This drink is designed for weightlifters, who need the extra energy.
This drink is designed for weightlifters that need the extra energy.

I always see the comma before who as needless.
Can anyone please extrapulate on this, please?
Thank you.
1 2
Sentence 1 tells you that the drink is designed for people who lift weights. As an aside, it also casually mentions the opinion that these people may need extra energy.

Sentence 2 tells you that the drink is designed for people who meet two simultaneous criteria, these being (1) they must lift weights, and (2) they must need the extra energy. Anyone not meeting both criteria simultaneously is not a person for whom the drink was designed.

in my opinion the "," make the sentences different,without the comma they will mean the same
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I was not aware that it had to do with criteria meeting.
Thank you.
I think you are missing the main problem, I would say it is not standard to use the second wording, at all! It is usual to use "who" for people in that sort of context. You dont need the comma either!
Accrding to a gramma book, I don't need a comma?
I come across a lot of sentences, which don't have commas before who or which(as in this sentence), and I consider them wrong according to a grammar book's rules.
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I have a good grammar book, but, to be honest, I can't really use it in comma usage, because I never know what qualifies as an independent clause or anyhitng like that!
I am going for a walk, would you join me? Have I used the comma correctly?
Listen guys - this is a difference between British English and American English. Your grammar book will depend on where it was published. (And Microsoft Word's grammar checker will use the American rules even if you tell it you live in Britain, because it's stupid).

What I said in my earlier post on this thread is correct FOR AMERICA, which I shall repeat here, hopefully more clearly -

"That" (without a comma) is used to connect two clauses, where the clause on the left is incomplete without the clause on the right. For example, as in "This is the house that Jack built".

"Which," and "who," (with a required comma) is used to connect two clauses, where the clause on the left is complete in itself, and where the clause on the right merely provides additional descriptive information. For example "This is my house, which I built".

In the first form, "that" is used for both people and things. In the second form, "which," is used for things and "who," is used for people.

On the other side of the Atlantic, the rules are different. In Britain, "which" and "that" are completely interchangable in this context, as are "who" and "that". In Britain, the difference between the two meanings is carried ENTIRELY by the comma.

I gave you the American rules because the American rules will be correct everywhere in the world, even in Britain, whereas the British rules would be considered incorrect in America.

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