标量函数和多语句表值用户定义函数会导致性能问题，因为它们隐式地将基于集合的操作转换为基于游标的操作.

Scalar functions and multi statement table valued user defined functions can cause performance issues, because they implicitly turn your set based operation into a cursor based operation.

然而，内联表值的用户定义函数不会遇到这个问题.他们很快.

However, inline table valued user defined functions do not suffer from this problem. They're fast.

区别在于您如何声明函数，以及它们内部的代码是什么样的.多语句函数执行它在罐头上所说的 - 它允许您有多个语句.像这样:

The difference is how you declare the fuction, and what the code looks like inside them. A multi statement function does what it says on the tin - it lets you have multiple statements. Like this:

create function slow() returns @t table(j int, k int) as 
begin
    declare @j int = 1; -- statement 1
    declare @k int = 2; -- statement 2
    insert @t values (@j, @k); -- statement 3
    return; -- statement 4
end

内联表值函数不会返回填充在函数内部的命名表.它返回一个选择语句:

An inline table valued function does not return a named table which is populated inside the function. It returns a select statement:

create function quick() returns table as
return
(
    select j = 1, k = 2
);

内联表值函数可以内联"；进入外部 select 语句，与视图的方式大致相同.当然，区别在于 UDF 可以接受参数，而视图不能.

The inline table valued function can be "inlined" into the outer select statement, in much the same way as a view. The difference, of course, being that the UDF can take parameters, whereas a view cannot.

您还必须以不同的方式使用它们.使用交叉应用:

You also have to use them differently. Use cross apply:

select       t.columnA, t.columnB, u.j, u.k
from         MyTable t
cross apply  quick(t.columnA, t.columnB) u

如果不清楚 - 是的，在您的情况下，您只需要一个标量"返回值，但这只是一个表值函数，它返回单列和单行.因此，与其编写标量函数，不如编写一个执行相同工作的内联表值函数，然后交叉应用它.

In case it's not clear - yes, in your case you only want a "scalar" value back, but that's just a table valued function which returns a single column and a single row. So instead of writing a scalar function, write an inline table valued function that does the same job, and cross apply it.