Query optimization: connecting meta data to a value list table - sql

Question

I have a database containing a table with data and a meta data table. I want to create a View that selects certain meta data belonging to an item and list it as a column.
The basic query for the view is: SELECT * FROM item. The item table is defined as:
CREATE TABLE item (
id INTEGER PRIMARY KEY AUTOINCREMENT
UNIQUE
NOT NULL,
traceid INTEGER REFERENCES trace (id)
NOT NULL,
freq BIGINT NOT NULL,
value REAL NOT NULL
);
The meta data to be added follow the schema "metadata.parameter='name'"
The meta table is defined as:
CREATE TABLE metadata (
id INTEGER PRIMARY KEY AUTOINCREMENT
UNIQUE
NOT NULL,
parameter STRING NOT NULL
COLLATE NOCASE,
value STRING NOT NULL
COLLATE NOCASE,
datasetid INTEGER NOT NULL
REFERENCES dataset (id),
traceid INTEGER REFERENCES trace (id),
itemid INTEGER REFERENCES item (id)
);
The "name" parameter should be selected this way:
if a record exists where parameter is "name" and itemid matches item.id, then its value should be included in the record.
otherwise, if a record exists where parameter is "name", "itemid" is NULL, and traceid matches item.traceid, its value should be used
otherwise, the result should be NULL, but the record from the item table should be included anyway
Currently, I use the following query to achieve this goal:
SELECT i.*,
COALESCE (
MAX(CASE WHEN m.parameter='name' THEN m.value END),
MAX(CASE WHEN m2.parameter='name' THEN m2.value END)
) AS itemname
FROM item i
JOIN metadata m
ON (m.itemid = i.id AND m.parameter='name')
JOIN metadata m2
ON (m2.itemid IS NULL AND m2.traceid = i.traceid AND m2.parameter='name')
GROUP BY i.id
This query however is somewhat inefficient, as the metadata table is used twice and contains many more records than just the "name" ones. So I am looking for a way to improve speed, especially regarding the case that some extensions are about to be implemented:
there is a third level "dataset" that should be included: a "parameter=name" should be used if it has the same datasetid as the item (will be looked up for the items by searching another which connects traceid and datasetid), if no "parameter=name" exists with either "itemid" matching or "traceid" matching
more meta data should be queried by the view following the same schema
Any help is appreciated.

Answer 1

First of all, you can use one join instead of 2, like this:
JOIN metadata m ON (m.parameter='name' AND (m.itemId = i.id OR (m.itemId IS NULL AND m.traceid = i.traceid)))
Then you can remove COALESCE, using simple select:
SELECT i.*, m.value as itemname
Result should look like this:
SELECT i.*, m.value as itemname
FROM item i
JOIN metadata m ON (m.parameter='name' AND (m.itemId = i.id OR (m.itemId IS NULL AND m.traceid = i.traceid)))
GROUP BY i.id