Question

I have a table that needs each row to store an ordered collection of rows of the same type.

e.g., I am imagining something similar to:

class blah
{
   float yetMoreData;
};

class foo
{
   int data;
   String moreData;
   blah[] anOrderedCollectionOfBlahClasses;
};

What would be the best way to achieve this?

Answer 1

You want Blah and Foo to be separate tables, where Blah has a many-to-one relationship with Foo. The records in the Blah table should have a foreign key foo_id, identifying their parent Foo record. Blah also has an index_in_array attribute that allows the results to be sorted.

CREATE TABLE Foo (
    id BIGINT,
    data INT,
    moreData CHAR(50)
);

CREATE TABLE Blah (
    id BIGINT,
    yetMoreData DOUBLE,
    foo_id BIGINT,
    index_in_array INT
);

If you want to get all the Blahs of a given Foo (say, the Foo with the ID of 5), use:

SELECT * FROM Blah
WHERE Blah.foo_id = 5
ORDER BY Blah.index_in_array;

If you’re ordering records, usually it shouldn’t be by something arbitrary (unless that arbitrary order was chosen specifically by the user, e.g., sorting the items on a to-do list). Normally, you sort on some “naturally-occurring” property of the data, like the date it was added or alphabetically by name, etc.

N.B.: This answer is meant to be pseudocode SQL — make sure you adapt it to your specific database syntax before you toss it into a script. (And don’t forget to add some keys and indexes!)