だけ・しか～ない: the difference
だけN5
Meaning
Only, Just
Past papers
Tested 6 times in past papers
Example
一人だけです。
Only one person.
しか～ないN4
Meaning
There's only, Nothing but
Past papers
Tested 10 times in past papers
Example
座るところがここしかありません。
There is nowhere else to sit but here.
How to tell them apart
「だけ」 vs 「しか〜ない」: 「千円だけある」 just states that there is 1,000 yen; 「千円しかない」 always takes a negative and carries regret: 'only 1,000 yen, it's not enough'.
「ばかり」 means 'nothing but' and often carries dissatisfaction: 「肉ばかり食べる」 (eat nothing but meat); 「肉だけ食べる」 just says meat is the only thing eaten.