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SQL中的ISNULL函数使用介绍 - 站长搜索
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SQL中的ISNULL函数使用介绍

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2022-09-23 17:15:54 转载来源: 网络整理/侵权必删

ISNULL使用指定的替换值替换NULL。语法ISNULL(check_expression,replacement_value)参数check_expression将被检查是否为NULL的表达式

ISNULL

使用指定的替换值替换 NULL。

语法
ISNULL ( check_expression , replacement_value )

参数
check_expression

将被检查是否为 NULL的表达式。check_expression 可以是任何类型的。

replacement_value

在 check_expression 为 NULL时将返回的表达式。replacement_value 必须与 check_expresssion 具有相同的类型。

返回类型
返回与 check_expression 相同的类型。

注释
如果 check_expression 不为 NULL,那么返回该表达式的值;否则返回 replacement_value。

示例
A. 将 ISNULL 与 AVG 一起使用
下面的示例查找所有书的平均价格,用值 $10.00 替换 titles 表的 price 列中的所有 NULL 条目。

USE pubs
GO
SELECT AVG(ISNULL(price, $10.00))
FROM titles
GO

下面是结果集:

--------------------------
14.24

(1 row(s) affected)

B. 使用 ISNULL
下面的示例为 titles 表中的所有书选择书名、类型及价格。如果一个书名的价格是 NULL,那么在结果集中显示的价格为 0.00。

USE pubs
GO
SELECT SUBSTRING(title, 1, 15) AS Title, type AS Type,
ISNULL(price, 0.00) AS Price
FROM titles
GO

下面是结果集:

Title Type Price
--------------- ------------ --------------------------
The Busy Execut business 19.99
Cooking with Co business 11.95
You Can Combat business 2.99
Straight Talk A business 19.99
Silicon Valley mod_cook 19.99
The Gourmet Mic mod_cook 2.99
The Psychology UNDECIDED

SQL ISNULL()、NVL()、IFNULL() 和 COALESCE() 函数

请看下面的 "Products" 表:

P_Id ProductName UnitPrice UnitsInStock UnitsOnOrder
1 computer 699 25 15
2 printer 365 36  
3 telephone 280 159 57

假如 "UnitsOnOrder" 是可选的,而且可以包含 NULL 值。

我们使用如下 SELECT 语句:

SELECT ProductName,UnitPrice*(UnitsInStock+UnitsOnOrder)FROM Products

在上面的例子中,如果有 "UnitsOnOrder" 值是 NULL,那么结果是 NULL。

微软的 ISNULL() 函数用于规定如何处理 NULL 值。

NVL(), IFNULL() 和 COALESCE() 函数也可以达到相同的结果。

在这里,我们希望 NULL 值为 0。

下面,如果 "UnitsOnOrder" 是 NULL,则不利于计算,因此如果值是 NULL 则 ISNULL() 返回 0。

SQL Server / MS Access

SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0))FROM Products

Oracle

Oracle 没有 ISNULL() 函数。不过,我们可以使用 NVL() 函数达到相同的结果:

SELECT ProductName,UnitPrice*(UnitsInStock+NVL(UnitsOnOrder,0))FROM Products

MySQL

MySQL 也拥有类似 ISNULL() 的函数。不过它的工作方式与微软的 ISNULL() 函数有点不同。

在 MySQL 中,我们可以使用 IFNULL() 函数,就像这样:

SELECT ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0))FROM Products

或者我们可以使用 COALESCE() 函数,就像这样:

SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0))FROM Products

标签: SQL 中的 ISNULL 函数 使用 介绍


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