. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 812 63.4 查询函数 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813 63 Julia 支持表示统计意义上的缺失值，即某个变量在观察中没有可用值，但在理论上存在有效值的情 况。缺失值由 missing 对象表示，该对象是 Missing 类型的唯一实例。missing 等价于 SQL 中的 NULL 以及 R 中的 NA，并在大多数情况下表现得与它们一样。 21.1 缺失值的传播 missing values propagate automatically when passed 逻辑（或布尔）运算符 |、& 和 xor 是另一种特殊情况，因为它们只有在逻辑上是必需的时传递 missing 值。对于这些运算符来说，结果是否不确定取决于具体操作，其遵循三值逻辑的既定规则， 这些规则也由 SQL 中的 NULL 以及 R 中的 NA 实现。这个抽象的定义实际上对应于一系列相对自然的 行为，这最好通过具体的例子来解释。 让我们用逻辑「或」运算符 | 来说明这个原理。按照布尔逻辑的规则，如果其中一个操作数是

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situa�ons. 27.1 Propaga�on of Missing Values The behavior following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract defini�on actually corresponds to a rela�vely natural behavior which is Nothing} arguments or fields. To represent missing data in the sta�s�cal sense (NA in R or NULL in SQL), use the missing object. See the Missing Values sec�on for more details. The empty tuple (()) is

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situa�ons. 27.1 Propaga�on of Missing Values The behavior following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract defini�on actually corresponds to a rela�vely natural behavior which is Nothing} arguments or fields. To represent missing data in the sta�s�cal sense (NA in R or NULL in SQL), use the missing object. See the Missing Values sec�on for more details. The empty tuple (()) is

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situa�ons. 27.1 Propaga�on of Missing Values The behavior following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract defini�on actually corresponds to a rela�vely natural behavior which is Nothing} arguments or fields. To represent missing data in the sta�s�cal sense (NA in R or NULL in SQL), use the missing object. See the Missing Values sec�on for more details. The empty tuple (()) is

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situa�ons. 28.1 Propaga�on of Missing Values The behavior following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract defini�on actually corresponds to a rela�vely natural behavior which is Nothing} arguments or fields. To represent missing data in the sta�s�cal sense (NA in R or NULL in SQL), use the missing object. See the Missing Values sec�on for more details. The empty tuple (()) is

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situations. 27.1 Propagation of Missing Values The behavior following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract definition actually corresponds to a relatively natural behavior which is Nothing} arguments or fields. To represent missing data in the statistical sense (NA in R or NULL in SQL), use the missing object. See the Missing Values section for more details. The empty tuple (()) is

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situa�ons. 29.1 Propaga�on of Missing Values The behavior following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract defini�on actually corresponds to a rela�vely natural behavior which is Nothing} arguments or fields. To represent missing data in the sta�s�cal sense (NA in R or NULL in SQL), use the missing object. See the Missing Values sec�on for more details. The empty tuple (()) is

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situations. 20.1 Propagation of Missing Values missing following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract definition actually corresponds to a relatively natural behavior which is Nothing} arguments or fields. To represent missing data in the statistical sense (NA in R or NULL in SQL), use the missing object. See the Missing Values section for more details. In some languages, the

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situations. 20.1 Propagation of Missing Values missing following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract definition actually corresponds to a relatively natural behavior which is Nothing} arguments or fields. To represent missing data in the statistical sense (NA in R or NULL in SQL), use the missing object. See the Missing Values section for more details. In some languages, the

missing object, which is the singleton instance of the type Missing. missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situations. 20.1 Propagation of Missing Values missing following the well-established rules of three-valued logic which are also implemented by NULL in SQL and NA in R. This abstract definition actually corresponds to a relatively natural behavior which is Nothing} arguments or fields. To represent missing data in the statistical sense (NA in R or NULL in SQL), use the missing object. See the Missing Values section for more details. In some languages, the