Oracle常用技巧及一些函数的总结

1、将字符串转换为日期:可以不用TO_DATE

SELECT * FROM EMP E WHERE E.HIREDATE < DATE '2012-08-09'

2、从子查询中删除数据

DELETE FROM (SELECT * FROM DUAL)

3、查询所有的系统默认参数

SELECT * FROM NLS_DATABASE_PARAMETERS;
SELECT * FROM NLS_SESSION_PARAMETERS;

4、查询数据库的信息

SELECT * FROM v$database;

4、查询实例的相关信息:包括主机名，启动时间，当前的状态，版本信息

SELECT * FROM v$instance;

--使用CAST函数:

     CAST converts one built-in datatype or collection-typed value into another built-in datatype or collection-typed value.(操作集合类型和内部支持的数据类型)

     CAST lets you convert built-in datatypes or collection-typed values of one type into another built-in datatype or collection type(将内部类型或集合类型的值转换为另一种集合类型或者内部类型). You can cast an unnamed operand (such as a date or the result set of a subquery) or a named collection (such as a varray or a nested table) into a type-compatible datatype or named collection. The type_name must be the name of a built-in datatype or collection type and the operand must be a built-in datatype or must evaluate to a collection value:

When you use CAST ... MULTISET to get a collection value, each select list item in the query passed to the CAST function is converted to the corresponding attribute type of the target collection element type.

User-defined types such as collections cannot be implicitly converted, but must be explicitly converted using CAST ... MULTISET

语法:

SELECT CAST ('1' AS NUMBER) FROM dual

--使用collect函数:

结合cast使用的collect

 参考地址:http://www.cnblogs.com/caroline/archive/2012/04/27/2474281.html

SELECT Deptno,
       Tab_To_String(CAST(COLLECT(Ename) AS t_Varchar2_Tab)) AS Employees
  FROM Emp
 GROUP BY Deptno;

使用cast聚合成集合:

SELECT CAST(COLLECT(Ename) AS t_Varchar2_Tab) AS Employees
  FROM Emp
 GROUP BY Deptno;
 

使用table函数，把集合变为一行一行的记录:

SELECT *
  FROM TABLE (SELECT CAST(COLLECT(Ename) AS t_Varchar2_Tab) AS Employees
                FROM Emp);