Need a MySQL query for selecting from a table storing key value pairs
-
28-09-2019 - |
Pergunta
I need to store few items and its properties in form of a key value pairs in the database (mySQL). I am planning to do it as following.
I'll use two tables items
and item_properties
.
items
itemId | itemName ------------------- 1923 | AC 1235 | Fridge 8273 | Heater
item_properties
itemId | property | value -------------------------------- 1923 | effect | cooling 1923 | consumption | efficient 1923 | type | split 1235 | effect | cooling 1235 | volume | 20 liters 8273 | effect | heating 8273 | consumption | efficient 8273 | heatMethod | coil
Now, if I have to select items whose 'effect' is 'cooling', I can do that using following query (which will give me 'AC' and 'Fridge' in result).
SELECT itemName FROM items i, item_properties p WHERE i.itemId=p.itemId AND (p.property = 'effect' AND p.value ='cooling');
I would like to know how write queries to select items that match multiple properties like
- select all items whose 'effect' is 'cooling' AND 'consumption' is 'efficient' (which would match item 'AC').
- select all items whose 'type' is 'split' OR 'heatMethod' is 'coil' OR 'consumption' is 'effecient' (which would match items 'AC' and 'Heater').
Kindly Help... Thanks in advance!!
Solução
Here is an example query:
SELECT
itemName
FROM
items i,
JOIN
item_properties effect
ON i.itemId = effect.itemId AND effect.property = 'effect'
JOIN
item_properties consumption
ON i.itemId = consumption.itemId AND consumption.property = 'consumption'
WHERE effect.value = 'cooling' AND consumption.value = 'efficient';
I'll leave the oR
query as something you can try yourself. It's simply adding more tables and using OR
instead of AND
in the WHERE
.
Outras dicas
Greetings!
I think your original query is maybe not right.. if your item_properties table has columns named (itemId, property, value) then your query should be:
SELECT i.itemName FROM items i, item_properties p
WHERE i.itemId=p.itemId AND p.property='cooling';
Also, you're doing "implicit joins" here, and I don't know how much you want to learn about SQL vs. just wanting to get something to work, but there is another way to write your queries that I think is perhaps better if you're planning to stick with SQL databases for a while. It's no big deal, just that the second form is easier to read for me.
Your original query:
SELECT itemName FROM items i, item_properties p
WHERE i.itemId=p.itemId AND p.property='cooling';
Would be rewritten using join syntax as:
SELECT i.itemName FROM items i
JOIN item_properties p ON i.itemId=p.itemId WHERE p.property='cooling';
I'll try to give both forms in answer to your questions...
select all items whose 'effect' is 'cooling' AND 'consumption' is 'efficient' (which would match item 'AC').
select itemName FROM items i, item_properties p
WHERE i.itemId=p.itemId AND (p.property='cooling' AND p.value='consumption');
select i.itemName FROM items i JOIN item_properties p ON i.itemId=p.itemId
WHERE p.property='cooling' AND p.value='consumption';
select all items whose 'type' is 'split' OR 'heatMethod' is 'coil' OR 'consumption' is 'effecient' (which would match items 'AC' and 'Heater').
select itemName FROM items i, item_properties p
WHERE i.itemId=p.itemId AND ((p.property='type' AND p.value='split') OR
(p.property='heatMethod' AND p.value='coil') OR
(p.property='consumption' AND p.value='efficient'));
select itemName FROM items i JOIN item_properties p ON i.itemId=p.itemId
WHERE (p.property='type' AND p.value='split') OR
(p.property='heatMethod' AND p.value='coil') OR
(p.property='consumption' AND p.value='efficient');
Hope that helps!