INSERT ON DUPLICATE KEY UPDATE

https://mariadb.com/kb/en/library/insert-on-duplicate-key-update/

Syntax

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
  [INTO] tbl_name [PARTITION (partition_list)] [(col,...)]
  {VALUES | VALUE} ({expr | DEFAULT},...),(...),...
  [ ON DUPLICATE KEY UPDATE
    col=expr
      [, col=expr] ... ]

Or:

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [PARTITION (partition_list)]
    SET col={expr | DEFAULT}, ...
    [ ON DUPLICATE KEY UPDATE
      col=expr
        [, col=expr] ... ]

Or:

INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [PARTITION (partition_list)] [(col,...)]
    SELECT ...
    [ ON DUPLICATE KEY UPDATE
      col=expr
        [, col=expr] ... ]

Contents

1. Syntax
2. Description
3. Examples
4. See Also

Description

INSERT ... ON DUPLICATE KEY UPDATE is a MariaDB/MySQL extension to the INSERT statement that, if it finds a duplicate unique or primary key, will instead perform an UPDATE.
The row/s affected value is reported as 1 if a row is inserted, and 2 if a row is updated, unless the API's CLIENT_FOUND_ROWS flag is set.
If more than one unique index is matched, only the first is updated. It is not recommended to use this statement on tables with more than one unique index.
If the table has an AUTO_INCREMENT primary key and the statement inserts or updates a row, the LAST_INSERT_ID() function returns its AUTO_INCREMENT value.
The VALUES() function can only be used in a ON DUPLICATE KEY UPDATE clause and has no meaning in any other context. It returns the column values from the INSERT portion of the statement. This function is particularly useful for multi-rows inserts.
The IGNORE and DELAYED options are ignored when you use ON DUPLICATE KEY UPDATE.

MariaDB starting with 10.0

The PARTITION clause was introduced in MariaDB 10.0. See Partition Pruning and Selection for details.

This statement activates INSERT and UPDATE triggers. See Trigger Overview for details.
See also a similar statement, REPLACE.

Examples

CREATE TABLE ins_duplicate (id INT PRIMARY KEY, animal VARCHAR(30));
INSERT INTO ins_duplicate VALUES (1,'Aardvark'), (2,'Cheetah'), (3,'Zebra');

If there is no existing key, the statement runs as a regular INSERT:

INSERT INTO ins_duplicate VALUES (4,'Gorilla') ON DUPLICATE KEY UPDATE animal='Gorilla';
Query OK, 1 row affected (0.07 sec)

SELECT * FROM ins_duplicate;
+----+----------+
| id | animal   |
+----+----------+
|  1 | Aardvark |
|  2 | Cheetah  |
|  3 | Zebra    |
|  4 | Gorilla  |
+----+----------+

A regular INSERT with a primary key value of 1 will fail, due to the existing key:

INSERT INTO ins_duplicate VALUES (1,'Antelope');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

However, we can use an INSERT ON DUPLICATE KEY UPDATE instead:

INSERT INTO ins_duplicate VALUES (1,'Antelope') ON DUPLICATE KEY UPDATE animal='Antelope';
Query OK, 2 rows affected (0.09 sec)

Note that there are two rows reported as affected, but this refers only to the UPDATE.

SELECT * FROM ins_duplicate;
+----+----------+
| id | animal   |
+----+----------+
|  1 | Antelope |
|  2 | Cheetah  |
|  3 | Zebra    |
|  4 | Gorilla  |
+----+----------+

Adding a second unique column:

ALTER TABLE ins_duplicate ADD id2 INT;
UPDATE ins_duplicate SET id2=id+10;
ALTER TABLE ins_duplicate ADD UNIQUE KEY(id2);

Where two rows match the unique keys match, only the first is updated. This can be unsafe and is not recommended unless you are certain what you are doing. Note that the warning shown below appears in MariaDB 5.5 and before, but has been removed in MariaDB 10.0, as MariaDB now assumes that the keys are checked in order, as shown in SHOW CREATE TABLE.

INSERT INTO ins_duplicate VALUES (2,'Lion',13) ON DUPLICATE KEY UPDATE animal='Lion';
Query OK, 2 rows affected, 1 warning (0.06 sec)

SHOW WARNINGS;
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                  |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1592 | Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT. INSERT... ON DUPLICATE KEY UPDATE  on a table with more than one UNIQUE KEY is unsafe |
+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

SELECT * FROM ins_duplicate;
+----+----------+------+
| id | animal   | id2  |
+----+----------+------+
|  1 | Antelope |   11 |
|  2 | Lion     |   12 |
|  3 | Zebra    |   13 |
|  4 | Gorilla  |   14 |
+----+----------+------+

Although the third row with an id of 3 has an id2 of 13, which also matched, it was not updated.
Changing id to an auto_increment field. If a new row is added, the auto_increment is moved forward. If the row is updated, it remains the same.

ALTER TABLE `ins_duplicate` CHANGE `id` `id` INT( 11 ) NOT NULL AUTO_INCREMENT;
ALTER TABLE ins_duplicate DROP id2;
SELECT Auto_increment FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='ins_duplicate';
+----------------+
| Auto_increment |
+----------------+
|              5 |
+----------------+

INSERT INTO ins_duplicate VALUES (2,'Leopard') ON DUPLICATE KEY UPDATE animal='Leopard';
Query OK, 2 rows affected (0.00 sec)

SELECT Auto_increment FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='ins_duplicate';
+----------------+
| Auto_increment |
+----------------+
|              5 |
+----------------+

INSERT INTO ins_duplicate VALUES (5,'Wild Dog') ON DUPLICATE KEY UPDATE animal='Wild Dog';
Query OK, 1 row affected (0.09 sec)

SELECT * FROM ins_duplicate;
+----+----------+
| id | animal   |
+----+----------+
|  1 | Antelope |
|  2 | Leopard  |
|  3 | Zebra    |
|  4 | Gorilla  |
|  5 | Wild Dog |
+----+----------+

SELECT Auto_increment FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME='ins_duplicate';
+----------------+
| Auto_increment |
+----------------+
|              6 |
+----------------+

Refering to column values from the INSERT portion of the statement:

INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
    ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);

See the VALUES() function for more.