For the "general" solution, I'd suggest that you let the database engine handle it, so you can avoid race conditions, etc. You can give it to the database engine by writing an appropriate stored procedure, or a simple if statement. Most database engines I'm aware of will let you do something similar to:

if exists (select * from table where PK='value')
   update table set col1=val1, col2=val2 where PK='value'
else
   insert table (PK, col1, col2)
   values ('value', val1, val2)
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