Question

All the 3 following tables have a “date_v_end” field which is a validity date. As long as it’s NULL this means it’s the current “good” value.

For example, to select the current values of the table categorie we can simply do SELECT * FROM categorie WHERE date_v_end IS NULL.

I have 3 tables:

• a table categorie (category in English)
• a table “one to many” categorie_produit (category <=> product in English)
• a table produit (product in English)

I’d like to make a query of all the current categories, and their one to many produit, and I still need one result even if the category has no product (= no produit_categorie row).

Before having the “date_v_end” field I did it this way:

SELECT
  c.id AS c_id,
  c.titre AS c_titre,
  c.description AS c_description,
  cp.id_categorie AS cp_id_categorie,
  cp.id_produit AS cp_id_produit,
  p.id AS p_id,
  p.titre AS p_titre,
  p.description AS p_description
FROM categorie_produit cp
LEFT OUTER JOIN categorie c
ON c.id=cp.id_categorie
LEFT OUTER JOIN produit p
ON p.id=cp.id_produit
ORDER BY c_id,p_id;

It worked like a charm. Now I’m trying to modify the query with the new “date_v_end” field. I’ve added the three clauses WHERE c.date_v_fin IS NULL AND cp.date_v_fin IS NULL AND p.date_v_fin IS NULL. There you go:

SELECT
  c.id AS c_id,
  c.titre AS c_titre,
  c.description AS c_description,
  cp.id_categorie AS cp_id_categorie,
  cp.id_produit AS cp_id_produit,
  p.id AS p_id,
  p.titre AS p_titre,
  p.description AS p_description
FROM categorie_produit cp
LEFT OUTER JOIN categorie c
ON (c.id=cp.id_categorie AND c.date_v_fin IS NULL)
LEFT OUTER JOIN produit p
ON (p.id=cp.id_produit AND p.date_v_fin IS NULL)
WHERE cp.date_v_fin IS NULL
ORDER BY c_id,p_id;

This works fine except in one circumstance: when there’s a categorie_produit and its “date_v_end” is not NULL: before adding this damn “date_v_end” field, I got one row with c_id, c_titre, and c_description filled and the other fields NULL (cp_id_categorie, cp_id_produit,p_id,p_titre,p_description). Now there’s no result.

I really don’t know how I can modify my query to make it work. Any idea?

Answer 1

You need to move the IS NULL check into the JOINs.

In your version you join, regardless of the date_v_fin field, and then filter the results.

In the query below a record is only joined if the date_v_fin field IS NULL.

SELECT
  c.id AS c_id,
  c.titre AS c_titre,
  c.description AS c_description,
  cp.id_categorie AS cp_id_categorie,
  cp.id_produit AS cp_id_produit,
  p.id AS p_id,
  p.titre AS p_titre,
  p.description AS p_description
FROM
  categorie c
LEFT OUTER JOIN
  categorie_produit cp
    ON  c.id=cp.id_categorie
    AND cp.date_v_fin IS NULL
LEFT OUTER JOIN
  produit p
    ON  p.id=cp.id_produit
    AND p.date_v_fin IS NULL
WHERE
  c.date_v_fin IS NULL
ORDER BY
  c_id,
  p_id