customerID WHEN a record is deleted from the customer Table. (I am new to triggers)
I INSERT Sample Data as shown below
INSERT INTO customer
(customerID, lastname, firstname, phone, category)
VALUES
(1, 'Picard', 'Corey', 1234567890, 'A'),
(2, 'Bond', 'Devon', 9876543210, 'B'),
(3, 'Douglas', 'Bryan', 6549871230, 'C')
INSERT INTO package
(packageCode, destination, price, passportRequired)
VALUES
('YFK001', 'Toronto', 1000.57, 'N'),
('YFK002', 'Orlando', 3000.98, 'Y')
INSERT INTO booking
(customerID, bookingDate, amountPaid)
VALUES
(1, GETDATE(), 1548),
(2, GETDATE(), 1586),
(3, GETDATE(), 1350),
(4, GETDATE(), 1650)
And Finally I delete the Customer from the TABLE customer with the customerID of 1 by using
DELETE customer
WHERE customerID = 1
However, when I attempt to see the results by using
SELECT * FROM customer
--WHERE customerID = 1 OR customerID = 2 OR customerID = 3
SELECT * FROM package
--WHERE packageCode = 'YFK001' OR packageCode = 'YFK002'
SELECT * FROM booking
--WHERE customerID = 1 OR customerID = 2 OR customerID = 3 OR customerID = 4
It displays bookings with customerID 1 and 4.
Can you let me know what I'm doing wrong?
The trigger is essentially used for the purpose of deleting the bookings with the same customerID of the customer we delete from the customer TABLE
All help is greatly appreciated.
Thanks,
Bryan

Answer 1

Change your delete to this:
DELETE B
FROM booking B
INNER JOIN DELETED D
ON B.customerID = D.customerID;

Answer 2

My Answer is not trigger, [ If you specially want to use Trigger then You can use Lamak's & Pradeep's Answer]
Here best way you can do is use Cacade on foreign Key in your case
Here is your Query I just updated cascade in it
CREATE TABLE customer
(
customerID INT,
lastname VARCHAR(70) NOT NULL,
firstname VARCHAR(70) NOT NULL,
phone VARCHAR(10) CONSTRAINT phoneCheck CHECK ((phone LIKE '[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]')),
category VARCHAR(7) NOT NULL CONSTRAINT categoryDefault DEFAULT 'A',
CONSTRAINT categoryCheck CHECK (category IN ('A', 'B', 'C')),
CONSTRAINT customerPK
PRIMARY KEY (customerID)
)
CREATE TABLE package /*Still need to do the Zero Padding*/
(
packageCode VARCHAR(6),
destination VARCHAR(70),
CONSTRAINT packageCodeCheck CHECK (packageCode LIKE ('YFK%')),
price MONEY NOT NULL CONSTRAINT priceCheck CHECK ((price BETWEEN 1000 AND 10000)),
passportRequired VARCHAR(7) NOT NULL CONSTRAINT passportRequiredDefault DEFAULT 'Y',
CONSTRAINT passportCheck CHECK (passportRequired IN ('Y', 'N')),
CONSTRAINT packagePK
PRIMARY KEY (packageCode)
ON DELETE CASCADE
ON UPDATE CASCADE
)
CREATE TABLE booking /*Still need to do the Customer and Package delete*/
(
customerID VARCHAR(6),
bookingDate VARCHAR(70) NOT NULL DEFAULT GETDATE(),
amountPaid MONEY CONSTRAINT amountPaidDefault DEFAULT 0.00,
CONSTRAINT bookingPK
PRIMARY KEY (customerID)
ON DELETE CASCADE
ON UPDATE CASCADE
)
This way when you are deleting or update [keyvalue] any entry of Table holding primary key will update its related foreign childs