MySQL 8.0

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CREATE TABLE customer (id SERIAL PRIMARY KEY, phone VARCHAR(20));
ALTER TABLE customer
ADD CONSTRAINT CHECK(phone REGEXP '^[0-9]{3}-[0-9]{2}-[0-9]{3}$');
INSERT INTO customer (phone) VALUES ('123-45-678')
INSERT INTO customer (phone) VALUES ('123-456-789')
Check constraint 'customer_chk_1' is violated.

INSERT INTO customer (phone) VALUES ('123-xx-678')
Check constraint 'customer_chk_1' is violated.

INSERT INTO customer (phone) VALUES (' 123-45-678')
Check constraint 'customer_chk_1' is violated.

INSERT INTO customer (phone) VALUES ('123-45-678 ')
Check constraint 'customer_chk_1' is violated.

INSERT INTO customer (phone) VALUES (NULL)
SELECT * FROM customer


id
phone


1
123-45-678

2
null