count(*)===>表的记录数，与某个字段是否为空无关
count(column_name)===>如果当前列有空，则会剔除掉该列为空的那些行
可以参看以下实验结果：
SQL> select count(*) from table_name t where t.lvx_id is null;
COUNT(*)
----------
9
SQL> select count(t.lvx_id) from table_name t where t.lvx_id is null;
COUNT(T.LVX_ID)
---------------
0
SQL>
【编写于 2009-01-20】
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