Query to return values from table B when newer than record in Table A

Question

I have a table with a member's name, address, etc. and a time stamp of the last time the record was updated. I have a second table that holds updates to the member record, a holding table, until changes are approved by staff.

I have a query that returns data from the member table. I now need to check the updates table, and if the member's record in the updates table has a more recent time stamp, return that record instead of the record in the member table.

I tried a few things such as a UNION with Top 1 but it's not quite right. I could make a complex CASE statement but is that going to perform well?

It sounds simple, get the most recent record from table A, and the most recent from table B and return the one record that is the newest.

SELECT name, address, city, state, zipcode, time_stamp
FROM Member
WHERE ID = 123

SELECT name, address, city, state, zipcode, time_stamp
FROM MemberUpdates
WHERE ID = 123

EDIT:

OK, with the help so far, I was able to get the results I expected. Then, I went to add the extra where clauses and I broke it. Tried several different ways including using a CTE and could not quite get it right. Here is a query that works and returns the expected results, however notice I have to pass name_last/birth_year/memNum twice. Is there a better way?

SELECT TOP 1 m.abn,
            m.aliases,
            m.birth_year,
            m.user_stamp, 

            q.updatePending,
            q.name_first,
            q.name_last,
            q.company,
            q.address1,
            q.mailing_address,
            q.city,
            q.state,
            q.zipcode,
            q.email_address

FROM (
                        SELECT  TOP 1   
                            1 AS updatePending, 
                            a.entity_number,
                            a.name_first,
                            a.name_last,
                            NULLIF(LTRIM(RTRIM(
                            LTRIM(RTRIM(ISNULL(a.company, ''))) +
                            LTRIM(RTRIM(ISNULL(a.firm_name, ''))))),'') AS company,
                            a.address1,
                            a.mailing_address,
                            a.city,
                            a.state,
                            a.zip_code AS zipcode,
                            a.internet_address AS email_address,
                            a.time_stamp
            FROM        statebar.dbo.STAGING_Address_Change_Request a
                INNER JOIN Member m ON m.entity_number = a.entity_number
            WHERE       a.entity_number = (
    SELECT m.entity_number
    FROM Member m
        INNER JOIN  Named_Entity ne ON  (ne.entity_number = m.entity_number)
    WHERE   ne.name_last = 'jones'
        AND m.birth_year = '1975'
        AND m.memNum = '12345'
        )
                AND a.time_stamp > m.time_stamp

            UNION ALL

            SELECT TOP 1 
                    0 AS updatePending,
                    ne.entity_number,
                    ne.name_first,
                    ne.name_last,
                    NULLIF(LTRIM(RTRIM(
                    LTRIM(RTRIM(ISNULL(ne.company, ''))) +
                    LTRIM(RTRIM(ISNULL(ne.firm_name, ''))))),'') AS company,
                    ne.address1,
                    ne.mailing_address,
                    ne.city,
                    ne.state,
                    ne.zip_code,
                    ne.internet_address AS email_address,
                    m.time_stamp
            FROM    Member m
                INNER JOIN  Named_Entity ne ON  (ne.entity_number = m.entity_number)
                LEFT JOIN   statebar.dbo.STAGING_Address_Change_Request a ON a.entity_number = m.entity_number
            WHERE   ne.entity_number = (
                                        SELECT m.entity_number
                                        FROM Member m
                                            INNER JOIN  Named_Entity ne ON  (ne.entity_number = m.entity_number)
                                        WHERE   ne.name_last = 'jones'
                                            AND m.birth_year = '1975'
                                            AND m.memNum = '12345'
                                            )
                AND m.time_stamp > a.time_stamp
            ORDER BY updatePending DESC, a.time_stamp DESC) q
    INNER JOIN Member m on m.entity_number = q.entity_number

ORDER BY q.time_stamp DESC 

Answer 1

Here is a simple query that will help you return the most recent record:

--Only selects the top row with the most recent record
SELECT TOP 1 * FROM record
(
    --Select rows with the same ID
    SELECT name, address, city, state, zipcode, time_stamp
    FROM Member
    WHERE ID = 123
    UNION ALL
    SELECT name, address, city, state, zipcode, time_stamp
    FROM MemberUpdates
    WHERE ID = 123
) t
ORDER BY t.time_stamp DESC --Order the table by time_stamp to get the most recent record
-- DESC is used because datetime is ordered by oldest first in ascending order.

Answer 2

The union approach is a good idea, but you'd want to use the row_number() window function and not just top. Also, union all can be used instead of union . You don't care about duplicates between A and B , and union all will just perform better:

SELECT name, address, city, state, zipcode, time_stamp
FROM   (SELECT name, address, city, state, zipcode, time_stamp, 
               ROW_NUMBER() OVER (PARTITION BY name ORDER BY time_stamp DESC) rn
        FROM   (SELECT name, address, city, state, zipcode, time_stamp
                FROM   Member
                UNION ALL
                SELECT name, address, city, state, zipcode, time_stamp
                FROM   MemberUpdates) t
        ) q
WHERE   rn = 1

Answer 3

consider:

SELECT
    id,
    MAX(CASE WHEN u.mx_ts IS NULL THEN m.mx_ts ELSE u.mx_ts end)
FROM 
    (SELECT 
        id, 
        MAX(time_stamp) AS mx_ts 
    FROM 
        MEMBER 
    GROUP BY 
        id) m
    LEFT OUTER JOIN
    (SELECT 
        id, 
        MAX(time_stamp) AS mx_ts 
    FROM 
        MemberUpdates 
    GROUP BY 
        id) u ON
    m.id = u.id AND
    u.mx_ts > m.mx_ts
GROUP BY 
    id

this will join a later timestamp per id from memberupdates , if there is one. Otherwise, you can use the latest timestamp per id from the member table.