at least与at the least

问题描述:

at least与at the least
什么情况下用前面的那个,什么时候用后面的那个?

2个都是是至少的意思.
我举2个例子来说明在句子中的区别吧:
THERE'S AT LEAST 5 WORDS IN THIS SENTENCE.
AT THE LEAST,CAN YOU BRING ME SOME WATER?