Question

I have a database full of simple note data, with columns for title, due date, priority, and details. There is also a _id column PRIMARY KEY int.

Say I have a note in the table already with some data filled and the rest null. I also have a set of data that will fill all those fields.

Is there a way that I can only write data to the fields that are NULL?

I can’t overwrite existing data, but I’d like to add data to NULL columns.

I know the rowId of the target row.

If my target row had rowId of 5, I could do something like this:

UPDATE SET duedate='some date', priority='2', details='some text' WHERE _id=5

But that would overwrite all the data in that row, and I don’t want to lose any data that might be there. How can I change this statement to avoid writing to non-null fields?

Answer 1

Suppose you start with

CREATE TABLE "t" ("a" , "b" , "c" );
INSERT INTO "t" ("a", "c") VALUES (1, 3);

Then

update t set a = coalesce(a,9), b = coalesce(b,10), c = coalesce(c,11);

Will update only the null values, ie. only column B will be set to 10. A and C will be left alone because they contain values.

Coalesce means pick the first item in the list that is not null.