trying to get the number of months

Question

membership table

• membership_startdate (2011-01-12)
• membership_dueday values like only dates (09,08,07)
• member_id

Answer 1

The 1054 error is because the column does not exist in the table(s) defined in the FROM clause. Additionally, the WHERE clause is not used to set a variable, or column alias -- it's for filtering rows returned.

Use DATEDIFF:

SELECT t.member_id,
       DATEDIFF(LEAST(NOW(), t.membership_dueday), t.membership_startdate) / 30
  FROM MEMBERSHIP t

The LEAST function will return the lowest of the two dates, so it will use the current date if the due date is in the future. You can use the GREATEST function if you want that reversed.