Mysql 内连接与 OR 条件？

Question

I have 2 tables like below

location_distance

----------------------------------------------
id   | fromLocid    | toLocid   |  distance
----------------------------------------------
1    |  3           |  5        |   70
2    |  6           |  8        |   15
3    |  2           |  4        |   63
...

other_table

--------------------------------------------
Id  | fromLocid   | toLocid    | otherdata
--------------------------------------------
12  |  5          | 3          | xxxx
22  |  2          | 4          | xxxx   
56  |  8          | 6          | xxxx
78  |  3          | 5          | xxxx

I would like to retrieve the distance b/w the locations in other_table for each row. Here's what i've tried

SELECT ot.*, ld.distance FROM other_table AS ot 
    INNER JOIN location_distance ld ON ld.fromLocid = ot.fromLocid AND ld.toLocid = ot.toLocid

This doesnt return the rows if the locations values are vice versa. How can i rewrite the above query to produce expected result? Should i inlude OR condition on the join clause? like below?

SELECT ot.*, ld.distance FROM other_table AS ot 
    INNER JOIN location_distance ld ON (ld.fromLocid = ot.fromLocid OR ld.fromLocid = ot.toLocid) AND (ld.toLocid = ot.fromLocid OR ld.toLocid = ot.fromLocid)

but this query Explain says "Range checked for each record". .. is this a bad practise?

Result

--------------------------------------------------------
Id  | fromLocid | toLocid    | otherdata   | distance
--------------------------------------------------------
22  |  2        |   4        | xxxx        | 63
78  |  3        |   5        | xxxx        | 70

Expected Result should be

-----------------------------------------------------
Id  | fromLocid   | toLocid   | otherdata  | distance
-----------------------------------------------------
12  |   5         |    3      | xxxx       | 70
22  |   2         |    4      | xxxx       | 63
56  |   8         |    6      | xxxx       | 15 
78  |   3         |    5      | xxxx       | 70

Answer 1

You can join on the location_distance table twice using a LEFT JOIN and then use the COALESCE() function to return the correct value for the distance :

select ot.id,
  ot.fromlocid,
  ot.tolocid,
  ot.otherdata,
  coalesce(ld1.distance, ld2.distance) distance
from other_table ot
left join location_distance ld1
  on ld1.fromLocid = ot.toLocid
  and ld1.toLocid = ot.fromLocid 
left join location_distance ld2
  on ld2.toLocid = ot.toLocid
  and ld2.fromLocid = ot.fromLocid 

See SQL Fiddle with Demo

This returns the result:

| ID | FROMLOCID | TOLOCID | OTHERDATA | DISTANCE |
---------------------------------------------------
| 12 |         5 |       3 |      xxxx |       70 |
| 22 |         2 |       4 |      xxxx |       63 |
| 56 |         8 |       6 |      xxxx |       15 |
| 78 |         3 |       5 |      xxxx |       70 |

Answer 2

It will probably be faster to join the distance table twice like

INNER JOIN location_distance ld1 ON ld1.fromLocid = ot.fromLocid AND ld1.toLocid = ot.toLocid
INNER JOIN location_distance ld2 ON ld2.toLocid = ot.fromLocid AND ld2.fromLocid = ot.toLocid

and then use an IF to determine which one to select

IF(ld1.fromLocid, ld1.distance, ld2.distance) as distance