Summary: in this tutorial, you will learn how to use MySQL UPDATE JOIN statement to perform cross-table update. We will show you step by step how to use INNER JOIN clause and LEFT JOIN clause with the UPDATE statement.

MySQL UPDATE JOIN syntax

You often use JOIN clauses to query records in a table that have (in case of INNER JOIN) or do not have (in case of LEFT JOIN) corresponding records in another table. In MySQL, you can use the JOIN clauses in the UPDATE statement to perform cross-table update.

The syntax of the MySQL UPDATE JOIN is as follows:

UPDATE T1, T2, [INNER JOIN | LEFT JOIN] T1 ON T1.C1 = T2. C1 SET T1.C2 = T2.C2,      T2.C3 = expr WHERE condition

Let’s examine the MySQL UPDATE JOIN syntax in greater detail:

• First, you specify the main table ( T1) and the table that you want the main table to join to ( T2) after the UPDATE clause. Notice that you must specify at least one table after the UPDATE clause. The data in the table that is not specified after the UPDATE clause is not updated.

• Second, you specify a kind of join you want to use i.e., either INNER JOIN or LEFT JOIN and a join condition. Notice that the JOINclause must appear right after the UPDATE clause.

• Third, you assign new values to the columns in T1 and/or T2 tables that you want to update.

• Fourth, the condition in the WHERE clause allows you to limit the rows to update.

If you follow the UPDATE statement tutorial, you notice that there is another way to update data cross-table using the following syntax:

UPDATE T1, T2 SET T1.c2 = T2.c2,       T2.c3 = expr WHERE T1.c1 = T2.c1 AND condition

This UPDATE statement works the same as UPDATE JOIN with implicit INNER JOIN clause. It means you can rewrite the above statement as follows:

UPDATE T1,T2 INNER JOIN T2 ON T1.C1 = T2.C1 SET T1.C2 = T2.C2,       T2.C3 = expr WHERE condition

Let’s take a look at some examples of using the UPDATE JOIN statement to having a better understanding.

MySQL UPDATE JOIN examples

We are going to use a new sample database in these examples. The sample database contains 2 tables:

• employees table stores employee data with employee id, name, performance and salary.

• merits table stores performance and merit’s percentage.

The SQL script for creating and loading data in this sample database is as follows:

CREATE DATABASE IF NOT EXISTS empdb; -- create tables CREATE TABLE merits (   performance INT(11) NOT NULL,   percentage FLOAT NOT NULL,   PRIMARY KEY (performance) ); CREATE TABLE employees (   emp_id INT(11) NOT NULL AUTO_INCREMENT,   emp_name VARCHAR(255) NOT NULL,   performance INT(11) DEFAULT NULL,   salary FLOAT DEFAULT NULL,   PRIMARY KEY (emp_id),   CONSTRAINT fk_performance   FOREIGN KEY(performance)    REFERENCES merits(performance) ); -- insert data for merits table INSERT INTO merits(performance,percentage) VALUES(1,0),       (2,0.01),       (3,0.03),       (4,0.05),       (5,0.08); -- insert data for employees table INSERT INTO employees(emp_name,performance,salary)       VALUES('Mary Doe', 1, 50000), ('Cindy Smith', 3, 65000), ('Sue Greenspan', 4, 75000), ('Grace Dell', 5, 125000), ('Nancy Johnson', 3, 85000), ('John Doe', 2, 45000), ('Lily Bush', 3, 55000);

MySQL UPDATE JOIN example with INNER JOIN clause

Suppose you want to adjust the salary of employees based on their performance. The merit’s percentages are stored in the merits table therefore you have to use UPDATE INNER JOIN statement to adjust the salary of employees in the employees table based on the percentage stored in the merits table. The link between the employees and merit tables is performance field. See the following query:

UPDATE employees INNER JOIN merits ON employees.performance = merits.performance SET salary = salary + salary * percentage

How the query works.

• We specify only the employees table after UPDATE clause because we want to update data in the  employees table only.

• For each employee record in the employees table, the query checks the its performance value against the performance value in the merits table. If it finds a match, it gets the percentage in the merits table and update the salary column in the employees table.

• Because we omit the WHERE clause in the UPDATE statement, all the records in the employees table get updated.

MySQL UPDATE JOIN example with LEFT JOIN

Suppose the company hires two more employees:

INSERT INTO employees(emp_name,performance,salary) VALUES('Jack William',NULL,43000), ('Ricky Bond',NULL,52000);

Because these employees are new hires so their performance data is not available or NULL.

To increase the salary for new hires, you cannot use the UPDATE INNER JOIN statement because their performance data is not available in the merit table. This is why the UPDATE LEFT JOIN comes to the rescue.

The UPDATE LEFT JOIN statement basically updates a record in a table when it does not have a corresponding record in another table. For example, you can increase the salary for a new hire by 1.5%  using the following statement:

UPDATE employees LEFT JOIN merits ON employees.performance = merits.performance SET salary = salary + salary * 0.015; WHERE merits.percentage IS NULL

In this tutorial, we have shown you how to use MySQL UPDATE JOIN with INNER JOIN and LEFT JOIN to perform cross-table update.

原文链接：http://outofmemory.cn/mysql/mysql-update-join