Question

In MySQL database I have table item with some columns describing items, and another table itemExtendedFields. The second one has columns: id, itemId, name, value. In the itemExtendedFields additional information about items are stored.

Some items have expiry date that can be constructed from two values kept in two rows of itemExtendedFields table.
For example for an item with itemId = 34 i have following two rows in itemExtendedFields:

id | itemId | name | value

--------------------------

87 | 34 | ExpiryYear | 2014

88 | 34 | ExpiryMonth | 2


I need to write query that selects all items from items along with expiryYear and ExpiryMonth values from itemExtendedFields table in separate columns. Moreover i need to select items that will expire on given date (for example February 2014). I have no idea how to construct proper query. I cannot change the construction of the tables.

Answer 1

this will list all columns from items table along with the ExpiryYear and ExpiryMonth from the itemExtendedFields table.

SELECT  a.*, b.ExpiryYear, b.ExpiryMonth
FROM    items a
        INNER JOIN 
        (
            SELECT  itemID,
                    MAX(CASE WHEN name = 'ExpiryYear' THEN `value` ELSE NULL END) ExpiryYear,
                    MAX(CASE WHEN name = 'ExpiryMonth' THEN `value` ELSE NULL END) ExpiryMonth 
            FROM    itemExtendedFields
            GROUP BY itemID
        ) b ON a.itemID = b.itemID
WHERE   b.ExpiryYear = 2012 AND 
        b.ExpiryMonth = 2