当连接两张表时，用来关联的字段都不为空时，不会出现问题。但是假使其中一个连接字段可以为空，那么必须分开来处理。
select T1.A from T1 left outer join T2 on T1.A = T2.A
=>
select T1.A from T1 left outer join T2 on (T1.A is null and T2.A is null) or (T1.A = T2.A)
select T1.A from T1 left outer join T2 on T1.A = T2.A
=>
select T1.A from T1 left outer join T2 on (T1.A is null and T2.A is null) or (T1.A = T2.A)
