(select concat(chr(64)||chr(52)||chr(51)||chr(55)||chr(48)||chr(49)||user||chr(64)||chr(49)||chr(48)||chr(55)||chr(51)||chr(52) from DUAL))

Decoded explanation:

• chr(64) = @, chr(52) = 4, chr(51) = 3, chr(55) = 7, chr(48) = 0, chr(49) = 1, and chr(64) = @, chr(49) = 1, chr(48) = 0, chr(55) = 7, chr(51) = 3, chr(52) = 4.
• The expression builds the string: @43701 || user || @10734 In Oracle, user (unquoted) is the current username (the USER value). So the final result is: "@43701" + USER + "@10734"

Thus the output is something like:

• If you’re connected as SCOTT: @43701SCOTT@10734
• If you’re connected as HR: @43701HR@10734

Correct, simpler form to achieve the same:

• SELECT '@43701' || USER || '@10734' FROM dual;

If you really need to use CONCAT (two-argument form), nest it:

• SELECT CONCAT(CONCAT('@43701', USER), '@10734') FROM dual;

Notes:

• The outer CONCAT around a single expression in your snippet would typically be a syntax error; the above forms are the standard ways to achieve the same result.
• Depending on the context, USER is a function returning the current database user. If you intended a literal column named user, you’d need to quote it as "USER" or refer to it unambiguously.