MySQL UPDATE

 MySQL UPDATE

Summary: updating data is one of the most important tasks when you work with the database. In this tutorial, you will learn how to use the MySQL UPDATE statement to update data in a table.

Introduction to MySQL UPDATE statement

The UPDATE statement updates data in a table. It allows you to change the values in one or more columns of a single row or multiple rows.

The following illustrates the basic syntax of the UPDATE statement:

UPDATE [LOW_PRIORITY] [IGNORE] table_name 
SET 
    column_name1 = expr1,
    column_name2 = expr2,
    ...
[WHERE
    condition];

In this syntax:

• First, specify the name of the table that you want to update data after the UPDATE keyword.
• Second, specify which column you want to update and the new value in the SET clause. To update values in multiple columns, you use a list of comma-separated assignments by supplying a value in each column’s assignment in the form of a literal value, an expression, or a subquery.
• Third, specify which rows to be updated using a condition in the WHERE clause. The WHERE clause is optional. If you omit it, the UPDATE statement will modify all rows in the table.

Notice that the WHERE a clause is so important that you should not forget. Sometimes, you may want to update just one row; However, you may forget the WHERE clause and accidentally update all rows of the table.

MySQL supports two modifiers in the UPDATE statement.

1. The LOW_PRIORITY modifier instructs the UPDATE statement to delay the update until there is no connection reading data from the table. The LOW_PRIORITY takes effect for the storage engines that use table-level locking only such as MyISAM, MERGE, and MEMORY.
2. The IGNORE modifier enables the UPDATE statement to continue updating rows even if errors occurred. The rows that cause errors such as duplicate-key conflicts are not updated.

MySQL UPDATE examples

Let’s practice the UPDATE statement.

1) Using MySQL UPDATE to modify values in a single column example

See the following employees table from the sample database.

In this example, we will update the email of Mary Patterson to the new email mary.patterso@classicmodelcars.com.

First, find Mary’s email from the employees table using the following SELECT statement:

SELECT 
    firstname, 
    lastname, 
    email
FROM
    employees
WHERE
    employeeNumber = 1056;

Second, update the email address of Mary to the new email mary.patterson@classicmodelcars.com :

UPDATE employees 
SET 
    email = 'mary.patterson@classicmodelcars.com'
WHERE
    employeeNumber = 1056;

MySQL issued the number of rows affected:

1 row(s) affected

In this UPDATE statement:

• The WHERE the clause specifies the row with employee number 1056 will be updated.
• The SET the clause sets the value of the email column to the new email.

Third,  execute the SELECT the statement again to verify the change:

SELECT 
    firstname, 
    lastname, 
    email
FROM
    employees
WHERE
    employeeNumber = 1056;

2) Using MySQL UPDATE to modify values in multiple columns

To update values in the multiple columns, you need to specify the assignments in the SET clause. For example, the following statement updates both the last name and email columns of employee number 1056:

UPDATE employees 
SET 
    lastname = 'Hill',
    email = 'mary.hill@classicmodelcars.com'
WHERE
    employeeNumber = 1056;

Let’s verify the changes:

SELECT 
    firstname, 
    lastname, 
    email
FROM
    employees
WHERE
    employeeNumber = 1056;

3) Using MySQL UPDATE to replace string example

The following example updates the domain parts of emails of all Sales Reps with office code 6:

UPDATE employees
SET email = REPLACE(email,'@classicmodelcars.com','@mysqltutorial.org')
WHERE
   jobTitle = 'Sales Rep' AND
   officeCode = 6;

In this example, the REPLACE() function replaces @classicmodelcars.com in the email column with @mysqltutorial.org.

4) Using MySQL UPDATE to update rows returned by a SELECT statement example

You can supply the values for the SET clause from a SELECT a statement that queries data from other tables.

For example, in the customers table, some customers do not have any sales representatives. The value of the column saleRepEmployeeNumber is NULL as follows:

SELECT 
    customername, 
    salesRepEmployeeNumber
FROM
    customers
WHERE
    salesRepEmployeeNumber IS NULL;

We can take a sales representative and update those customers.

To do this, we can select a random employee whose job title is Sales Rep from the  employees table and update it for the  employees table.

This query selects a random employee from the table employees whose job title is the Sales Rep.

SELECT 
    employeeNumber
FROM
    employees
WHERE
    jobtitle = 'Sales Rep'
ORDER BY RAND()
LIMIT 1;

To update the sales representative employee number  column in the customers table, we place the query above in the SET clause of the UPDATE a statement as follows:

UPDATE customers 
SET 
    salesRepEmployeeNumber = (SELECT 
            employeeNumber
        FROM
            employees
        WHERE
            jobtitle = 'Sales Rep'
        ORDER BY RAND()
        LIMIT 1)
WHERE
    salesRepEmployeeNumber IS NULL;

If you query data from the  employees table, you will see that every customer has a sales representative. In other words, the following query returns no row.

SELECT 
     salesRepEmployeeNumber
FROM
    customers
WHERE
    salesRepEmployeeNumber IS NULL;

In this tutorial, you have learned how to use MySQL UPDATE statement to update data in a database table.