SELECT 语句的 SQL 别名

Question

I would like to do something like

(SELECT ... FROM ...) AS my_select
WHERE id IN (SELECT MAX(id) FROM my_select GROUP BY name)

Is it possible to somehow do the "AS my_select" part (ie assign an alias to a SELECT statement)?

(Note: This is a theoretical question. I realize that I can do it without assign an alias to a SELECT statement, but I would like to know whether I can do it with that.)

Answer 1

Not sure exactly what you try to denote with that syntax, but in almost all RDBMS-es you can use a subquery in the FROM clause (sometimes called an "inline-view"):

SELECT..
FROM (
     SELECT ...
     FROM ...
     ) my_select
WHERE ...

In advanced "enterprise" RDBMS-es (like oracle, SQL Server, postgresql) you can use common table expressions which allows you to refer to a query by name and reuse it even multiple times:

-- Define the CTE expression name and column list.
WITH Sales_CTE (SalesPersonID, SalesOrderID, SalesYear)
AS
-- Define the CTE query.
(
    SELECT SalesPersonID, SalesOrderID, YEAR(OrderDate) AS SalesYear
    FROM Sales.SalesOrderHeader
    WHERE SalesPersonID IS NOT NULL
)
-- Define the outer query referencing the CTE name.
SELECT SalesPersonID, COUNT(SalesOrderID) AS TotalSales, SalesYear
FROM Sales_CTE
GROUP BY SalesYear, SalesPersonID
ORDER BY SalesPersonID, SalesYear;

(example from http://msdn.microsoft.com/en-us/library/ms190766(v=sql.105).aspx )

Answer 2

You can do this using the WITH clause of the SELECT statement:

;
WITH my_select As (SELECT ... FROM ...) 
SELECT * FROM foo
WHERE id IN (SELECT MAX(id) FROM my_select GROUP BY name)

That's the ANSI/ISO SQL Syntax. I know that SQL Server, Oracle and DB2 support it. Not sure about the others...

Answer 3

Yes, but you can select only one column in your subselect

SELECT (SELECT id FROM bla) AS my_select FROM bla2

Answer 4

You could store this into a temporary table.

So instead of doing the CTE/sub query you would use a temp table.

Good article on these here http://codingsight.com/introduction-to-temporary-tables-in-sql-server/