Could a sentence with relative clause be simple sentence?

Question

It sounds a bit basic and simple question but I want to learn if a sentence as the following can be classified as simple sentence?

They decided that it was a good idea to invest in the company.

And if there is a special name for this part of sentence when a "independent sentence" , what I mean is that the part it was a good idea to invest in the company, follows that or is it simply called as object.

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Edit :

Another sentence could be as the following:

I liked the book that I bought last weekend.

or

It tasted so good that I could not help but have another one.

Answer 1

Could a sentence with relative clause be simple sentence?

No.

There are three types of sentences in English.

1. Simple: One independent clause with a subject and a verb (predicate).

2. Compound: Two independent clauses joined by a coordinator such as for, and, but, etc.

3. Complex: One independent clause with one or more dependent clauses. A dependent clause could be

(1) a noun clause. One example is a clause with the complementizer that:

They decided that it was a good idea to invest in the company.

(2) a relative clause with relative pronouns such as which, that, who and whom, etc.

I liked the book that I bought last weekend.

(3) an adverbial clause with conjunctions such as because, if, as, since and when, etc.

I couldn't finish homework because it was too late.

As John Lawler commented, a simple sentence is rare in English and you can Google its examples.