Sqlite self join using alias - sql

Question

I have such sql query:
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
it works ok, but I have two similar sub-queries.
Also I tried something like this:
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
But it doesn't work.
So is it possible to get rid of sub-query duplication?

Answer 1

In SQLite 3.8.3 or later, you could use a common table expression:
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
Alternatively, use a temporary view.