Question

Some time ago I asked a question about nested loops on SO and as it was, there were queries inside the loops of my example and I got a clear answer:

NEVER EVER NEVER put an SQL query inside a loop

I’ve tried ever since and mostly it works. Just need to make an effort and write a query that retrieves all you need at once.

BUT what do you do when you have a dataset from a JOIN query which contains nested data which you need to output in a nested way?

Example join from table A and B:

A.a     |  B.a     |  B.b
--------|----------|-------------
fruits  |  banana  |  yellow
fruits  |  apple   |  red
animals |  zebra   |  black&white
animals |  elefant |  gray
animals |  fox     |  red
planets |  earth   |  blue
planets |  mars    |  red

ok, now I got that all in an array or rowset and now I need to display something like that:

fruits

• yellow banana
• red apple

animals

• black&white zebra
• gray elefant
• red fox

planets

• blue earth
• red mars

it seems obvious that it should work but I’ve tried to wrap my mind around it several times now and I just can’t come up with a solution.

At the moment I do it my old way:

query groups

foreach groups
{
    query animals in group
    foreach animal
}

but hey, NEVER EVER NEVER put sql inside a loop. so what shold I do? I do PHP but I think this is a meta question.

Answer 1

Use the control break algorithm.

I’d return a result set exactly as you show in the question:

A.a     |  B.a     |  B.b
--------|----------|-------------
fruits  |  banana  |  yellow
fruits  |  apple   |  red
animals |  zebra   |  black&white
animals |  elefant |  gray
animals |  fox     |  red
planets |  earth   |  blue
planets |  mars    |  red

loop over all the rows:

• when A.a changes, output the title
• then always output the B.b + B.a value

pseudo code for application calling SQL:

set last_A = null
exec query

loop over result set {
    if last_A == null or fetch_A!=last_A {
        last_A=fetch_A
        display fetch_a
    }
    display fetch_Bb + fetch_Ba
    }
}//loop