Messing with 'when not matched' in SQL (how to merge properly) - sql

Question

I'm creating several tables and merging data from them into one. These tables have similar columns col1 and col2 but the values in these columns can be different, ex. col1 on row 1 from tab1 may have "0" and col1 from tab2 may have on the same row in col1 "1". ONLY in this case I need to update the value from tab1 in col1 on the row 1. Take a look at the following example:
BEFORE MERGING
tab1:
id col1 col2
1 0 0
2 0 1
3 0 0
AFTER MERGING WITH tab2
tab2: tab1:
id col1 col2 id col1 col2
1 1 0 ---> 1 1 0
2 0 0 2 0 1
3 0 0 3 0 0
For that purpose I use the following query:
merge into tab1 st
using (select id, col1, col2 from tab2) ss
ON (st.id=ss.id)
when matched then
update set st.col1 = ss.col1,
st.col2 = ss.col2
where ss.col1 != 0 or ss.col2 != 0
when not matched then
insert (st.id, st.col1, st.col2)
values (ss.id, ss.col1, ss.col2)
;
I'm just learning sql so only now I noticed that this query is not correct. It can overwrite the "1" value in the tab1 with "0" coming from tab2. Help me to improve this query please, I'm stuck.

Answer 1

You can use a CASE expression when setting the values to control which value gets assigned. Try something like:
merge into tab1 st
using (select id, col1, col2 from tab2) ss
ON (st.id=ss.id)
when matched then
update set st.col1 = CASE WHEN st.col1 = 0 THEN ss.col1 ELSE st.col1 END,
st.col2 = CASE WHEN st.col2 = 0 THEN ss.col2 ELSE st.col2 END
where ss.col1 != 0 or ss.col2 != 0
when not matched then
insert (st.id, st.col1, st.col2)
values (ss.id, ss.col1, ss.col2)
;
Thus in the WHEN MATCHED situation, if the value from tab1 (st) is zero you change it to the value from tab2 (ss), otherwise you just set the value from tabl1 to itself.
Share and enjoy.

Answer 2

You should update with the max of {ss.coln, st.coln}. For example, using GREATEST:
MERGE INTO tab1 st
USING (SELECT id, col1, col2 FROM tab2) ss
ON (st.id = ss.id)
WHEN MATCHED THEN
UPDATE
SET st.col1 = greatest(ss.col1, st.col1),
st.col2 = greatest(ss.col2, st.col2)
WHERE ss.col1 > st.col1
OR ss.col2 > st.col2
WHEN NOT MATCHED THEN
INSERT (st.id, st.col1, st.col2)
VALUES (ss.id, ss.col1, ss.col2);