MySQL：如何将别名委托给子查询的子查询？

Question

How can I delegate my DV_ID or DV.ID into the second (deeper) subquery? Is it possible? (If I'd have just one level of nesting it would work.)

SELECT DV.ID DV_ID, (
SELECT COUNT(*) FROM (SELECT * MYTABLE VSV
WHERE VSV.ID = DV_ID //DV_ID is not seen...
GROUP BY VSV.ID) A ) B
FROM MYTABLE2 DV

Answer 1

When you use an alias in SELECT field list is not seen in the subquery. You can use DV.ID (DV is alias of your table).

In your example:

SELECT DV.ID DV_ID

you can replace your output alias with a string, like this:

SELECT DV.ID "Hi my dear"

but is not correct read that information inside other part of query

Answer 2

You can rewrite your particular query as:

SELECT DV.ID as DV_ID,
       (SELECT COUNT(DISTINCT VSV.ID)
        FROM MYTABLE VSV
        WHERE VSV.ID = DV.ID //DV_ID is not seen...
       ) as B
FROM MYTABLE2 DV;

(NOTE: If VSV.ID can take on NULL values, the expression would be SELECT COUNT(DISTINCT vsv.ID) + MAX(vsv.id is null) ).