MSSQL 自定义函数详解

自定义函数分为：标量值函数或表值函数

如果 RETURNS 子句指定一种标量数据类型，则函数为标量值函数。可以使用多条 Transact-SQL 语句定义标量值函数。

如果 RETURNS 子句指定 TABLE，则函数为表值函数。

表值函数又可分为：内嵌表值函数（行内函数）或多语句函数

如果 RETURNS 子句指定的 TABLE 不附带列的列表，则该函数为内嵌表值函数。

如果 RETURNS 子句指定的 TABLE 类型带有列及其数据类型，则该函数是多语句表值函数。

标量值函数示例

CREATE FUNCTION dbo.Foo
(
)
RETURNS INT
AS
BEGIN
	DECLARE @n INT
	SELECT @n = 3
	RETURN @n
END 

内嵌表值函数示例

CREATE FUNCTION dbo.Foo
(
)
RETURNS TABLE
AS
	RETURN
	SELECT id,
	       title
	FROM   msgs 

多语句表值函数示例（部分）

CREATE FUNCTION dbo.fn_FindReports (@InEmpID INTEGER)
RETURNS @retFindReports TABLE
(
    EmployeeID int primary key NOT NULL,
    Name nvarchar(255) NOT NULL,
    Title nvarchar(50) NOT NULL,
    EmployeeLevel int NOT NULL,
    Sort nvarchar (255) NOT NULL
)
--Returns a result set that lists all the employees who report to the
--specific employee directly or indirectly.*/
AS
BEGIN
   WITH DirectReports(Name, Title, EmployeeID, EmployeeLevel, Sort) AS
    (SELECT CONVERT(Varchar(255), c.FirstName + ' ' + c.LastName),
        e.Title,
        e.EmployeeID,
        1,
        CONVERT(Varchar(255), c.FirstName + ' ' + c.LastName)
     FROM HumanResources.Employee AS e
          JOIN Person.Contact AS c ON e.ContactID = c.ContactID
     WHERE e.EmployeeID = @InEmpID
   UNION ALL
     SELECT CONVERT(Varchar(255), REPLICATE ('| ' , EmployeeLevel) +
        c.FirstName + ' ' + c.LastName),
        e.Title,
        e.EmployeeID,
        EmployeeLevel + 1,
        CONVERT (Varchar(255), RTRIM(Sort) + '| ' + FirstName + ' ' +
                 LastName)
     FROM HumanResources.Employee as e
          JOIN Person.Contact AS c ON e.ContactID = c.ContactID
          JOIN DirectReports AS d ON e.ManagerID = d.EmployeeID
    )
-- copy the required columns to the result of the function
   INSERT @retFindReports
   SELECT EmployeeID, Name, Title, EmployeeLevel, Sort
   FROM DirectReports
   RETURN
END;
GO
在以下示例中，调用了此函数。
-- Example invocation
SELECT EmployeeID, Name, Title, EmployeeLevel
FROM dbo.fn_FindReports(109)
ORDER BY Sort;

注意其 RETURNS 部分。

多语句函数的主体中允许使用以下语句。未在下面的列表中列出的语句不能用在函数主体中。

赋值语句。

控制流语句。

DECLARE 语句，该语句定义函数局部的数据变量和游标。

SELECT 语句，该语句包含带有表达式的选择列表，其中的表达式将值赋予函数的局部变量。

游标操作，该操作引用在函数中声明、打开、关闭和释放的局部游标。只允许使用以 INTO 子句向局部变量赋值的 FETCH 语句；不允许使用将数据返回到客户端的 FETCH 语句。

INSERT、UPDATE 和 DELETE 语句，这些语句修改函数的局部 table 变量。

EXECUTE 语句调用扩展存储过程。

ALTER FUNCTION [dbo].[AmountRole1]
(
    @Amount numeric(18,2)
)
RETURNS numeric(18,2)
AS
BEGIN
    DECLARE @i FLOAT
    IF @Amount >= 2000
        SET @i = @Amount * 0.98 --满2000打9.8折
    ELSE
        SET @i = @Amount
    RETURN (@i)
END 

