Oracle SQL TO_CHAR Function return values

Question

I don't understand why these two code results are identically the same? I thought if I have quotes it just concatenate strings. Why not first one is 300.5100? I know second one is 400.5 anyway. Thank you

Select to_char('300.5' + '100') From Dual;

Select to_char(300.5 + 100) From Dual;

Answer 1

To concatenate strings in SQL you have to use || . The + is only there to add numbers. If you didn't pass '300.5' + '100' you would simply get an error, eg the following is invalid SQL:

select '300.5' + '100'
from dual;

But as to_char() expects a number as the input parameter Oracle implicitly converts those strings to numbers and then adds them, just like in the second statement.

Answer 2

concatenation is

'xxx' || 'yyy'

your example allows the literals to be converted to numeric then treated as normal numbers.