Join one row to multiple rows in another table - sql

Question

I have a table to entities (lets call them people) and properties (one person can have an arbitrary number of properties). Ex:
People
Name Age
--------
Jane 27
Joe 36
Jim 16
Properties
Name Property
-----------------
Jane Smart
Jane Funny
Jane Good-looking
Joe Smart
Joe Workaholic
Jim Funny
Jim Young
I would like to write an efficient select that would select people based on age and return all or some of their properties.
Ex: People older than 26
Name Properties
Jane Smart, Funny, Good-looking
Joe Smart, Workaholic
It's also acceptable to return one of the properties and total property count.
The query should be efficient: there are millions of rows in people table, hundreds of thousands of rows in properties table (so most people have no properties). There are hundreds of rows selected at a time.
Is there any way to do it?

Answer 1

Use:
SELECT x.name,
GROUP_CONCAT(y.property SEPARATOR ', ')
FROM PEOPLE x
LEFT JOIN PROPERTIES y ON y.name = x.name
WHERE x.age > 26
GROUP BY x.name
You want the MySQL function GROUP_CONCAT (documentation) in order to return a comma separated list of the PROPERTIES.property value.
I used a LEFT JOIN rather than a JOIN in order to include PEOPLE records that don't have a value in the PROPERTIES table - if you only want a list of people with values in the PROPERTIES table, use:
SELECT x.name,
GROUP_CONCAT(y.property SEPARATOR ', ')
FROM PEOPLE x
JOIN PROPERTIES y ON y.name = x.name
WHERE x.age > 26
GROUP BY x.name
I realize this is an example, but using a name is a poor choice for referencial integrity when you consider how many "John Smith"s there are. Assigning a user_id, being a unique value per user, would be a better choice.

Answer 2

You can use INNER JOIN to link the two tables together. More info on JOINs.
SELECT *
FROM People P
INNER JOIN Properties Pr
ON Pr.Name = P.Name
WHERE P.Name = 'Joe' -- or a specific age, etc
However, it's often a lot faster to add a unique primary key to tables like these, and to create an index to increase speed.
Say the table People has a field id
And the table Properties has a field peopleId to link them together
Then the query would then look something like this:
SELECT *
FROM People P
INNER JOIN Properties Pr
ON Pr.id = P.peopleId
WHERE P.Name = 'Joe'

Answer 3

SELECT x.name,(select GROUP_CONCAT(y.Properties SEPARATOR ', ')
FROM PROPERTIES y
WHERE y.name.=x.name ) as Properties FROM mst_People x
try this