SQL-对另一个查询的结果执行查询？

Question

I have a column(varchar) with date values, I need to find those dates which are expiring in next 30 days.

ExpiringDate
===================
20171208,
20171215,samples
20171130,tested
N/A
No
(empty row)

So, First I need to get values before comma. On the resultset, I need to filter out rows that has only numbers(no 'N/A' or 'No' or empty rows) & then I need to filter those dates which are expiring in next 30 days.

Edited

I have tried the following & resultset seems to be inappropriate

SELECT
  DocName,
  CategoryName,
  AttributeName,
  CAST(SUBSTRING_INDEX(AttributeValue, ',', 1) AS DATE) AS ExpiredDate
FROM myDB
WHERE (AttributeName = 'Date of last vessel OVID' OR AttributeName = 'Next Statutory docking' OR
       AttributeName = 'Last statutory docking') AND AttributeValue LIKE '%[^0-9]%' AND
      DATEDIFF(now(), AttributeValue) <= 30;

Answer 1

Because you are not only storing dates as text, but mixing those dates with entirely non date information, this complicates things. In this case, we can do two checks, one to ensure that the record starts with an actual expected date, and the second to make sure that the date diff is within 30 days from now.

SELECT ExpiringDate
FROM
(
    SELECT ExpiringDate
    FROM yourTable
    WHERE ExpiringDate REGEXP '^[0-9]{8}'
) t
WHERE
    DATEDIFF(LEFT(ExpiringDate, 8), NOW()) BETWEEN 0 AND 30;

Note that I use a subquery to first remove rows that do not even have a parseable date. The reason for this is that DATEDIFF will error out if not passed valid dates for both parameters.

Demo