MySQL primary key and unique key repetition usage

Table name Heyf_t10, which is used to test the statement

INSERT into Heyf_t10 (Empid, DeptID, salary)
VALUES
(1, 2, 3) On DUPLICATE KEY UPDATE empid = 1,
DeptID = 2,
Salary = 5

The original data is

, the unique key is set, then executes the above SQL, because it touches a unique key

If the unique key is set, the original data and the resulting data are as follows, because the Union is a single key and must be repeated before it goes into the UPDATE statement, which is the federated unique key

If you set the primary key, the effect is the same as the federated unique key

MySQL primary key and unique key repetition usage