Oracle 11g Release 2

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with 
  a(alias, level_20, points, date_) as (
    select 'Jbax', 300, 325, date '2018-06-20' from dual union all
    select 'Cvg2', 100, 103, date '2018-06-20' from dual union all
    select 'Deg1', 200, 281, date '2018-06-20' from dual union all
    select 'pqrs', 100, 103, date '2018-06-20' from dual),
  b(alias, code_level, prize, date_) as (
    select 'Jbax', 'Level_20', 500, date'2017-01-15' from dual union all
    select 'Jbax', 'Level_10', 200, date'2017-03-20' from dual union all
    select 'Cvg2', 'Level_20', 100, date'2017-04-05' from dual union all
    select 'Deg1',  null,      null, null             from dual union all    
    select 'Cvg2', 'Level_20',   50, date'2017-02-01' from dual union all
    select 'pqrs', 'Level_20',  200, date'2017-04-05' from dual union all
    select 'pqrs', 'Level_20',  100, date'2017-02-01' from dual 
    ),   
  c(alias, name) as (
    select 'Jbax', 'Jessie' from dual union all 
    select 'Cvg2', 'Carol'  from dual union all
    select 'Deg1', 'Danny'  from dual union all
    select 'pqrs', 'Peter'  from dual)  
select name, level_20 
  from (
    select a.alias, a.level_20, c.name, b.code_level, b.prize, b.date_,
           row_number() over (partition by a.alias, c.name order by b.date_ desc) rn
      from a join c on c.alias = a.alias
      left join b on b.alias = a.alias and b.code_level = 'Level_20')
  where rn = 1 and (prize is null or prize <> level_20)

NAME	LEVEL_20
Danny	200
Jessie	300
Peter	100