Returning a single row from query joining multi-row tables

Question

I have a database design that holds data about houses. There are 2 (relevant) tables - one holds property code, name, description etc, and the second holds information about attributes of the property.

Currently I have got columns in the property table (MaxDoubles, MaxSingles etc) that hold denormalised data that I need to now hold (for various reasons) in the attributes table. In effect I am swapping a series of columns in the Properties table for a series of rows in the Attributes table. So my existing query

SELECT MaxDoubles, MaxSingles FROM Properties

that returns one row per property needs re-writing to also produce one row per property when joined to the Attributes. If I try

SELECT A.MaxDoubles, A.MaxSingles FROM Properties P, Attributes A

then I obviously get multiple rows returned per property.

Is there a clever way of joining these tables so the query result returns a single row?

Thanks

Answer 1

Assuming a schema like @Konerak's example, you'll need to "pivot" the attributes records if you want one row containing a property and all of its attributes.

Fortunately, there is no shortage of info on SO about how to do that. :)

Answer 2

What you now have is called an EAV data structure and what you want to do is called "to pivot". Besides using a subselect, there are two possibilities.

Using a GROUP BY:

SELECT P.Property_ID,
       MAX(IF(A.Type = 'maxsingles',A.Value,0)) AS MaxSingles,
       MAX(IF(A.Type = 'maxdoubles',A.Value,0)) AS MaxDoubles
FROM Properties P
JOIN Attributes A USING (Property_ID)
GROUP BY Property_ID

Using one JOIN for each attribute:

SELECT P.Property_ID, A1.Value AS MaxSingles, A2.Value AS MaxDoubles
FROM Properties P
JOIN Attributes A1 ON (A1.Property_ID = P.Property_ID AND A1.Type = 'maxsingles')
JOIN Attributes A2 ON (A2.Property_ID = P.Property_ID AND A2.Type = 'maxdoubles')

Answer 3

We'll need more information from you to handle your question. For example:

What do your tables look like ( SHOW CREATE TABLE Properties ) What queries do you want to execute? How do you join your tables?

This schema is normalized enough to allow typical all queries:

Table Properties:

• PropertyID
• PropertyName
• Propertydescription

Table Attributes:

• AttributeID
• PropertyID
• AttributeName
• AttributeValue

So if you have Property 1, a white house with 3 bedrooms, you could have

PropertyID: 1
PropertyName: Charming white landhouse with 3 spacious bedrooms
Propertydescription: This charming landhouse will ...

Attributes
AttributeID: 1
PropertyID: 1
AttributeName: NrBedrooms
AttributeValue: 3

AttributeID: 2
PropertyID: 1
AttributeName: NrBathrooms
AttributeValue: 2

AttributeID: 3
PropertyID: 1
AttributeName: Kitchen
AttributeValue: Fully Equiped

Now if you want to know how many bedrooms your house has, you can ask

SELECT A.AttributeValue from Attributes A
INNER JOIN Properties P
ON P.PropertyID = A.PropertyID
WHERE P.PropertyID = 1
and A.AttributeName = 'NrBedrooms'