来源:https://www.mysqltutorial.org/mysql-insert-or-update-on-duplicate-key-update/

Introduction to the MySQL INSERT ON DUPLICATE KEY UPDATE statement

The INSERT ON DUPLICATE KEY UPDATE is a MySQL’s extension to the SQL standard’s INSERT statement.

When you insert a new row into a table if the row causes a duplicate in UNIQUE index or PRIMARY KEY , MySQL will issue an error.

However, if you specify the ON DUPLICATE KEY UPDATE option in the INSERT statement, MySQL will update the existing row with the new values instead.

The syntax of INSERT ON DUPLICATE KEY UPDATE statement is as follows:

INSERT INTO table (column_list)
VALUES (value_list)
ON DUPLICATE KEY UPDATE
   c1 = v1, 
   c2 = v2,
   ...;

The only addition to the INSERT statement is the ON DUPLICATE KEY UPDATE clause where you specify a list of column-value-pair assignments in case of duplicate.

Basically, the statement first tries to insert a new row into the table. If a duplicate error occurs, it will update the existing row with the value specified in the ON DUPLICATE KEY UPDATE clause.

MySQL returns the number of affected-rows based on the action it performs:

• If the new row is inserted, the number of affected-rows is 1.
• If the existing row is updated, the number of affected-rows is 2.
• If the existing row is updated using its current values, the number of affected-rows is 0.

To use the values from the INSERT clause in the DUPLICATE KEY UPDATE clause, you use the VALUES()function as follows:

INSERT INTO table_name(c1)
VALUES(c1)
ON DUPLICATE KEY UPDATE c1 = VALUES(c1) + 1;

The statement above sets the value of the c1 to its current value specified by the expression VALUES(c1) plus 1 if there is a duplicate in UNIQUE index or PRIMARY KEY.

MySQL INSERT ON DUPLICATE KEY UPDATE example

Let’s take a look at an example of using the INSERT ON DUPLICATE KEY UPDATE to understand how it works.

First, create a table named devices to store the network devices.

CREATE TABLE devices (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(100)
);

Next, insert rows into the devices table.

INSERT INTO devices(name)
VALUES('Router F1'),('Switch 1'),('Switch 2');

Then, query the data from the devices table to verify the insert:

SELECT 
    id, 
    name
FROM    
    devices;

Now, we have three rows in the devices table.

After that, insert one more row into the devices table.

INSERT INTO 
   devices(name) 
VALUES 
   ('Printer') 
ON DUPLICATE KEY UPDATE name = 'Printer';

Because there is no duplicate, MySQL inserts a new row into the devices table. The statement above has the same effect as the following statement:

INSERT INTO devices(name) 
VALUES ('Printer');

Finally, insert a row with a duplicate value in the id column.

INSERT INTO devices(id,name) 
VALUES 
   (4,'Printer') 
ON DUPLICATE KEY UPDATE name = 'Central Printer';

MySQL issues the following message:

2 row(s) affected

Because a row with id 4 already exists in the devices table, the statement updates the name from Printer to Central Printer.

In this tutorial, you have learned how to insert or update data in a table using the ON DUPLICATE KEY UPDATE option of the INSERT statement.