Question

If I have a table like this:

TABLEA:

id| object | type
1 | greg | person
2 | mary | person
3 | jared | person
4 | kelly | person
5 | melissa | person
6 | william | person
7 | skiing | hobby
8 | biking | hobby

TABLEB:

id | husband | wife
1 | greg | mary
2 | jared | kelly
3 | william | kelly

TABLEC:

id | female | hobby
1 | mary | skiing
2 | kelly | biking

Is there some way I could get a result table of:

TABLED:

id | a | b | link
1 | 1 | 2 | related
2 | 3 | 4 | related
3 | 6 | 4 | related
4 | 1 | 4 | related
5 | 2 | 7 | likes
6 | 4 | 8 | likes

Using only MySQL query(/ies)?

• Logic would basically start iterating from TABLE B and all rows of table B.
• The third column is related when the table being selected is TableB, and is likes when the table being processed is TABLE B.

Sample logic would be:

Looking at the first row of TABLEB (husband) greg and wife (mary), it looks up from TABLEA to see that greg is at row 1 (id 1) and mary is at row 2 (id 2) and creates a new TableD with the first row being 1 | 2.

Is there a query that can do some join or something that would be able to do this without having to programmatically iterate through all rows of TABLEB, then all rows of TABLEC to produce the desired TABLED?

Answer 1

You will need to JOIN the table tablea two times with the tableb:

• one for husband > object and another,
• one for wife > object.

So that you can get the two ids of the husband and wife in the same row.

The same with Tablec with tablea. Then use UNION(implicit distinct) or UNION ALL to union the two result sets.

Something like:

SELECT
  (@rownum := @rownum + 1) AS id,
  sub.*
FROM
(
  SELECT 
    ah.id     AS a,
    aw.id     AS b,
    'related' AS link
  FROM tableb       AS b
  INNER JOIN Tablea AS ah ON ah.object = b.husband AND ah.type = 'person'
  INNER JOIN Tablea AS aw ON aw.object = b.wife    AND aw.type = 'person'
  UNION ALL
  SELECT
    a.id  AS a,
    a2.id AS b,
    'hobby'
  FROM tablec       AS c
  INNER JOIN tablea AS a  ON a.object  = c.female 
  INNER JOIN tablea AS a2 ON a2.object = c.hobby AND a2.type = 'hobby'
) AS Sub, (SELECT @rownum := 0) AS t;

SQL Fiddle Demo

This will give you:

| ID | A | B |    LINK |
------------------------
|  1 | 1 | 2 | related |
|  2 | 3 | 4 | related |
|  3 | 6 | 4 | related |
|  4 | 2 | 7 |   hobby |
|  5 | 4 | 8 |   hobby |

Note that:

• This query will give you only 5 rows, where the expected result set that you are looking for is 6. It is missing the row 4 | 1 | 4 | related, because there is no entry for those greg(id 1) and keyll (id 4) in the tableb. As you explained in your question.

• The new column id is auto incremental id, generated from the result set, it is not selected from the tables.

---

If you need to create a brand new table from this select, use the following syntax:

CREATE TABLE Tabled
AS
SELECT ...
... -- the same previous select query

and you will have a new table tabled having the same structure of this SELECT.

Like in this updated fiddle.