sql - 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

在选定列的列表中设置为变量的计算值 BalanceDue 不能在 WHERE 子句中使用。

有什么办法可以实现吗？在这个相关问题( Using a variable in MySQL Select Statment in a Where Clause )中，答案似乎是，实际上，不，您只需写出计算(并在查询中执行该计算)两次，其中没有一个很满意。

Answer 1

除了 ORDER BY 之外，您不能引用别名，因为 SELECT 是计算的倒数第二个子句。两种解决方法:

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。如果表达式极其复杂(或计算成本高昂)，您可能应该考虑使用计算列(并且可能是持久化的)，特别是当大量查询引用同一表达式时。

PS你的担心似乎没有根据。至少在这个简单的例子中，SQL Server 足够聪明，只执行一次计算，即使您已经引用了两次。继续比较计划；你会发现它们是相同的。如果您遇到更复杂的情况，即多次计算表达式，请发布更复杂的查询和计划。

下面是 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: