Question

I have a problem with the correct syntax to use UNION and GROUP_CONCAT in this situation:

I have 4 tables:

• base: Is the main table with a lot of columns.
• mm: Is a mm table that points to the next two tables using a ‘tablenames’ field.
• t1 and t2 that stores data related.

Records in ‘base’ tables can have many related records in t1 and t2 through the mm table.

I’m creating a VIEW in MySQL and I need all those related records are displayed in a single column separated by commas.

This is the base MySQL code:

SELECT base.uid, t1.nombre_es
FROM base
INNER JOIN mm 
ON mm.uid_local=base.uid
INNER JOIN t1 
ON mm.uid_foreign=t1.uid WHERE mm.tablenames = 't1'

UNION

SELECT base.uid, t2.nombre_es
FROM base
INNER JOIN mm 
ON mm.uid_local=base.uid
INNER JOIN t2
ON mm.uid_foreign=t2.uid WHERE mm.tablenames = 't2'

Thanks in advance.

---

I could do it using two VIEWS, the first using the code above with the name ‘viewtest”, and the second with this code:

SELECT base.uid,
GROUP_CONCAT(DISTINCT vi.nombre_es ORDER BY vi.nombre_es SEPARATOR ',') as nombre

FROM base

INNER JOIN viewtest as vi
ON vi.uid=base.uid

GROUP BY uid

Now the question is ¿How can I join this two views in a single view?

Answer 1

You can use derived tables from queries. Next is an example of how you can use them.

SELECT GROUP_CONCAT( f ) 
FROM (
  SELECT 1 AS f  # <-- QUERY #1
  UNION 
  SELECT 2 AS f  # <-- QUERY #2
  UNION 
  SELECT 3 AS f  # <-- QUERY #3
) AS T

Basically, you can use any SELECT query as an aliased table. Then, you can apply any aggregate functions you need to that aliased query.