MySQL：根据另一行更新多行的列

Question

I have a database of Tweets with the following columns

+------------------------+--------------+
| Field                  | Type         | 
+------------------------+--------------+
| id                     | varchar(128) |
| message_id             | bigint(20)   |
| message                | text         |
| from_id                | int(12)      |
| cnty                   | int(5)       |
| county                 | varchar(64)  |
| city                   | varchar(64)  |
| state                  | varchar(2)   |
+------------------------+--------------+

The database contains multiple messages for each distinct from_id . There is one row for each from_id that contains cnty , county , city . and state and all other rows with the same from_id have NULL values for those columns. I would like to fill them in with the same values for each from_id . Is this possible in MySQL? My first thought was to write a Python script to do this, but maybe that is overkill.

Answer 1

You could use a subquery that gets all non null values, then use an UPDATE query with a join:

UPDATE
  tweets INNER JOIN (
    SELECT from_id, MAX(cnty) AS cnty, MAX(country) AS country, MAX(city) AS city, ...
    FROM tweets
    GROUP BY from_id) m
  ON tweets.from_id = m.from_id
SET
  tweets.cnty=m.cnty,
  tweets.country=m.country,
  tweets.city=m.city,
  ...