MySQL加入3个级联表

Question

I have seen plenty of examples to join multiple tables back to one master table but this query is different in that it cascades.

I have 3 tables similar to the below (simplified for ease):

**CONTACTS**     
--------------------------------------------
Cid    name
--------------------------------------------
1      John
2      Peter
3      Karl


**OPPORTUNITIES**
--------------------------------------------
Oid   Cidlink   title
--------------------------------------------
1     2         php lookup script
2     2         php facial recognition
3     3         html email template
4     1         javascript verification

**ATTACHMENTS**
--------------------------------------------
Aid    Oidlink  attachment
--------------------------------------------
1     3         received enquiry
2     3         header and footer done
3     3         pixel trace image done on server
4     2         database structure done      
5     2         html get form done

As you can see, attachments link back to opportunities and opportunities relate to contacts. Attachments do not link directly to contacts except via the opportunity table.

I need to create a recordset including fields from all 3 tables and am struggling.

SELECT CONTACTS.*, OPPORTUNITIES.*, ATTACHMENTS.*
FROM ATTACHMENTS
    INNER JOIN OPPORTUNITIES
        ON CONTACTS.Cid = OPPORTUNITIES.Cidlink
    INNER JOIN ATTACHMENTS
        ON OPPORTUNITIES.Oid = ATTACHMENTS.Oidlink

ORDER BY ****whatever****

Am I close or am I approaching this the wrong way?

Answer 1

I think you mean to have contacts be the first table not attachments...

SELECT CONTACTS.*, OPPORTUNITIES.*, ATTACHMENTS.*
FROM ATTACHMENTS  --- This is wrong
INNER JOIN OPPORTUNITIES
  ON CONTACTS.Cid = OPPORTUNITIES.Cidlink
INNER JOIN ATTACHMENTS
  ON OPPORTUNITIES.Oid = ATTACHMENTS.Oidlink
ORDER BY ****whatever****

I think you mean (and use aliases easier to read in the long run)

SELECT C.*, O.*, A.*
FROM Contacts C
INNER JOIN OPPORTUNITIES O
   ON C.Cid = O.Cidlink
INNER JOIN ATTACHMENTS A
   ON O.Oid = A.Oidlink
ORDER BY ****whatever****

Since you indicated you need fields from all 3 tables I'm assuming INNER JOIN is appropriate here; but maybe you want to use an OUTER JOIN to include all contacts? Include all contacts and their opportunities if they exist. and include all attachments for opportunities if if they exist)

SELECT C.*, O.*, A.*
FROM Contacts C
LEFT JOIN OPPORTUNITIES O
   ON C.Cid = O.Cidlink
LEFT JOIN ATTACHMENTS A
   ON O.Oid = A.Oidlink
ORDER BY ****whatever****

For an improved understanding of joins this Venn Diagram approach from CodingHorror is a good start.