sql 内左连接 + 性能

Question

I have 3 tables:

room
room location
room_storys

If anyone creates a room, everytime comes a row automaticlly to room location. room_storys table can be empty.

Now I want to inner join the tables.

If I make this I get no results:

SELECT
r.name, 
r.date, 

rl.city, 
rl.street, 
rl.number, 
rl.name,

rs.source,
rs.date

FROM room r

INNER JOIN room_location rl
ON rl.room_id = 67

INNER JOIN room_storys rs
ON rs.room_id = 67

LIMIT 1;

If I make this:

INNER JOIN room_storys rs
ON rs.room_id = 67

to this:

LEFT JOIN room_storys rs
ON rs.room_id = 67
```

then it works. But I heard that left join has no good performance, how you would perform this query above? Or is that okey?

Answer 1

Every JOIN type has its pros and cons but, for a query like yours, the “cost” is negligible. One recommendation would be to have the tables reference each other rather than a specific id as part of the ON clause. Specificity can be crucial with OUTER JOIN s in some situations. Your query can be written like this:

SELECT r.`name`, 
       r.`date`, 

       rl.`city`, 
       rl.`street`, 
       rl.`number`, 
       rl.`name`,

       rs.`source`,
       rs.`date`
  FROM `room` r INNER JOIN `room_location` rl ON r.`id` = rl.`room_id`
           LEFT OUTER JOIN `room_storys` rs ON rl.`room_id` = rs.`room_id`
 WHERE r.`id` = 67;

This change solves the problem of returning everything in room , which was mitigated by the LIMIT 1 . It also ensures a record is returned even if there is nothing in room_stories for the room_id .

Hope this gives you something to think about with future queries

Answer 2

Think of it this way:

ON says how the tables are related, such as

 ON room.id = room_story.room_id

WHERE is used for filtering, such as

 WHERE room.id = 67

Also, JOIN (or INNER JOIN requires the matching rows in each of the 2 tables to both exist. LEFT JOIN says that the matching row in the right table is optionally missing.

Putting those together, I think this is what you needed:

FROM room r
INNER JOIN room_location rl  ON rl.room_id = r.id
INNER JOIN room_storys rs    ON rs.room_id = r.id
WHERE r.id = 67

This becomes irrelevent (I think): LIMIT 1