可以存储一个选择列的值并将其用于下一个吗？

Question

是否可以存储或缓存属于一个选择列的一部分的值，然后用于下一个？例如，

select 
       FirstColumn = (complex query returns a value based on ThirdColumn),
       SecondColumn = (uses the same value returned from above + does some additional calculations.)
from SomeTable

是否可以这样做，这样我就不必编写两次相同的复杂查询？

Answer 1

我认为在这种情况下，您应该使用 CTE（公用表表达式）。例如

;WITH CTE1 AS
(
 select col1 from table ---- your query 
),
CTE2 as 
(
 select col2 from table --- your query
)
select col1, Col2
from CTE1 join CTE2 on --- do anything you want

Answer 2

子查询是最简单的形式。您可以将子查询嵌套到您喜欢执行计算的任何级别：

select 
    ComplexValue1 as FirstColumn
    , ComplexValue1 + ComplexValue2 as SecondColumn
from (
  select 
    {complex query returns a value} ComplexValue1
    , {complex query returns a value} ComplexValue2
    , {any other required columns}
  from SomeTable
) X;

注意：CTE 本质上是一样的，只是更漂亮（也许吧）。

横向连接是另一种选择，因为它允许您引用基表中的列并对其执行计算，然后可在结果集中使用：

select
    X.ComplexValue1 as FirstColumn
    , X.ComplexValue1 + ST.ComplexValue2 as SecondColumn
from SomeTable ST
cross apply (select {complex query returns a value referencing ST}) as X (ComplexValue1);

Answer 3

一种方法是选择结果集并从那里开始工作。

这通过选择第一列及其所有复杂内容并将其返回到名为t 的结果集中。

select t.FirstColumn,
from   ( select FirstColumn --(complex query returns a value based on ThirdColumn)
         from SomeTable
       ) t

然后您可以非常简单地从t 中进行选择，它会返回FirstColumn，就像它是一个普通的简单列一样，因此您可以添加到它而无需重复复杂的内容。

select t.FirstColumn,
       secondcolum (that uses t.FirstColum as it where a normal field)
from   ( select FirstColumn
         from SomeTable
       ) t

一个例子：

select t.FirstColumn,
       (t.FirstColum / 123.0) * 50 as SecondColumn
from   ( select (s.A + s.B) * s.C as FirstColumn
         from   sometable s
       ) t

如您所见，我不需要重复计算 t.Firstcolumn 来为我的 SecondColumn 添加东西。

另一种选择是使用cross apply（这里的另一个答案中有解释，所以我不会重复）。
但是请测试哪种解​​决方案最适合您，交叉应用在某些情况下可能会降低性能，我的解决方案也是如此。因此，只需测试最适合您的方法。

Answer 4

这里需要CROSS APPLY，它可以引用外部引用，不需要烦人的子查询或CTE：

select col1, col2
from table1 as outer_table

-- can also have multi-row values
cross apply (values (complex_expression_1) ) as v1 (col1)
cross apply (values (expression_referring_to_col1) ) as v2 (col2)

-- alternate syntax, select without from returns a single row
cross apply (select complex_expression_1 as col1 ) AS v1
cross apply (select expression_referring_to_col1 as col2 ) as v2

-- you can also do anything you like in there, can be one or multiple rows
cross apply (
    select complex_expression_1 as col1 
    from othercomplexjoin as o
    where o.join_column = outer_table.join_column
) AS v1

你可以用APPLY做更多的技巧：

1.每组子表前 1 名：

“每组前 1 名”的经典解决方案是使用 row_number()。这通常会导致大量扫描，尤其是当不同外部值的数量相对于子表而言较少时。

select
    o.id,
    lastPayment.Date
from order_header as o
join
( select *, row_number() over (partition by order_id order by date desc) as rn
 from payments
) as lastPayment on ...
where lastPayment.rn = 1

我们可以这样做：

select
    o.id,
    lastPayment.Date
from order_header as o
cross apply
( select top (1) *
 from payments as p
 where p.order_id = o.id
 order by date desc
) as lastPayment

注意：OUTER APPLY 在概念上替换左连接，即返回空值而不是无行。

---

2。不旋转

select
    o.id,
    customer.*
from order_header as o
cross apply ( values    -- This returns two rows for every order_header
    ( 'DeliveryCustomer', o.deliveryCustomer ),
    ( 'billingCustomer', o.billingCustomer )
) as customer (type, name)

---

3.以可变次数分解一行：

假设我们要取一个金额，并将其分成不同的行。如果是amount <= 50，则为一排amount，如果为> 50，则为两排，50 之一，其余各之一：

select t.id, v.amount
from table as t
cross apply (
    select case when amount > 50 then 50 else amount end as amount
    union all
    select amount - 50   -- note this row will not appear if amount < 50
    where amount > 50
) v