只有函数和某些扩展存储过程才能从函数内部执行

The types of statements that are valid in a function include:

DECLARE statements can be used to define data variables and cursors that are local to the function.

Assignments of values to objects local to the function, such as using SET to assign values to scalar and table local variables.

Cursor operations that reference local cursors that are declared, opened, closed, and deallocated in the function. FETCH statements that return data to the client are not allowed. Only FETCH statements that assign values to local variables using the INTO clause are allowed.

Control-of-flow statements.

SELECT statements containing select lists with expressions that assign values to variables that are local to the function.

UPDATE, INSERT, and DELETE statements modifying table variables that are local to the function.

EXECUTE statements calling an extended stored procedure.
The number of times that a function specified in a query is actually executed can vary between execution plans built by the optimizer. An example is a function invoked by a subquery in a WHERE clause. The number of times the subquery and its function is executed can vary with different access paths chosen by the optimizer.

Built-in functions that can return different data on each call are not allowed in user-defined functions. The built-in functions not allowed in user-defined functions are:

@@CONNECTIONS @@PACK_SENT GETDATE
@@CPU_BUSY @@PACKET_ERRORS GetUTCDate
@@IDLE @@TIMETICKS NEWID
@@IO_BUSY @@TOTAL_ERRORS RAND
@@MAX_CONNECTIONS @@TOTAL_READ TEXTPTR
@@PACK_RECEIVED @@TOTAL_WRITE