参考资料：http://docs.oracle.com/database/122/SQLRF/Functions.htm#SQLRF006
Single-row functions return a single result row for every row of a queried table or view. These functions can appear in select lists, WHEREclauses, START WITH and CONNECT BY clauses, and HAVING clauses.
单行函数返回查询表或视图的每一行的单一结果行。单行函数可以出现在SELECT列中，WHERE子句，START WITH 和CONNECT BY子句以及HAVING子句中。
NULL-Related Functions：
The NULL-related functions facilitate null handling.
The NULL-related functions are:
NVL:
语法： NVL(expr1,expr2)
功能：
NVL lets you replace null (returned as a blank) with a string in the results of a query. If expr1 is null, then NVL returns expr2. If expr1 is not null, then NVL returns expr1.
说明：
1.The arguments expr1 and expr2 can have any data type.
参数expr1和expr2可以是任何数据类型。
2. If their data types are different, then Oracle Database implicitly converts one to the other.
如果俩参数数据类型不同，oracle数据库将进行隐式转换。
3.If they cannot be converted implicitly, then the database returns an error.
如果俩参数不能进行隐式转换，数据库将返回一个error。
4.The implicit conversion is implemented as follows:(隐式转换如下)
1>.If expr1 is character data, then Oracle Database converts expr2 to the data type of expr1 before comparing them and returns VARCHAR2 in the character set of expr1.
如果expr1是字符数据，则数据库会将expr2转换为expr1的数据类型，并返回expr1的字符集中的VARCHAR2。
2>.If expr1 is numeric, then Oracle Database determines which argument has the highest numeric precedence, implicitly converts the other argument to that data type, and returns that data type.
如果expr1是数字，则Oracle数据库确定哪个参数具有最高的数字优先级，将另一个参数隐式转换为该数据类型，并返回该数据类型。
用途：
最主要的是格式化数据，比如涉及到数字的，不想出现空数据时，可用nvl(num,0)来得到0。由于 null+[或-,*,/]数字 等于null，所以在表达式中对可能为空的值要使用nvl。
NVL2:
语法： NVL2(expr1,expr2,expr3)
功能：
NVL2 lets you determine the value returned by a query based on whether a specified expression is null or not null. If expr1 is not null, then NVL2 returns expr2. If expr1 is null, then NVL2 returns expr3.
说明：
1.The argument expr1 can have any data type. The arguments expr2 and expr3 can have any data types except LONG.
参数expr1可以是任何数据类型，参数expr2和expr3可以是除了LONG类型外的任何数据类型。
2.If the data types of expr2 and expr3 are different, then Oracle Database implicitly converts one to the other.
如果参数expr2和expr3是不同的数据类型，数据库将会隐式的将其中一个转换为另一个。
3.If they cannot be converted implicitly, then the database returns an error.
如果俩参数不能进行隐式转换，数据库将返回一个error。
4.If expr2 is character or numeric data, then the implicit conversion is implemented as follows:
(如果expr2是字符或是数字数据，则隐式转换如下实现)
1>.If expr2 is character data, then Oracle Database converts expr3 to the data type of expr2 before returning a value unless expr3 is a null constant. In that case, a data type conversion is not necessary, and the database returns VARCHAR2 in the character set of expr2.
如果expr2是字符数据，数据库会将expr3(null除外)转换为expr2的数据类型。如果expr3为空常量，则不需要进行数据类型转换，数据库会返回expr2的字符集中的VARCHAR2。
2>.If expr2 is numeric data, then Oracle Database determines which argument has the highest numeric precedence, implicitly converts the other argument to that data type, and returns that data type.
如果expr2是数字数据，那么Oracle数据库将确定哪个参数具有最高的数字优先级，将另一个参数隐式转换为该数据类型，并返回该数据类型。
NULLIF:
语法： NULLIF(expr1,expr2)
功能： NULLIF compares expr1 and expr2. If they are equal, then the function returns null. If they are not equal, then the function returns expr1. You cannot specify the literal NULL for expr1.
说明：
If both arguments are numeric data types, then Oracle Database determines the argument with the higher numeric precedence, implicitly converts the other argument to that data type, and returns that data type. If the arguments are not numeric, then they must be of the same data type, or Oracle returns an error.
如果俩参数是数字数据类型，则Oracle数据库将确定具有较高数字优先级的参数，将另一个参数隐式转换为该数据类型，并返回该数据类型。 如果参数不是数字，则它们必须是相同的数据类型，否则Oracle返回一个error。
The NULLIF function is logically equivalent to the following CASE expression:
CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END
LNNVL:
语法： LNNVL(condition)
功能： LNNVL provides a concise way to evaluate a condition when one or both operands of the condition may be null.
The function can be used in the WHERE clause of a query, or as the WHEN condition in a searched CASE expression.
It takes as an argument a condition and returns TRUE if the condition is FALSE or UNKNOWN and FALSE if the condition is TRUE.
LNNVL can be used anywhere a scalar expression can appear, even in contexts where the IS [NOT] NULL, AND, or OR conditions are not valid but would otherwise be required to account for potential nulls.
Oracle Database sometimes uses the LNNVL function internally in this way to rewrite NOT IN conditions as NOT EXISTS conditions.
In such cases, output from EXPLAIN PLAN shows this operation in the plan table output.
The condition can evaluate any scalar values but cannot be a compound condition containing AND, OR, or BETWEEN.
NANVL:
语法： NANVL(n2,n1)
功能：
The NANVL function is useful only for floating-point numbers of type BINARY_FLOAT or BINARY_DOUBLE.
It instructs Oracle Database to return an alternative value n1 if the input value n2 is NaN (not a number). If n2 is not NaN, then Oracle returns n2.
This function takes as arguments any numeric data type or any nonnumeric data type that can be implicitly converted to a numeric data type.
Oracle determines the argument with the highest numeric precedence, implicitly converts the remaining arguments to that data type, and returns that data type.