数据库入门的脚本1

案例1，认识左联的条件，可在最后一行on条件中，将b.id修改成a.id，会得到不同的结果，说明左联的on条件选取很重要，否则数据会丢失

select * from 
       ( select 1 as id, '1' as vaa from dual
         union all
         select 2 as id, '2' as vaa from dual
) a
left join
       ( select 1 as id, '1' as vbb from dual
         union all
         select 3 as id, '3' as vbb from dual
) b
on a.id = b.id
left join
       ( select 2 as id, '2' as vcc from dual
         union all
         select 3 as id, '3' as vcc from dual
) c
on b.id = c.id

案例2， decode中可以嵌套一个 select 

select decode(a.id, 1, a.vaa, 
                    2, (select b.vbb from ( select 1 as id, '1' as vbb from dual
                                           union all
                                            select 3 as id, '3' as vbb from dual
                                          ) b
                                          where b.id = 3),
              null)
from ( select 1 as id, '1' as vaa from dual
       union all
       select 2 as id, '2' as vaa from dual
) a