I making table “clients” and thing I can’t understand.
How to add constraint unique values only for “client_id”?

Here’s my query:

CREATE TABLE clients
(
    client_id UInt16 NOT NULL,
    second_name String,
    first_name String,
    patronymic String,
    phone_number String,
    email String,
    discount Float32,
    comment String,
    PRIMARY KEY (client_id)
)
ENGINE = MergeTree
ORDER BY client_id
SETTINGS index_granularity = 8192;