摘要： 1.建立环境create table t1 (id int,name varchar(10),age int);create table t2 (id int,name varchar(10),age int);insert into t1 values (1,’abc’,33);insert into t1 values (2,’’,31);insert into t2 values (1,’abc’,33);insert into t2 values (2,’’,31);insert into t1 values (3,’xyz’,34);2. Null的问题SQL> select 阅读全文