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I am working with QGIS 3.10 and would like to create a field in the attribute table which is a unique ID for a feature. I have read that using the @row_number as the expression should provide this capability, but the resulting value is always NULL (both in the Preview and once a feature is created). If I select the field from the Attribute table and apply the @row_number in the expression box and select 'Update All' the table is correctly updated with the row numbers. How can I get the correct row number to show up when the feature is selected and then displayed in the Attribute table?

imre
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mjldbr
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  • Can you explain better your problem? Are you trying to create an ID field that updates automatically when you create a new feature? You can create it with the expression if (maximum("id") is NULL, 1 , maximum("id") + 1) Check this post : https://gis.stackexchange.com/questions/132346/is-there-a-way-of-auto-increment-for-the-id-column-in-qgis. In particular the second answer. – Val P Apr 27 '20 at 20:09
  • Yes, you understand the problem. And your suggestion does work but if the feature is updated, the ID will change. This is why I was hoping to use @row_number. Once assigned, I do not want this number to change. – mjldbr Apr 27 '20 at 20:41
  • New features have NULL for this field. But the Preview looks correct when I am updating the field in the editor. – mjldbr Apr 27 '20 at 21:03
  • With this system, the number shouldn't change. Because you are referring to the attribute already recorded and not to the variable. You can call your field in any way, e.g. "Field_ID" and use this name instead "id" in the expression. Only ], be sure that the option Apply default value on update is not selected. – Val P Apr 27 '20 at 21:04
  • New features always will have NULL using @row_number, because the system can not calculate the row before that the new feature is created. – Val P Apr 27 '20 at 21:07
  • When I unselected the 'Apply default value on update' option, the number was as I had hoped - and did not change from edits! There was a warning when this was unchecked, but apparently it did not matter. Thank you for your help! – mjldbr Apr 27 '20 at 21:11
  • You're welcome. I have created an answer with the info from our chat in the comments. – Val P Apr 27 '20 at 21:33

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To achieve your request you need to create, on the layer Proprieties-> Attribute Form, a default value by expression.

You can use the expression 

IF (maximum("field_id") is NULL, 1 , maximum("field_id") + 1)

where field_id is the name of your autoincremental field.

Also, select the Constrains Not null and Unique.

enter image description here

In this way, for any new feature, you will have a new number autoincrementate by 1. The first part of the expression helps to avoid to have a null number with the first feature created when the field is still empty.

NB. To avoid the number changing, be sure that the option Apply default value on update is not selected.

Val P
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