When using case when in QGIS 3, even without else, all other previously filled cells are replaced by NULL

Question

I would like to update a column with different strings based on other strings in another column.
For instance : if A, B, C in column 1, write Cat1 in column 2. So I use "case when". But even without writing 'else' when it always replace all other values by NULL. Which means that if I then want to update again the column with "if X, Y, Z in column 1, write Cat2 in column 2", "Cat1" gets deleted in all the cells where it had been added. If I don't want to update all cells at the same time, how can I conditionally update certain cells while ignoring others (i.e. not deleting their content)?

Erik · Accepted Answer

if("Column 1" IN ('X','Y','Z'),'Cat2',"Column 2")
This checks whether the specified condition Column 1 is like X, Y, or Z is fulfilled and then either writes Cat2 into Column 2, or uses preexisting values or Column 2 otherwise.
You could also chain several if statements, or run a comprehensive case when when when when expression.

Similar to your question from earlier:
CASE
   WHEN "l_co" in ('Romainville','Bagnolet' ,'Bobigny','Bondy' ,'Les Lilas','Montreuil','Noisy-le-Sec' ,'Le Pré-Saint-Gervais','Pantin')
   THEN 'Est Ensemble'
   WHEN "l_co" in (several other city names)
   THEN 'Plaine Commune group'
   ELSE "EPCI"
END

M Bain · Answer

Why not not just sort and select, or filter, on the values you want to update and use "update selected" or "update filtered"?

When using case when in QGIS 3, even without else, all other previously filled cells are replaced by NULL

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