sql pulling a row for next or previous row of a current row

I considered id as primary key in the table (and as "row number"), and used it to compare each record with the record before. The following code must work.

CREATE SCHEMA temp
create table temp.emp (id integer,name varchar(50), salary  varchar(50));
insert into temp.emp values(1,'a','25000');
insert into temp.emp values(2,'b','30000');
insert into temp.emp values(3,'c','35000');
insert into temp.emp values(4,'d','40000');
insert into temp.emp values(5,'e','45000');
insert into temp.emp values(6,'f','20000');

select * from temp.emp

SELECT
    current.id, current.name, current.salary,
    case 
        when current.id = 1 then current.salary 
        else 
            case
                when current.salary > previous.salary then previous.salary
                else current.salary  
            end
     end
FROM
    temp.emp AS current
    LEFT OUTER JOIN temp.emp AS previous
    ON current.id = previous.id + 1