simple PL/SQL syntax question.

Can someone explain why this works, please?

I want to set the value of one local string variable in PL/SQl to:  

    USER_ID = 'another_local_string'

including the single ticks. Through trial and error I've found the following assignment statement works.

a_local_string := 'USER_ID = ''' || another_local_string || '''' ;

In case this is hard to read with your typeface, that is: one single tick at the beginning, three single ticks in the middle, and four single ticks at the end.

Does this make sense to anyone? If so please enlighten me. Thanks in advance.

-- 
richard_davies_at_retek.com