关于外连接的问题

t1:
code value
---------------
B0006 2123629.03
B0005 0
B0007 456573.38
t2:
code value
---------------
B0006 2123629.03


1.select t1.code_t1 code_t1,
  t1.value_t1 value_t1,
  t2.code_t2 code_t2,
  t2.value_t2 value_t2
  from t1
  left join t2
  on t1.code_t1 = 'B0006';

code_t1 value_t1 code_t2 value_t2
--------------------------------------------
B0006 2123629.03 B0006 2123629.03
B0005 0
B0007 456573.38



2.select t1.code_t1 code_t1,
  t1.value_t1 value_t1,
  t2.code_t2 code_t2,
  t2.value_t2 value_t2
  from t1
  left join t2
  on t1.code_t1 = t2.code_t2
  where t1.code_t1 = 'B0006'

code_t1 value_t1 code_t2 value_t2
--------------------------------------------
B0006 2123629.03 B0006 2123629.03


3.select t1.code_t1 code_t1,
  t1.value_t1 value_t1,
  t2.code_t2 code_t2,
  t2.value_t2 value_t2
  from t1
  left join t2
  on t1.code_t1 = t2.code_t2
  and t1.code_t1 = 'B0006'

code_t1 value_t1 code_t2 value_t2
--------------------------------------------
B0006 2123629.03 B0006 2123629.03
B0005 0
B0007 456573.38

4.select t1.code_t1 code_t1,
  t1.value_t1 value_t1,
  t2.code_t2 code_t2,
  t2.value_t2 value_t2
  from t1
  left join t2
  on t1.code_t1 = t2.code_t2
  and t2.code_t2 = 'B0006'

code_t1 value_t1 code_t2 value_t2
--------------------------------------------
B0006 2123629.03 B0006 2123629.03
B0005 0
B0007 456573.38


可以解释一下这些结果产生的原理吗？

我也不会 
这个问题极深 极深
拿下Oracle认证的我都怀疑能否讲得清