Need a MySQL query for selecting from a table stor

问题:

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!!

回答1:

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.

回答2:

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!