Useful

If Code

 {{#if:x| p | q }} gives "p" 

{{#if:  {{{4}}}|[[{{{4}}}]]|'''N/A'''}}

http://www.mediawiki.org/wiki/Help:Extension:ParserFunctions#.23if

If Equal

{{#ifeq: string 1 | string 2 | value if identical | value if different }}

If Exist

{{#ifexist: {{{{{1}}}/list|{{{2}}}}} Guests|[[{{{{{1}}}/list|{{{2}}}}} Guests|Guests]]<br>}}