MySQL 5.7

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CREATE TABLE customers (
    id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(20) NOT NULL
);
INSERT INTO customers VALUES (1, 'Customer 1'), (2, 'Customer 2'), (3, 'Customer 3');

CREATE TABLE tickets (
    customer_id INT UNSIGNED NOT NULL,
    ticket_id INT UNSIGNED NOT NULL,
    PRIMARY KEY (customer_id, ticket_id)
);
CREATE TABLE customer_ticket_sequence (
    customer_id INT UNSIGNED NOT NULL PRIMARY KEY,
    seq INT UNSIGNED NOT NULL DEFAULT 0,
    FOREIGN KEY (customer_id) REFERENCES customers (id) ON DELETE CASCADE
);
INSERT INTO customer_ticket_sequence (customer_id) VALUES (1), (2), (3);

Records: 3  Duplicates: 0  Warnings: 0

Records: 3  Duplicates: 0  Warnings: 0

-- let's insert some tickets for customer 1
UPDATE customer_ticket_sequence SET seq = LAST_INSERT_ID(seq + 1) WHERE customer_id = 1;
INSERT INTO tickets VALUES (1, LAST_INSERT_ID());

UPDATE customer_ticket_sequence SET seq = LAST_INSERT_ID(seq + 1) WHERE customer_id = 1;
INSERT INTO tickets VALUES (1, LAST_INSERT_ID());

UPDATE customer_ticket_sequence SET seq = LAST_INSERT_ID(seq + 1) WHERE customer_id = 1;
INSERT INTO tickets VALUES (1, LAST_INSERT_ID());

-- let's insert some tickets for customer 2
UPDATE customer_ticket_sequence SET seq = LAST_INSERT_ID(seq + 1) WHERE customer_id = 2;
INSERT INTO tickets VALUES (2, LAST_INSERT_ID());

UPDATE customer_ticket_sequence SET seq = LAST_INSERT_ID(seq + 1) WHERE customer_id = 2;
INSERT INTO tickets VALUES (2, LAST_INSERT_ID());

-- let's insert another ticket for customer 1
UPDATE customer_ticket_sequence SET seq = LAST_INSERT_ID(seq + 1) WHERE customer_id = 1;
INSERT INTO tickets VALUES (1, LAST_INSERT_ID());

Rows matched: 1  Changed: 1  Warnings: 0

Rows matched: 1  Changed: 1  Warnings: 0

Rows matched: 1  Changed: 1  Warnings: 0

Rows matched: 1  Changed: 1  Warnings: 0

Rows matched: 1  Changed: 1  Warnings: 0

Rows matched: 1  Changed: 1  Warnings: 0

-- now let's check
SELECT * FROM customer_ticket_sequence;
SELECT * FROM tickets;

customer_id
seq

1
4

2
2

3
0

customer_id
ticket_id

1
1

1
2

1
3

1
4

2
1

2
2