Create New Feature_ID following Series of numbers

Question

I have two datasets of data let say set_1 and set_2. set_1 is original dataset and set_2 is modified dataset (duplicates and wrong data has been removed from the data). Due to the modification some of the data has been removed from the set_2. Now i am adding few more new features to the data. so i want new ids for the new features.

if i use $id the features id's i have already deleted in set_2 will again regenerated. if i vlook up data from the set_1 in case the data wont join correctly. thats why i dont want any of the id which are in the set_1 should not repeat. thats why i want new ids for my new features................hope you understand my question

Answer 1

I don't know how you created your data. However, the behaviour is not normal. Normally, you should not need to care about the id this way, as QGIS would handle this automatically the way you want. Saving your data as Geopackage, you automatically get a fid that exactly does what you describe.

So you probably miss a point here, but you don't provide enough information to see what goes wrong. Still, to answer your question in case you really want to continue to do this manually using the following expression (see below for explanation). Be aware: in my case, fid is the name of the id-value in Set_1; in Set_2 it's called id (change this accordingly):

if (
    "id" is NULL,
    aggregate ('Set_1', 'max', "fid") +
    @row_number -
    array_length ( 
        aggregate( 
            @layer,
            'array_agg',
            "id", 
            filter:= "id" is not NULL
        )
    ),
    "id"
)

1. Find the largest existing id value in Set_1 with aggregate( 'Set_1','max',fid). In your screenshot, this would return 8.

2. Find the number of remaining features (rows, lines) that already have an id value in Set_2 with array_length (array_agg ("id", filter:= "id" is not NULL)). Returns 6 in your example.

3. Now for each of the additional rows (lines 7, 8,...), calculate a sequential series of how much you have to add: add 1 for line 7, 2 for line 8 etc. To calculate this, use @row_number - [result from step 2] - replace the part in angular braces [] with the expression from step 2.

4. You want to start with id=9 (next larger value from step 1) in line 7 (next feature from step 2). Thus add value from step 1 with value from step 3.

5. You want to update only id's that are empty, but keep pre existing values

Answer 2

1. Select the new features. You can use Select by Expression with fid is NULL.

2. Run Field calculator and check the box next to Only update [n] selected features.

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You could even update $id for all fid fields, including those that already have a fid value. If this pre existing fid is based on $id, it will return the same value as before.