数据库中NULL对排序和选择条件的影响（Oracle and DB2）

 

NULL 最大，但即不大于1，也不小于1

 

-----------ORACLE

 

SQL> create table a(i integer,j integer);

Table created.

SQL> insert into a values(1,2);

1 row created.

SQL> insert into a(i) values(2);

1 row created.

SQL> commit;

Commit complete.

SQL> select * from a;

         I          J
---------- ----------
         1          2
         2

SQL> select * from a order by j desc;

         I          J
---------- ----------
         2
         1          2

SQL> c/desc/asc
  1* select * from a order by j asc
SQL> /

         I          J
---------- ----------
         1          2
         2

SQL> select * from a where j>1;

         I          J
---------- ----------
         1          2

SQL> c/>/<
  1* select * from a where j<1
SQL> /

no rows selected
 

 

 

 

 

 

 

关于大于小于order by的试验：----------DB2

 

db2 =>
db2 => create table a(i integer,j integer);
create table a(i integer,j integer)
DB20000I  The SQL command completed successfully.

db2 =>  insert into a values(1,2);
insert into a values(1,2)
DB20000I  The SQL command completed successfully.

db2 =>  insert into a(i) values(2);
insert into a(i) values(2)
DB20000I  The SQL command completed successfully.

db2 =>  select * from a order by j desc;
select * from a order by j desc

I           J         
----------- -----------
          2           -
          1           2

  2 record(s) selected.


db2 => select * from a order by j asc
db2 (cont.) => ;
select * from a order by j asc

I           J         
----------- -----------
          1           2
          2           -

  2 record(s) selected.


db2 => select * from a where j>1;
select * from a where j>1

I           J         
----------- -----------
          1           2

  1 record(s) selected.


db2 => select * from a where j<1;
select * from a where j<1

I           J         
----------- -----------

  0 record(s) selected.
 

 

 

ORACLE 与DB2的处理是一模一样的。

 

 

 

 

---------------------------关于NULL 与 ''  -------------------DB2------------

 

db2 => create table b(i integer,j char(10));
create table b(i integer,j char(10))
DB20000I  The SQL command completed successfully.

db2 => insert into b values (1,'111');
insert into b values (1,'111')
DB20000I  The SQL command completed successfully.

db2 => insert into b values (2,'');
insert into b values (2,'')
DB20000I  The SQL command completed successfully.

db2 => insert into b(i) values (3);
insert into b(i) values (3)
DB20000I  The SQL command completed successfully.

db2 => commit;
commit
DB20000I  The SQL command completed successfully.

db2 => select * from b;
select * from b

I           J        
----------- ----------
          1 111      
          2          
          3 -        

  3 record(s) selected.


db2 => select * from b where j is null;
select * from b where j is null

I           J        
----------- ----------
          3 -        

  1 record(s) selected.


db2 => select * from b where j='';
select * from b where j=''

I           J        
----------- ----------
          2          

  1 record(s) selected.