SQL> create table nvl (a int, b int);

表已创建。

SQL> insert into nvl values (1,1);

已创建 1 行。

SQL> insert into nvl values (2,null);

已创建 1 行。

SQL> insert into nvl values (3,3);

已创建 1 行。

SQL> commit;

提交完成。

SQL> select count(nvl(b,0))
2 from nvl where b is null;

COUNT(NVL(B,0))
---------------
1

SQL> select * from nvl where b is null;

A B
---------- ----------
2

SQL> select count(*) from nvl where b is null;

COUNT(*)
----------
1

SQL> select count(b) from nvl wehre b is null;
select count(b) from nvl wehre b is null
*
第 1 行出现错误:
ORA-00933: SQL 命令未正确结束

SQL> select count(b) from nvl wehre b = null;
select count(b) from nvl wehre b = null
*
第 1 行出现错误:
ORA-00933: SQL 命令未正确结束

SQL> select count(distinct b) from nvl wehre b is null;
select count(distinct b) from nvl wehre b is null
*
第 1 行出现错误:
ORA-00933: SQL 命令未正确结束

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