MySQL REPLACE

Summary: in this tutorial, you will learn how to use the MySQL REPLACE statement to insert or update data in database tables.

Introduction to MySQL REPLACE statement

The MySQL REPLACE statement is an extension to the SQL Standard. The MySQL REPLACE statement works as follows:

Step 1. Insert a new row into the table, if a duplicate key error occurs.

Step 2. If the insertion fails due to a duplicate-key error occurs:

Delete the conflicting row that causes the duplicate key error from the table.
Insert the new row into the table again.

To determine whether the new row that already exists in the table, MySQL uses PRIMARY KEY or UNIQUE KEY index. If the table does not have one of these indexes, the REPLACE statement works like an INSERT statement.

To use the REPLACE statement, you need to have at least both INSERT and DELETE privileges for the table.

Notice that MySQL has the REPLACE string function which is not the REPLACE statement covered in this tutorial.

Using MySQL REPLACE to insert a new row

The following illustrates the syntax of the REPLACE statement:

REPLACE [INTO] table_name(column_list)
VALUES(value_list);Code language: SQL (Structured Query Language) (sql)

It is similar to the INSERT statement except for the keyword REPLACE. Let’s take a look at the following example of using the REPLACE statement to see how it works.

First, create a new table named cities as follows:

CREATE TABLE cities (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(50),
    population INT NOT NULL
);Code language: SQL (Structured Query Language) (sql)

Next, insert some rows into the cities table:

INSERT INTO cities(name,population)
VALUES('New York',8008278),
	  ('Los Angeles',3694825),
	  ('San Diego',1223405);Code language: SQL (Structured Query Language) (sql)

Then, query data from the cities table to verify the insert operation.

SELECT * FROM cities;Code language: SQL (Structured Query Language) (sql)

After that, use the REPLACE statement to update the population of the Los Angeles city to 3696820.

REPLACE INTO cities(id,population)
VALUES(2,3696820);Code language: SQL (Structured Query Language) (sql)

Finally, query the data of the cities table again to verify the replacement.

SELECT * FROM cities;Code language: SQL (Structured Query Language) (sql)

The value of the name column is NULL now.

The REPLACE statement works as follows:

First, the REPLACE statement attempted to insert a new row into cities the table. The insertion failed because the id 2 already exists in the cities table.
Then, the REPLACE statement deleted the row with id 2 and inserted a new row with the same id 2 and population 3696820. Because no value is specified for the name column, it was set to NULL.

Using MySQL REPLACE statement to update a row

The following illustrates how to use the REPLACE statement to update data:

REPLACE INTO table
SET column1 = value1,
    column2 = value2;Code language: SQL (Structured Query Language) (sql)

This statement is like the UPDATE statement except for the REPLACE keyword. In addition, it has no WHERE clause.

This example uses the REPLACE statement to update the population of the city Phoenix to 1768980:

REPLACE INTO cities
SET id = 4,
    name = 'Phoenix',
    population = 1768980;Code language: SQL (Structured Query Language) (sql)

Unlike the UPDATE statement, if you don’t specify the value for the column in the SET clause, the REPLACE statement will use the default value of that column.

SELECT * FROM cities;Code language: SQL (Structured Query Language) (sql)

Using MySQL REPLACE to insert data from a SELECT statement

The following illustrates the REPLACE statement that inserts data into a table with the data coming from a query.

REPLACE INTO table_1(column_list)
SELECT column_list
FROM table_2
WHERE where_condition;Code language: SQL (Structured Query Language) (sql)

Note that this form of the REPLACE statement is similar to INSERT INTO SELECT statement.

The following statement uses the REPLACE INTO statement to copy a row within the same table:

REPLACE INTO 
    cities(name,population)
SELECT 
    name,
    population 
FROM 
   cities 
WHERE id = 1;Code language: SQL (Structured Query Language) (sql)

In this tutorial, you’ve learned different forms of the MySQL REPLACE statement to insert or update data in a table.

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