mysql update table using data from another table - mysql

Question

i have a table structures in my database like this:
/*city*/
+----------+------------+
| id | name |
|-----------------------|
| 1 | Gotham |
| 2 | Metropolis |
| 3 | Smallville |
| 4 | Fawcett |
+----------+------------+
/*district*/
+----------+------------+------------+
| id | name | city_id |
|------------------------------------|
| 1 | A | 1 |
| 2 | B | 1 |
| 3 | C | 2 |
| 4 | D | 2 |
| 5 | E | 2 |
| 6 | F | 3 |
| 7 | G | 3 |
| 8 | H | 4 |
+----------+------------+------------+
/*distance*/
+----------+-------------+------------------+-------------------------+---------+
| id | origin_city | city_destination | district_destination | length |
|---------------------------------------------------------------------|---------|
| 1 | 2 | 2 | 1 | 4 |
| 2 | 3 | 3 | 1 | 5 |
| 3 | 1 | 1 | 2 | 6 |
| 4 | 2 | 2 | 3 | 5 |
| 5 | 4 | 4 | 1 | 8 |
| 6 | 4 | 2 | 4 | 9 |
| 7 | 4 | 3 | 5 | 11 |
| 8 | 1 | 4 | 6 | 13 |
+----------+-------------+------------------+-------------------------+---------+
the table district is connected to city table via city_id foreign key, and the distance table is connected to both city and district table, the problem is if in distance table, there are wrong city_destination data that don't match with the district_destination, i need to fix this, but i don't know how to use the update query for this kind of trouble, to show the wrong city_destination data i used this query:
SELECT a.* FROM distance a, district b WHERE a.district_destination = b.id AND a.city_destination != b.city_id

Answer 1

First, ditch the old-school comma syntax for the join operation. Use the JOIN keyword and move the join predicates to an ON clause. Write a SELECT query that returns the existing row to be updated (along with the PK, and the new value to be assigned. (Which looks to be as far as you got.)
Assuming that we want to replace the values in the city_destination column of distance table, and seeing that this this column is functionally dependent on district_destination...
Start with a query that returns the rows to be updated.
SELECT ce.id AS id
, ce.district_destination AS district_destination
, ce.city_destination AS old_city_destination
, ct.city_id AS new_city_destination
FROM distance ce
JOIN district ct
ON ct.id = ce.district_destination
AND NOT ( ct.city_id <=> ce.city_destination )
ORDER BY ce.id
In MySQL, a multi-table update is pretty straightforward. The syntax is documented in the MySQL Reference Manual.
First, we'll write it as a SELECT, using the previous query as an inline view
SELECT t.id
, s.new_city_destination
FROM ( SELECT ce.id AS id
, ce.district_destination AS district_destination
, ce.city_destination AS old_city_destination
, ct.city_id AS new_city_destination
FROM distance ce
JOIN district ct
ON ct.id = ce.district_destination
AND NOT ( ct.city_id <=> ce.city_destination )
ORDER BY ce.id
) s
JOIN distance t
ON t.id = s.id
Then we can convert that to an UPDATE statement. Replace SELECT ... FROM with UPDATE and add a SET clause at the end. (Before the WHERE clause if there was one.)
UPDATE ( SELECT ce.id AS id
, ce.district_destination AS district_destination
, ce.city_destination AS old_city_destination
, ct.city_id AS new_city_destination
FROM distance ce
JOIN district ct
ON ct.id = ce.district_destination
AND NOT ( ct.city_id <=> ce.city_destination )
ORDER BY ce.id
) s
JOIN distance t
ON t.id = s.id
SET t.city_destination = s.new_city_destination