This is impossible to search on google, bing, yahoo, etc, because it uses symbols. How annoying!
What's the difference between ::= and := in oracle's pl/sql?
I am not sure about ::= as I have not seen that in Oracle but the wiki says about :=
In computer programming languages, the equals sign typically denotes either a boolean operator to test equality of values (e.g. as in Pascal or Eiffel), which is consistent with the symbol's usage in mathematics, or an assignment operator (e.g. as in C-like languages). Languages making the former choice often use a colon-equals (:=) or ≔ to denote their assignment operator. Languages making the latter choice often use a double equals sign (==) to denote their boolean equality operator.
Also check here:
The assignment operator in PL/SQL is a colon plus an equal sign (:=). PL/SQL string literals are delimited by single quotes