When ON delete set NULL is active, when a record is deleted from the parent table, all records in the child table with matching foreign key values are set to NULL.
当ON DELETESETNULL规则被激活时，如果一条记录从父表中删除，子表中的所有记录以及匹配的外键值都被设置为NULL。
Any value except Null can serve as a key; any value can be assigned to a slot.
除Null以外的任何值都可以充当一个键；任何值都可以被分配给一个slot。
The first key is 21 bytes long (the byte length is shown in parentheses [()]) and is called GLOBALS/c_setprinter followed by a NULL, which is ASCII zero.
第一个键为21字节（字节长度显示在括号中 [()]），被称为GLOBALS/c_setprinter，后跟NULL，其为ASCII 码零。
You can also use the system catalog views to retrieve information about NOT NULL, unique, primary key, foreign key, and table check constraints. For example.
也可以使用系统编目视图来检索关于NOT NULL、惟一、主键、外键和表检查约束的信息。
Line 7 gets the session key from the session if it's already present, else it would be null.
如果会话已存在，第7行代码将从会话中获取会话键，否则它将为null。
If name does not correspond to a key in the hashtable, then null will simply be returned.
如果哈希表中没有与键值对应的名称，将返回null。
This becomes particularly important if it is acceptable for a given key to exist in the map but have a corresponding value of null, as is the case for the Los Angeles key in Listing 1.
如果它可以表示map上存在某个键，但是有对应的null值，这一点特别重要了。比如清单1中Losangeles键。
For that effect, JSON defines the following main data structures: number, string, Boolean (true and false), array (an ordered sequence of values), object (collection of key value pairs), and null.
为了实现该目的，JSON定义了以下主要数据结构：数字、字符串、布尔值(真或假)、数组(值的有序排列)、对象(键值对集合)和null。
Any thread can tell which state the queue is in by comparing tail.next to null, which is the key to enabling threads to help other threads "finish" their operation.
任何线程通过比较tail . next是否为null，就可以判断出队列的状态，这是让线程可以帮助其他线程“完成”操作的关键。
In the "key" element, the default values for "not-null" and "update" are false and true, respectively. The above code shows them for the purpose of clarity.
在“key”元素中，“not -null”和“update”对应的默认值是false和true，上述代码为了明确这些取值，将它们写了出来。
For example, if you are creating a Customer table and the CustomerID is marked as not null, then when you define the primary key, CustomerID will show as a candidate column.
例如，如果您要创建一个Customer表并且CustomerID 被标记为not null，则当您定义主键时，CustomerID将会显示为备选列。
Neither a key nor a value can be null (if a null value in a dictionary is required, use "NSNull" instead).
密钥或值可能是null(如果需要字典中的null值，请使用NSNull替代)。
Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.
未能启用约束。一行或多行中包含违反非空、唯一或外键约束的值。
"This behavior occurs because the primary key or the" not NULL "column of the database table was not selected in the data Form Wizard."
此行为的发生是由于在“数据窗体向导”中未选择数据库表的主键或“非null”列。
You can attach unique constraints to columns that allow null values, whereas you can attach primary key constraints only to columns that do not allow null values.
可将唯一约束附加到允许空值的列，但是只能将主键约束附加到不允许空值的列。
注册表项的名称为空。
When a change is made to a relationship, the related foreign-key property is set to a null value.
当一个变化是一个关系，相关的外键的属性设置为空值。
When a change is made to a relationship, the related foreign-key property is set to a null value.
当一个变化是一个关系，相关的外键的属性设置为空值。
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