TableB as posted doesn't have a primary key. Please post the proper DDL
(CREATE TABLE statements) for the tables.
Explain what differentiates these two rows in TableB:
(1, m, NULL(1))
(1, m, NULL)
how do you define which one to UPDATE?
Possibly the UPDATE statement will look something like this:
SET col3 = 1
WHERE col1 = TableB.col1 AND col2 = TableB.col2)
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