Question

I’m pretty sure that I need to be doing left outer joins on my tables, but i’m not sure if that is 100% true or the syntax to write them in mySQL.

I have this query written:

select m.mechanic_id, 
   m.mechanic_name, 
   m.city, 
   m.state, 
   count(mr.mechanic_id) as num_ratings, 
   round(avg(mr.quality_id),2) quality_rating
   round(avg(mr.friendly_id),2) friendly_rating,
   round(avg(mr.professional_id),2) professional_rating
from mechanic m, mechanic_rating mr, rating r
where m.mechanic_id in (1)
and m.mechanic_id = mr.mechanic_id
and mr.quality_id = r.rating_id(+) <-- these cause issues
and mr.friendly_id = r.rating_id(+) <-- these cause issues
and mr.professional_id = r.rating_id(+) <-- these cause issues
group by mechanic_id

The (+) are one way to do outer joins in oracle and i’m not sure how to manually write out the outer joins in this query. I’m not even sure if I have them on the right columns.

My table structure looks like this

Mechanic Table

|mechanic_id|mechanic_name|city|state|zip|
|PK         |

Rating Table

|rating_id|rating  |
|1        |terrible|
|2        |bad     |
etc.

Mechanic_Rating table

|mechanic_rating_id|mechanic_id|quality_id|friendly_id|professional_id|
|unique auto inc   |FK         |

The quality_id, friendly_id, and professional_id should all be foreign keys to the rating_id in the rating table.

If i take off (+) from my query, i get zero results so i’m thinking that the problem is that i need to do left outer joins. Let me know if you need more info.

Answer 1

You need to learn and use the ANSI-92 Standard syntax for doing Join predicates. Using ANSI-92, your query would be written as

 select m.mechanic_id,  m.mechanic_name, m.city,
     m.state, count(mr.mechanic_id) num_ratings, 
     round(avg(mr.quality_id),2) quality_rating   
     round(avg(mr.friendly_id),2) friendly_rating,   
     round(avg(mr.professional_id),2) professional_rating
from mechanic m
   Left Join mechanic_rating mr
       On mr.mechanic_id = m.mechanic_id
   Left Join rating r 
       On r.rating_id = mr.quality_id
           And r.rating_id = mr.friendly_id
           And r.rating_id = mr.friendly_id 
 where m.mechanic_id in (1)
 group by mechanic_id

NOTE: What does the (1) refer to ?? Are you trying to restrict this to
where m.mechanic_id = 1 ?