How to read out data from sql-database when several criteria match - mysql

Question

I have a normalized sql-database with several tables and an html-form to read out the data. There is one table that contains product-characteristics and any product can have one or more characteristics.
Main table: product_main pm,
Table with characteristics: product_characteristics pc,
product_id is the foreign key.
id | product_id | product_name | characteristic
1 | 27 | Product_C | characteristic5
2 | 27 | Product_C | characteristic11
3 | 27 | Product_C | characteristic3
4 | 27 | Product_C | characteristic47
5 | 27 | Product_C | characteristic34
6 | 28 | Product_D | characteristic29
7 | 28 | Product_D | characteristic63
8 | 28 | Product_D | characteristic2
Now, I want to read out all product_names that match the selected criteria. This works fine, when only one characteristic is selected:
SELECT pm.name
FROM product_main pm, product_characteristics pc
WHERE pc.characteristic = 'characteristic47'
AND pm.id = pc.product_id
The correct result is: Product_C.
If more than one characteristics are selected simultaneously, e.g.:
SELECT pm.name
FROM product_main pm, product_characteristics pc
WHERE pc.characteristic = 'characteristic47'
AND pc.characteristic = 'characteristic11'
AND pm.id = pc.product_id
there is no result, although Product_C matches both criteria.
Could someone please tell me, how I can solve this problem?

Answer 1

One option uses conditional aggregation:
SELECT product_id, product_name
FROM product_main
GROUP BY product_id, product_name
HAVING SUM(CASE WHEN characteristic = 'characteristic47' THEN 1 ELSE 0 END) > 0 AND
SUM(CASE WHEN characteristic = 'characteristic11' THEN 1 ELSE 0 END) > 0
If you also want to retain all original records in product_main which have both matching characteristics, then you can join the above table back to your original table:
SELECT t1.*
FROM product_main t1
INNER JOIN
(
SELECT product_id
FROM product_main
GROUP BY product_id
HAVING SUM(CASE WHEN characteristic = 'characteristic47' THEN 1 ELSE 0 END) > 0 AND
SUM(CASE WHEN characteristic = 'characteristic11' THEN 1 ELSE 0 END) > 0
) t2
ON t1.product_id = t2.product_id

Answer 2

Well pc.characteristic cannot be both characteristic47 AND characteristic11 at the same time, but it could be (characteristic47 OR characteristic11)
So amend the query like so
SELECT pm.name
FROM product_main pm, product_characteristics pc
WHERE ( pc.characteristic = 'characteristic47' OR
pc.characteristic = 'characteristic11' )
AND pm.id = pc.product_id
You sould also be using JOIN's like so
SELECT pm.name
FROM product_main pm
JOIN product_characteristics pc ON pm.id = pc.product_id
WHERE ( pc.characteristic = 'characteristic47' OR
pc.characteristic = 'characteristic11' )