帮忙翻译如下短文？译完马上结贴，谢谢

[bluesea07]

Working with NULL Values
Back in Lesson 4, "Filtering Data," you learned that NULL values are no values or the lack of a value. A column that allows NULL values also allows rows to be inserted with no value at all in that column. A column that does not allow NULL values does not accept rows with no value—in other words, that column will always be required when rows are inserted or updated.
Every table column is either a NULL column or a NOT NULL column, and that state is specified in the table definition at creation time. Take a look at the following example:
输入
CREATE TABLE Orders
(
order_num INTEGER NOT NULL,
order_date DATETIME NOT NULL,
cust_id CHAR(10) NOT NULL
);
分析
This statement creates the Orders table used throughout this book. Orders contains three columns: order number, order date, and the customer ID. All three columns are required, and so each contains the keyword NOT NULL. This will prevent the insertion of columns with no value. If someone tries to insert no value, an error will be returned, and the insertion will fail.
This next example creates a table with a mixture of NULL and NOT NULL columns:

[NinGoo]

就是说一个表中的列，要么是允许插入NULL值，要么不允许插入NULL值。然后给了个例子，表中三个列都不允许是NULL值。

[itblog]

Working with NULL Values
空值的作用
Back in Lesson 4, "Filtering Data," you learned that NULL values are no values or the lack of a value. A column that allows NULL values also allows rows to be inserted with no value at all in that column. A column that does not allow NULL values does not accept rows with no value—in other words, that column will always be required when rows are inserted or updated.
在第4课,"数据过滤,"了解到空值是没有值或是不存在的值，一个列如果允许为空，在没有值的时候也可以被插入行。如果一个列不允许空，将不接受没有值的数据插入或更新。换句话说就是这个列在插入或是更新行的时候总提示有值没有填写。
Every table column is either a NULL column or a NOT NULL column, and that state is specified in the table definition at creation time. Take a look at the following example:
每一个表中的列或者是一个允许为空的列或者是一个不允许为空的列，它可以在创建表的时候指定，例如下面这个例子：
输入
CREATE TABLE Orders
(
order_num INTEGER NOT NULL,
order_date DATETIME NOT NULL,
cust_id CHAR(10) NOT NULL
);
分析
This statement creates the Orders table used throughout this book. Orders contains three columns: order number, order date, and the customer ID. All three columns are required, and so each contains the keyword NOT NULL. This will prevent the insertion of columns with no value. If someone tries to insert no value, an error will be returned, and the insertion will fail.
这段语句是创建表通过书本创建一个orders表,表orders包括三列:定单号, 定购日期,客户ID，这三个列都不能少，并且每列都包含关键字 not null,这样可以阻止插入列的时候没有数值，如果有人试图插入空值，将返回错误，插入失败。
This next example creates a table with a mixture of NULL and NOT NULL columns:
接下来这个例子是以允许空和不允许空混合的列创建一个表

[liangpei2008]

楼上翻译的不错：）

[hyrongg]

Null值的操作：
回到第四课，“数据过滤”，你已经知道NULL意味着没有值或者值缺乏，一个允许NULL值的列同时也允许插入一条该字段为NULL值的纪录。一个不允许NULL的列也不允许插入一条该列为NULL的纪录，换句话说，如果该列不允许为空，则当插入一条记录或修改这列的时候，这列通常会需要赋不为NULL值。
每个表的列不管是允许NULL的还是不允许NULL的，它的状态通常会在创建表的时候定义，看一下以下的例子：
输入
CREATE TABLE Orders
(
order_num INTEGER NOT NULL,
order_date DATETIME NOT NULL,
cust_id CHAR(10) NOT NULL
);
分析
这段命令创建了Orders表，该表从始至终的被运用于本书，orders表包含了3个列
采购号码, 采购日期和客户编号，所有三个列都必须要有相关信息，所以每个列都设为NOT NULL,这样会避免列插入空值，如果试图插入空值，将会返回错误信息，同时插入操作也会失败。接下来的例子将要创建一个既有NULL列也有Not NULL 列的表
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看你这本书真得不增麽样：（