Merge 2 columns with numbers and NULL cells (QGIS)

Question

I have an attribut table with 2 columns like this :
Column A Column B
22 NULL
23 NULL
24 NULL
25 NULL
26 NULL
NULL 52
NULL 53
NULL 54
NULL 55

I want to take the numbers of B (52 to 55) and replace the NULLs of the column A with these numbers like this :
COLUMN A
22
23
24
25
26
52
53
54

Aggregate or concatenate or & or + doesn't work in the field calculator because there are NULL in the process.

Answer 1

Create a new field with an expression:

coalesce( "Column A" , "Column B" )

Then rename the field to COLUMN A.

Answer 2

Use this expression to update COLUMN A for all features:

IF( "COLUMN A" IS NOT NULL, "COLUMN A" , "COLUMN B" )

Result: