Oracle 避免在查询中使用函数

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一个查询可能要搜索上百万行数据，在查询中使用函数就可能被调用上百万次，这会严重影响性能，下面是一个简单的例子。

-- 创建表
CREATE TABLE Department 
(
	Department_Id    NUMBER(9,0),
	Department_Name  VARCHAR2(40)
);

CREATE TABLE Employee
(
	Employee_id    NUMBER(9,0),
	Employee_Name  VARCHAR2(40),
	Department_Id  NUMBER(9,0)
);

-- 定义函数
CREATE OR REPLACE FUNCTION getDepartmentNameById(
	DepartmentId    number   
)  
	RETURN varchar2  
AS    
	DepartmentName VARCHAR2(40);    
BEGIN    
	select Department_Name into DepartmentName from Department where Department_Id = DepartmentId;    
	return DepartmentName;      
END;

-- 查询 SQL -- 使用函数
SELECT getDepartmentNameById(Department_Id) DepartmentName, Employee_Name FROM Employee;

-- 查询 SQL -- 使用表连接
SELECT 
	d.Department_Name, 
	e.Employee_Name 
FROM 
	Department d,
	Employee e
WHERE
	d.Department_Id = e.Department_Id;

-- 更多参见：Oracle SQL 优化精萃

-- 声明：转载请注明出处

-- Last edited on 2015-06-29

-- Created by ShangBo on 2015-06-29

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