Question

我有这样的SQL查询：

SELECT LeftCurrency.LeftCurrency, RightCurrency.RightCurrency FROM 
(
  SELECT DISTINCT [SecurityData].[Value] AS 'LeftCurrency'
  FROM [SecurityData]
  JOIN [Fields] ON [Fields].[Id] = [SecurityData].[FieldId]
  WHERE [Fields].[Mnemonic] = 'CRNCY'
) AS LeftCurrency
JOIN 
(
  SELECT DISTINCT [SecurityData].[Value] AS 'RightCurrency'
  FROM [SecurityData]
  JOIN [Fields] ON [Fields].[Id] = [SecurityData].[FieldId]
  WHERE [Fields].[Mnemonic] = 'CRNCY'
) AS RightCurrency
ON LeftCurrency.LeftCurrency != RightCurrency.RightCurrency

它运作正常，但我有两个类似的子查询。

我也试过这样的事情：

SELECT * FROM 
(
  SELECT DISTINCT [SecurityData].[Value] AS 'Currency'
  FROM [SecurityData]
  JOIN [Fields] ON [Fields].[Id] = [SecurityData].[FieldId]
  WHERE [Fields].[Mnemonic] = 'CRNCY'
) AS leftCurrency, leftCurrency AS rightCurrency
WHERE leftCurrency.Currency != rightCurrency.Currency

但它不起作用。那么有可能摆脱子查询重复吗？

Answer 1

在SQLite 3.8.3或更高版本中，您可以使用公用表表达式：

WITH Currency(Currency) AS (
    SELECT DISTINCT [SecurityData].[Value]
    FROM [SecurityData]
    JOIN [Fields] ON [Fields].[Id] = [SecurityData].[FieldId]
    WHERE [Fields].[Mnemonic] = 'CRNCY'
)
SELECT LeftCurrency.Currency AS LeftCurrency,
       RightCurrency.Currency AS RightCurrency
FROM Currency AS LeftCurrency
JOIN Currency AS RightCurrency ON LeftCurrency.Currency != RightCurrency.Currency

或者，使用临时视图。

使用别名的Sqlite自联接

1 个答案: