What is the difference between != and <> operators?

Question

I don\'t like not knowing this as there may be situations when I need to use one instead of the other. It seems in most cases they produce the same results but I am taking a

Answer 1

Being curious I found out that the conversion is defined in /src/backend/parser/scan.l:

/* Convert "!=" operator to "<>" for compatibility */
if (strcmp(yytext, "!=") == 0)
    yylval->str = pstrdup("<>");
else
    yylval->str = pstrdup(yytext);
return Op;

scan.l is used almost at the beginning of parsing. There is also note in Create Operator chapter:

The operator != is mapped to <> on input, so these two names are always equivalent.

You can find <> operator in pg_operator table, but there is nothing for !=:

select * from pg_operator where oprname = '<>';
select * from pg_operator where oprname = '!=';

There is a lot on this topic in this similar question.