Selecting top two features with longest length in the line vector file in QGIS

Question

I have a line vector file and I need to select two lines with the longest length using expression. I am familiar with selecting the line with the longest length:

length($geometry) = maximum(length($geometry))

But how to select the longest and second-longest line using an expression?

Answer 1

Use this expression:

$length >= array_sort(array_agg($length), 0)[1]

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Explanation

1. Create an array of the length of each line with array_agg() and $length
2. Sort the value by a length with array_sort() with the ascending parameter set to 0 (false), thus ordering in descending order
3. Get the second element (second longest line): as array-index starts with 0 for the first element, use the index operator [1]
4. Check if the current feature's length ($length) is equal or longer (>=) than this second-longest line