Use value of another field as default value for left join - mysql

Question

I have the following 3 tables:
users
| id | name | address |
|----+-------+----------+
| 1 | user1 | address1 |
| 2 | user2 | address2 |
locations
| id | user_id | name |
|----+---------+-----------+
| 1 | 1 | location1 |
| 2 | 1 | location2 |
| 3 | 2 | location3 |
orders
| id | user_id | from_id | to_id |
|----+---------+---------+-------+
| 1 | 1 | 1 | 2 |
| 2 | 1 | 1 | 0 |
from_id or to_id can have 0, meaning that user's address was used in this case.
Executing a 'naive' join on these tables:
SELECT u.name uname, fl.location flocation, tl.location tlocation
FROM users u, orders o, locations fl, locations tl
WHERE u.id = o.user_id
AND o.from_id = fl.id
AND o.to_id = tl.id
doesn't show records with 0:
user1 | location1 | location2
what I would like to see is the following data:
user1 | location1 | location2
user1 | location1 | address1
Using mysql, is there a way to extend the join to show such results ?

Answer 1

The following SQL should do it:
SELECT u.name uname, IF(fl.id, fl.location, u.address) AS flocation, IF(tl.id, tl.location, u.address) AS tlocation
FROM users u
INNER JOIN orders o ON u.id = o.user_id
LEFT JOIN locations fl ON o.from_id = fl.id
LEFT JOIN locations tl ON o.to_id = tl.id;
And if I may make a suggestion, I would recommend that you not use the WHERE clause as a way to join tables.

Answer 2

SELECT u.name uname, fl.location flocation, tl.location tlocation
FROM users u
left join orders o
on u.id = o.user_id
left join locations fl
on o.from_id = fl.id
left join locations tl
on o.to_id = tl.id
This will give you data for all users.
And as Tim Burch suggested, its best to use the ANSI SQL Syntax.

Answer 3

To use the user's address as a default for both the from and the to names, you can do a left join and use COALESCE to replace nulls with the address;
SELECT
u.name uname,
COALESCE(fl.name, u.address) flocation,
COALESCE(tl.name, u.address) tlocation
FROM users u
JOIN orders o ON u.id = o.user_id
LEFT JOIN locations fl ON o.from_id = fl.id
LEFT JOIN locations tl ON o.to_id = tl.id
An SQLfiddle to test with.

Answer 4

I recommend abandoning the "comma" syntax for the join operation; use the JOIN keyword, and move the join predicates to an ON clause rather than the WHERE clause.
To get all rows from orders returned, whether or not there is a "matching" row from the other tables, you can use an outer join operation.
In the SELECT list, you can test whether the location column from locations IS NULL, and return the address column from the user table when it is, using the COALESCE function.
SELECT u.name uname
, COALESCE(fl.location,u.address) flocation
, COALESCE(tl.location,u.address) tlocation
FROM orders o
LEFT
JOIN users u
ON u.id = o.user_id
LEFT
JOIN locations fl
ON fl.id = o.from_id
LEFT
JOIN locations tl
ON tl.id = o.to_id
The COALESCE(x,y) function is essentially shorthand for 'CASE WHEN x IS NULL THEN y ELSE x END`.
There's a corner case there, what if a matching row in the locations table is found, but the location column actually contains a NULL.
To more precisely match your specification, you could test for a value of "0" in the to_id column of the orders table:
SELECT u.name uname
, IF(o.from_id = 0, u.address, fl.location) flocation
, IF(o.to_id = 0, u.address, tl.location) tlocation
FROM orders o
LEFT
JOIN users u
ON u.id = o.user_id
LEFT
JOIN locations fl
ON fl.id = o.from_id
LEFT
JOIN locations tl
ON tl.id = o.to_id
But there's a corner case there; what if there is a row in locations that has an id value of "0".
The normative pattern would be to test for whether a row was returned from the locations table by checking a column that is guaranteed to be NOT NULL if a match is found, and will be NULL if a match is not found:
SELECT u.name uname
, IF(fl.id IS NULL, u.address, fl.location) flocation
, IF(tl.id IS NULL, u.address, tl.location) tlocation
FROM orders o
LEFT
JOIN users u
ON u.id = o.user_id
LEFT
JOIN locations fl
ON fl.id = o.from_id
LEFT
JOIN locations tl
ON tl.id = o.to_id
All three of those queries will return identical results for the "normal" cases; they just differ in how the "corner cases" are handled.
In summary:
use LEFT [OUTER] JOIN operations with orders as the driving table.
use conditional expressions in the SELECT list to determine which column value to return