It is necessary to mount an array similar to the structure you want to display. Something similar to:
[
'categoria1' => ['sub1', 'sub2', 'sub3', 'sub4'],
'categoria2' => ['sub1', 'sub2', 'sub3', 'sub4']
]
What can be done like this:
<?php
//supondo que são duas colunas distintas (com indice númerico)
$dadosConsulta = [
['categoria1', 'sub1'],
['categoria1', 'sub2'],
['categoria1', 'sub3'],
['categoria2', 'sub1'],
['categoria2', 'sub2'],
['categoria1', 'sub1'],
['categoria3', 'sub1']
];
$dados = [];
for($i = 0; $i < count($dadosConsulta); $i++){
$dados[$dadosConsulta[$i][0]][] = $dadosConsulta[$i][1];
}
//estrutura
var_dump($dados);
//percorrendo
foreach($dados as $categoria => $subcategorias){
echo $categoria . '<br>';
foreach($subcategorias as $subcategoria){
echo $subcategoria . '<br>';
}
}
Generate output:
array (size=3)
'categoria1' =>
array (size=4)
0 => string 'sub1' (length=4)
1 => string 'sub2' (length=4)
2 => string 'sub3' (length=4)
3 => string 'sub1' (length=4)
'categoria2' =>
array (size=2)
0 => string 'sub1' (length=4)
1 => string 'sub2' (length=4)
'categoria3' =>
array (size=1)
0 => string 'sub1' (length=4)
categoria1
sub1
sub2
sub3
sub1
categoria2
sub1
sub2
categoria3
sub1