统计oracle表中字段的个数

select count（column_name） from user_tab_columns where table_name='emp'

dba权限对应的视图是dba_tab_columns 和all_tab_columns;

2.select 1 、select count(1) 、select sum(1)    可以理解count(1) 到底是怎么回事了

SQL> select * from BOOK;

   BOOK_ID BOOK_NAME
---------- ----------
	 1 book1
	 1 book1

SQL> select 1 from BOOK;

	 1
----------
	 1
	 1

SQL> select count(1) from BOOK;

  COUNT(1)
----------
	 2

SQL> select 10 from BOOK;

	10
----------
	10
	10

SQL> select count(10) from BOOK;

 COUNT(10)
----------
	 2

SQL> select sum(10) from BOOK;

   SUM(10)
----------
	20