sql server - SQL do inner join if condition met -

i want way improve sql code, have use inner join when condition met. replicates code:

@systemmerge bit  if (@systemmerge=1)   begin    select          .......       mytable       inner join table on table.param1=mytable.param1       inner join systemtable on systemtable.param2=mytable.param2    end else    begin       select          .......       mytable       inner join table on table.param1=mytable.param1    end 

and in way this:

@systemmerge bit begin    select       .......    mytable    inner join table on table.param1=mytable.param1    ***//the next 4 lines not working, pseudo of want:***    if (@systemmerge=1)      begin       inner join systemtable on systemtable.param2=mytable.param2    end 

edit: solution (thanks @damien_the_unbeliever):

left join systemtable on systemtable.param2=mytable.param2     ((@systemmerge=1 , systemtable.param2 not null)     or    (@systemmerge=0 or @systemmerge null)) 

this should (approxmately) same thing:

select      .......   mytable   inner join table on table.param1=mytable.param1   left join systemtable on systemtable.param2=mytable.param2 , @systemmerge = 1   (@systemmerge = 0 or systemtable.nonnullablecolumn not null) 

of course, means other references columns within systemtable must written expect such columns null.