WHERE子句中的引用别名（在SELECT中计算）

Question

SELECT (InvoiceTotal - PaymentTotal - CreditTotal) AS BalanceDue
FROM Invoices
WHERE BalanceDue > 0 --error

在WHERE子句中不能使用在所选列列表中设置为变量的计算值“BalanceDue”。

有没有办法可以？在这个相关的问题（Using a variable in MySQL Select Statment in a Where Clause）中，似乎答案是，实际上，不，你只会写出计算（和在查询中执行该计算）两次，没有其中令人满意。

Answer 1

除了ORDER BY之外，您不能引用别名，因为SELECT是已计算的第二个最后一个子句。两个解决方法：

SELECT BalanceDue FROM (
  SELECT (InvoiceTotal - PaymentTotal - CreditTotal) AS BalanceDue
  FROM Invoices
) AS x
WHERE BalanceDue > 0;

或者只是重复一下表达式：

SELECT (InvoiceTotal - PaymentTotal - CreditTotal) AS BalanceDue
FROM Invoices
WHERE  (InvoiceTotal - PaymentTotal - CreditTotal)  > 0;

我更喜欢后者。如果表达式非常复杂（或计算成本很高），您应该考虑使用计算列（也许可以保留），尤其是在大量查询引用同一表达式的情况下。

PS你的恐惧似乎没有根据。至少在这个简单的例子中，SQL Server足够聪明，只能执行一次计算，即使你已经引用了两次。继续比较计划;你会发现它们是完全相同的。如果你有一个更复杂的情况，你看到多次评估表达式，请发布更复杂的查询和计划。

以下是5个示例查询，它们都产生完全相同的执行计划：

SELECT LEN(name) + column_id AS x
FROM sys.all_columns
WHERE LEN(name) + column_id > 30;

SELECT x FROM (
SELECT LEN(name) + column_id AS x
FROM sys.all_columns
) AS x
WHERE x > 30;

SELECT LEN(name) + column_id AS x
FROM sys.all_columns
WHERE column_id + LEN(name) > 30;

SELECT name, column_id, x FROM (
SELECT name, column_id, LEN(name) + column_id AS x
FROM sys.all_columns
) AS x
WHERE x > 30;

SELECT name, column_id, x FROM (
SELECT name, column_id, LEN(name) + column_id AS x
FROM sys.all_columns
) AS x
WHERE LEN(name) + column_id > 30;

所有五个查询的结果计划：

Answer 2

您可以使用cross apply

执行此操作

SELECT c.BalanceDue AS BalanceDue
FROM Invoices
cross apply (select (InvoiceTotal - PaymentTotal - CreditTotal) as BalanceDue) as c
WHERE  c.BalanceDue  > 0;

Answer 3

实际上可以有效地定义一个可以在SELECT，WHERE和其他子句中使用的变量。

交叉连接并不一定允许对引用的表列进行适当的绑定，但是OUTER APPLY可以 - 并且更透明地处理空值。

SELECT
    vars.BalanceDue
FROM
    Entity e
OUTER APPLY (
    SELECT
        -- variables   
        BalanceDue = e.EntityTypeId,
        Variable2 = ...some..long..complex..expression..etc...
    ) vars
WHERE
    vars.BalanceDue > 0

感谢Syed Mehroz Alam。

Answer 4

作为强制在WHERE子句之前执行SELECT子句的评估的解决方法，您可以将前者放在子查询中，而后者保留 在主要查询中：

SELECT * FROM (
  SELECT (InvoiceTotal - PaymentTotal - CreditTotal) AS BalanceDue
  FROM Invoices) AS temp
WHERE BalanceDue > 0