Syntax for "IF" clause in SELECT

Greetings,

the following is an MySQL statement that I would like to
translate to PostgreSQL:

Could someone point me to a documentation of a coresponding
Systax for an "IF" clause in the a SELECT,
or is the some other way to do this....

select
 if(spektrum is null,' ','J'),
 if(s19 is null,' ','J'),
 if(OhneGrenze is null,' ','J'),
 from namen;


Do I need to create my own function to allow this behaviour!


my best regards,

Stefan
--
email: stefan@yukonho.de
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