自定义函数语法

Scalar Functions
CREATE FUNCTION [ schema_name. ] function_name
( [ { @parameter_name [ AS ][ type_schema_name. ] parameter_data_type
    [ = default ] }
    [ ,...n ]
  ]
)
RETURNS return_data_type
    [ WITH <function_option> [ ,...n ] ]
    [ AS ]
    BEGIN
                function_body
        RETURN scalar_expression
    END
[ ; ]

Inline Table-valued Functions
CREATE FUNCTION [ schema_name. ] function_name
( [ { @parameter_name [ AS ] [ type_schema_name. ] parameter_data_type
    [ = default ] }
    [ ,...n ]
  ]
)
RETURNS TABLE
    [ WITH <function_option> [ ,...n ] ]
    [ AS ]
    RETURN [ ( ] select_stmt [ ) ]
[ ; ]

Multistatement Table-valued Functions
CREATE FUNCTION [ schema_name. ] function_name
( [ { @parameter_name [ AS ] [ type_schema_name. ] parameter_data_type
    [ = default ] }
    [ ,...n ]
  ]
)
RETURNS @return_variable TABLE < table_type_definition >
    [ WITH <function_option> [ ,...n ] ]
    [ AS ]
    BEGIN
                function_body
        RETURN
    END
[ ; ]

CLR Functions
CREATE FUNCTION [ schema_name. ] function_name
( { @parameter_name [AS] [ type_schema_name. ] parameter_data_type
        [ = default ] }
    [ ,...n ]
)
RETURNS { return_data_type | TABLE <clr_table_type_definition> }
    [ WITH <clr_function_option> [ ,...n ] ]
    [ AS ] EXTERNAL NAME <method_specifier>
[ ; ]

Method Specifier
<method_specifier>::=
    assembly_name.class_name.method_name

Function Options
<function_option>::=
{
    [ ENCRYPTION ]
  | [ SCHEMABINDING ]
  | [ RETURNS NULL ON NULL INPUT | CALLED ON NULL INPUT ]
  | [ EXECUTE_AS_Clause ]
}

<clr_function_option>::=
}
    [ RETURNS NULL ON NULL INPUT | CALLED ON NULL INPUT ]
  | [ EXECUTE_AS_Clause ]
}

Table Type Definitions
<table_type_definition>:: =
( { <column_definition> <column_constraint>
  | <computed_column_definition> }
        [ <table_constraint> ] [ ,...n ]
) 

<clr_table_type_definition>::=
( { column_name data_type } [ ,...n ] )

<column_definition>::=
{
    { column_name data_type }
    [ [ DEFAULT constant_expression ]
      [ COLLATE collation_name ] | [ ROWGUIDCOL ]
    ]
    | [ IDENTITY [ (seed , increment ) ] ]
    [ <column_constraint> [ ...n ] ]
}
<column_constraint>::=
{
    [ NULL | NOT NULL ]
    { PRIMARY KEY | UNIQUE }
      [ CLUSTERED | NONCLUSTERED ]
        [ WITH FILLFACTOR = fillfactor
        | WITH ( < index_option > [ , ...n ] )
      [ ON { filegroup | "default" } ]
  | [ CHECK ( logical_expression ) ] [ ,...n ]
}

<computed_column_definition>::=
column_name AS computed_column_expression 

<table_constraint>::=
{
    { PRIMARY KEY | UNIQUE }
      [ CLUSTERED | NONCLUSTERED ]
            ( column_name [ ASC | DESC ] [ ,...n ] )
        [ WITH FILLFACTOR = fillfactor
        | WITH ( <index_option> [ , ...n ] )
  | [ CHECK ( logical_expression ) ] [ ,...n ]
}

<index_option>::=
{
    PAD_INDEX = { ON | OFF }   | FILLFACTOR = fillfactor
  | IGNORE_DUP_KEY = { ON | OFF }
  | STATISTICS_NORECOMPUTE = { ON | OFF }
  | ALLOW_ROW_LOCKS = { ON | OFF }
  | ALLOW_PAGE_LOCKS ={ ON | OFF }
}