sql server - Building where clause based on null/not null parameters in sql -

i trying create stored procedure return records based on input. if input parameters null, return entire table, else use parameters , return records:

 create procedure getrecords @parm1 varchar(10) = null, @parm2 varchar(10) = null, @parm3 varchar(10) = null   declare @whereclause varchar(500)  set @whereclause = ' 1 = 1 '   if (@parm1 null , @parm2 null , @parm3 null)     select * dummytable  else     begin       if (@parm1 not null)          set @whereclause += 'and parm1 = ' + '' + @parm1 + ''       if (@parm2 not null)          set @whereclause += 'and parm2 = ' + '' + @parm2 + ''       if (@parm3 not null)          set @whereclause += 'and parm3 = ' + '' + @parm3 + ''        select * dummytable @whereclause  <-- error      end 

error while creating procedure "an expression of non-boolean type specified in context condition"

please comment if approach wrong in building clause?

thanks

the entire query should in varchar , can executed using "exec" function.

set @query = "select * dummytable 1=1 "  ... if clauses ...  exec(@query) 

hth.