INSERT INTO … ON DUPLICATE KEY UPDATE …. with condition? - mysql

Question

I am still wondering if there is something like a conditional on duplicate update in MySQL 5.7
I have a table which is updated by different sources.
Let’s assume I have a table
CREATE TABLE t
(
name VARCHAR(100),
value INT,
last update DATETIME
)
I have 3 rows
name
value
lastupdate
a
10
2021-01-01
b
20
2021-02-01
c
30
2021-03-01
Now I have some data to be imported
name
value
lastupdate
a
20
2021-01-01
b
40
2021-01-01
c
60
2021-04-01
The result of the query should be
name
value
lastupdate
a
20
2021-01-01
b
20
2021-02-01
c
60
2021-03-01
Can this be done by one insert query or must I check first if the last update of the existing data in the table is newer then the date of the import data?

Answer 1

Assuming that name is the PRIMARY KEY of the table or is defined as UNIQUE, you can use a CASE expression:
INSERT INTO t (name, value, lastupdate) VALUES
('a', 20, '2021-01-01'),
('b', 40, '2021-01-01'),
('c', 60, '2021-04-01')
ON DUPLICATE KEY UPDATE
value = CASE WHEN VALUES(lastupdate) >= lastupdate THEN VALUES(value) ELSE value END;
See the demo.
Note that (from INSERT ... ON DUPLICATE KEY UPDATE Statement):
The use of VALUES() to refer to the new row and columns is deprecated
beginning with MySQL 8.0.20, and is subject to removal in a future
version of MySQL. Instead, use row and column aliases, as described in
the next few paragraphs of this section.
So, if your version of MySql is 8.0.20+ it is recommended to use an alias instead of VALUES():
INSERT INTO t (name, value, lastupdate) VALUES
('a', 20, '2021-01-01'),
('b', 40, '2021-01-01'),
('c', 60, '2021-04-01') AS new
ON DUPLICATE KEY UPDATE
t.value = CASE WHEN new.lastupdate >= t.lastupdate THEN new.value ELSE t.value END;
See the demo.