官方解释：

Purpose

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. Ifexpr1 is not null, then NVL returns expr1.

 

如果expr1是null，则返回expr2，如果expr1 is not null，则返回expr1.

The arguments expr1 and expr2 can have any data type. If their data types are different, then Oracle Database implicitly converts one to the other.

If they cannot be converted implicitly, then the database returns an error.

 

如果他们不是同一类型，则报错。

The implicit conversion is implemented as follows:

官方参考：http://docs.oracle.com/cd/E11882_01/server.112/e41084/functions119.htm#sthref1312

 

 

 

Purpose

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.

 

如果expr1非空，则返回 expr2，如果expr1是空值，则返回expr3

The argument expr1 can have any data type. The arguments expr2 and expr3 can have any data types except LONG.

 

If the data types of expr2 and expr3 are different, then Oracle Database implicitly converts one to the other.

 

如果expr2 and expr3 数据类型不同，则隐式转为相同

 

 If they cannot be converted implicitly, then the database returns an error.

如果不能隐式转换，则报错。

If expr2 is character or numeric data, then the implicit conversion is implemented as follows:

官方参考：http://docs.oracle.com/cd/E11882_01/server.112/e41084/functions120.htm#sthref1315

 

 

 

Purpose

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.

 

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.

The NULLIF function is logically equivalent to the following CASE expression:

 

CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END

官网参考：http://docs.oracle.com/cd/E11882_01/server.112/e41084/functions116.htm#sthref1303

 

 

 

Purpose

COALESCE (expression_1, expression_2, ...,expression_n)

 

COALESCE returns the first non-null expr in the expression list. You must specify at least two expressions. If all occurrences of expr evaluate to null, then the function returns null.

 

Oracle Database uses short-circuit evaluation. The database evaluates each expr value and determines whether it is NULL, rather than evaluating all of the expr values before determining whether any of them is NULL.

 

If all occurrences of expr are numeric data type or any nonnumeric data type that can be implicitly converted to a numeric data type, then Oracle Database determines the argument with the highest numeric precedence, implicitly converts the remaining arguments to that data type, and returns that data type.

This function is a generalization of the NVL function.

 

You can also use COALESCE as a variety of the CASE expression. For example,

 

COALESCE(expr1, expr2)

is equivalent to:

CASE WHEN expr1 IS NOT NULL THEN expr1 ELSE expr2 END

Similarly,

COALESCE(expr1, expr2, ..., exprn)

where n >= 3, is equivalent to:

 

CASE WHEN expr1 IS NOT NULL THEN expr1

   ELSE COALESCE (expr2, ..., exprn) END

官方参考：http://docs.oracle.com/cd/E11882_01/server.112/e41084/functions030.htm#sthref995

 

 

 

 

DECODE(expr,search1,result1[,search2,result2……,default])：

比较expr与search，如果等于search1则返回result1，如果等于search2则返回result2，依次类推，如果都不等于，如果有default则返回default，否则返回NULL.

 

ORACLE在比较之前，会自动把expr和每一个search隐式转换成第一个search（search1）的数据类型。自动把返回值转换成第一个result（result1）的数据类型。如果第一个result的数据类型为CHAR或者值是null，则Oracle转换返回值为VARCHAR2.

在DECODE函数中,NULL是相等的，如果expr为空，则Oracle将会返回第一个为NULL的search所对应的result。DECODE列表中的最大表达式个数为255个。

 

expr与search1同类型，searchn与searche1同类型

返回值与result1同类型