Some people say that the copula is an intransitive verb, but it can be said that it is a doctor.

First of all, verbs are divided into three categories: notional verbs, auxiliary verbs and modal verbs. Verbs with real meaning can be divided into transitive verbs and intransitive verbs. As the name implies, intransitive verbs are verbs that cannot directly follow the object, that is, verbs that can fully express their meaning without following the object. For example, die is dead. Die can express himself completely without following and combining. If you look at it this way, connecting verbs can also be classified as intransitive verbs.

Secondly, the sentence you quoted is actually the structure of the main table. The so-called cardinality table structure is a structure composed of a connecting verb and a predicative.

Predicates can be nouns, adjectives, infinitives and gerunds. But the most common forms in English are nouns and adjectives. For example, he is A, and the students here are making slogans. In your example, doctor is also a slogan, not a predicate, so it is correct.