SQL Select语句中的函数

Question

CREATE VIEW View1
  AS
  SELECT Col1,
         Col2,
         dbo.fn1(col2) as Col3
  FROM   TestTable

  /*
  --Instead of writing below query I created a new View View2


  SELECT Col1,
         Col2,
         dbo.fn1(col2) as Col3
         dbo.fn2(dbo.fn1(col2)) as Col4
  FROM   TestTable
  */

  CREATE VIEW View2
  AS
  SELECT Col1,
         Col2,
         Col3,
         dbo.fn3(col3) as Col4
  FROM   TestTable

我有一个像上面这样的查询。我的列是fn1的输出。我想将该输出用于其他函数fn2。在那种情况下我不能直接在fn2中使用col3，所以我拆分了两个函数并查看如下。我想知道我是朝着正确的方向前进，我正在做什么或者有更好的方法吗？在View1的注释部分，函数fn1会调用两次吗？或者SQL Server会处理优化吗？

Answer 1

你可以这样做......

SELECT
    Col1,
    Col2,
    col3,
    dbo.fn2(col3) AS Col4
FROM
    (
    SELECT
        Col1,
        Col2,
        dbo.fn1(col2) as Col3
    FROM
        TestTable
    ) foo