i = case j 
  when  1  then  10 
  when  2  then  20 
  else  0 
  end;

select case :j 
  when  1  then (select result from proc1)
  when  2  then (select result from proc2)
  else        (select result from proc3)
  end 
from rdb$database into :i;

Declare Variable Cnt Integer;
...
Select Count(*) from from Tbl where Id = :InpId Into :Cnt;
if (Cnt >  0 ) then begin
  ...
end

if (Exists (Select Id from Tbl where Id = :InpId)) then begin
  ...
end

if (i =  0 ) then begin
  var1 =  0 ;
  var2 = '-';
  var3 = Cuttent_Date;
  ...
end;

if (i =  0 ) then  
  Select  0 , '-', Current_Date, ... From Rdb$Database 
  Into :Var1, :Var2, :Var3 ...;

if (StartDat is Null) then
  StartDat =  Cast('1900-01-01' as date);
if (EndDat is Null) then
  EndDat =  Current_Date;

StartDat = Coalesce(StartDat, Cast('1900-01-01' as date);
EndDat = Coalesce(EndDat, Current_Date); 

Select Coalesce(StartDat, Cast('1900-01-01' as date)), Coalesce(EndDat, Current_Date) 
From Rdb$Database Into :StartDat, EndDat;

Function Iif(Const Expr : Boolean; Const ValT, ValF : Integer) : Integer; Overload;
begin	if Expr then Result := ValT else Result := ValF; end;
Function Iif(Const Expr : Boolean; Const ValT, ValF : String) : String; Overload;
begin	if Expr then Result := ValT else Result := ValF; end;
Function Iif(Const Expr : Boolean; Const ValT, ValF : DateTime) : DateTime; Overload;
begin	if Expr then Result := ValT else Result := ValF; end;
...
...

...
if (LikeValue[ 1 ] = '%') then
  WhereExpr := WhereExpr + 'Like '
else
  WhereExpr := WhereExpr + 'Starting Whith ';
...

WhereExpr := WhereExpr + Iif(LikeValue[ 1 ] = '%', 'Like ', 'Starting Whith ');