How to return table based on if condition in Postgesql

[saswat36]

CREATE Procedure [dbo].[GetMstDropdowndata]
@FieldType varchar(100)='Typeid'
AS
BEGIN
if @FieldType='IsOptionalid'
BEGIN
SELECT 'Yes' as ddlvalue,'Yes' AS ddltext
union
SELECT 'No' as ddlvalue,'No' AS ddltext
END
else if @FieldType='Categoryid'
BEGIN
select [CategoryId] as ddlvalue ,[CategoryName] as ddltext
from [CategoryMaster] order by CategoryName
END
END

How to convert above sp in Postgresql