sql语句笔记

从我的oracle群（14559844）里总结出来的学习笔记

1〉怎么写sql语句删除表中重复的数据

循序渐进，有以下几种写法，不过最后一个应该是效率最高的吧

delete from tab_nm a where rowid >(select min(rowid) from tab_nm b where
a.k=b.k)

INSERT INTO A
SELECT DISTINCT *　FROM　B

CREATE　TABLE A
AS
SELECT DISTINCT *　FROM　B ;
DROP　TABLE　B;
RANAME　A　TO　B;

2〉select length(to_char)用例参考

自己没有试过 直接从QQ记录里拷贝过来的

张三公子(65366533) 08:08:53

SQL> select length(to_char('1234','099999')) from dual;

LENGTH(TO_CHAR('1234','099999'

------------------------------

7

SQL> select (to_char('1234','099999')) from dual;
(TO_CHAR('1234','099999'))

--------------------------

001234

SQL> select (to_char('+1234','099999')) from dual;

(TO_CHAR('+1234','099999'))

---------------------------

001234

SQL> select (to_char('-1234','099999')) from dual;

(TO_CHAR('-1234','099999'))

---------------------------

-001234

张三公子(65366533) 08:10:45
是隐式的符号位...利用to_char进行数字型转换的 + 与 -的表示,空格表示正数，负的表示-

也可以利用 S来显示说明:

SQL> select (to_char('1234','S099999')) from dual;

(TO_CHAR('1234','S099999'))

---------------------------

+001234

SQL> select (to_char('-1234','S099999')) from dual;

(TO_CHAR('-1234','S099999'))

----------------------------

-001234

2009-08-06 16:35:59
sql里查询数据的时候进行格式化.可以用cast或者convert。举例如下，想把查询的结果按照decimal(18,2)格式输出。在mssql2005里测试通过。

SELECT CAST(AVG(mony) AS decimal(18, 2)) AS bb FROM t_table

SELECT CONVERT(decimal(10, 2), AVG(mony)) AS Expr1 FROM t_table