Creating an SSH Tunnel for MySQL Remote Access

Installing MySQL on Debian effectively creates a robust and flexible database (DB) infrastructure that accommodates a wide range of applications as well as services. It is renowned for its scalability, dependability, and durability. By setting it, individuals experience the operations in an efficient manner and enhance the overall efficiency of DB infrastructure. This combination is especially beneficial for administrators, DB analysts, and industries that demand a dependable database solution for dealing with huge data. Additionally, MySQL's comprehensive guide and supporters help make it simpler to troubleshoot problems and enhance operations.  In this guide, we will demonstrate the thorough procedure for installing and configuring MySQL on Debian. How to Install MySQL on Debian The default repositories do not contain the MySQL database server package on Debian. To install it on a  Linux system follow the below instructions. We will download the recent version of the MySQL. Step 1: Download MySQL Package Let us obtain the MySQL repository information package, which is in the .deb format: wget https://dev.mysql.com/get/mysql-apt-config_0.8.30-1_all.deb Note: To authenticate the most updated release, go to the MySQL repository webpage. Step 2: MySQL Configuration Package Installation Then, employ the .deb file for initializing the installation via dpkg: sudo dpkg -i mysql-apt-config_0.8.30-1_all.deb Respond to the prompt. For instance, pick MySQL Server & Cluster and hit Enter for starting configurations: For picking a version such as (mysql-8.4-lts), scroll downward and hit OK for the next step: Step 3: Refresh the System Now, update the server's package indexes to implement the updated MySQL info: sudo apt update Step 4: MySQL Installation Debian's default manager makes sure to install MySQL in an easier manner. Installing the package with this command: sudo apt install mysql-server -y You will see the interface for setting the root account. Input a stronger password to secure the database. In the end, hit the Ok button: Check the version on the server via the --version utility: mysql --version Step 5: Managing the Services Now, you can enable the MySQL service to initialize automatically at boot time: sudo systemctl enable mysql Activate the service via the systemctl utility: sudo systemctl start mysql Check if the system service is operational by viewing its status: sudo systemctl status mysql Step 6: MySQL Secure Installation The key or password that the individual created at the initialising procedure is currently protecting the root DB user on the server. MySQL also includes other insecure defaults, such as remote access to test databases and the root database user on the server.  It is vital to secure the MySQL installation after it has been completed as well as disable all unsafe default settings. There is a security script that can assist us in this procedure. Run the script: sudo mysql_secure_installation To activate the VALIDATE PASSWORD component and guarantee stringent password procedures, type Y and hit Enter. Next, you will need to configure several security settings: Set the Root Password: Select a strong password and make sure that it is correct. Configure the password policy for the DB server. For instance, type 2 to permit only the strong passwords on the server and hit Enter. When required to modify the root password, input N; alternatively, input Y to modify the password. Eliminate Anonymous Users: It is advised to eliminate the accessibility of anonymous users. For this, input Y and Enter when prompted. Prevent Accessibility of Remote Root: It is a better practice to avoid remote root login for multiple security concerns. To prevent the root user from having a remote access, input Y and hit Enter. Delete the Test DB: For enhancing security, the test database, which is utilized for testing, can be deleted. To do so, input Y and hit Enter. Refreshing Privilege Tables: It guarantees that all modifications are implemented instantly. To implement the configuration and edit the privileges table, hit Enter. Step 7: Access MySQL Utilizing the mysql client utility, MySQL establishes the connection and provides access to the database server console.  Now, access the shell interface and run general statements on the DB server. Let’s input the root and the password created at the time of the safe installation procedure: sudo mysql -u root -p Step 8: Basic MySQL Operations The creation of a DB and a new user for your applications rather than utilizing the root is a better practice. To accomplish the task, employ the given instructions: Create a Database: First, create a database. For instance, hostmandb is created via the below command: CREATE DATABASE hostmandb; Display All Databases: List all databases to make sure hostmandb is created: SHOW DATABASES; Create of a New User: Create a user and assign a strong password. In our example, we set Qwer@1234 as a password for the user  minhal. Replace these values with your data. CREATE USER 'minhal'@'localhost' IDENTIFIED BY 'Qwer@1234'; Give Permissions to the User: Give complete access to the hostmandb to the new user: GRANT ALL PRIVILEGES ON hostmandb.* TO 'minhal'@'localhost'; Flush Privileges: To implement the modifications, refresh the table: FLUSH PRIVILEGES; Exit the Shell: For closing the interface, utilize the EXIT statement: EXIT; Access MySQL Console as the Particular User For the purpose of testing hostmandb access, log in to MySQL as the new user, in our case minhal. sudo mysql -u minhal -p It accesses the console after entering the minhal user password when prompted: For verification, display all DBs and confirm that the hostmandb is available: SHOW DATABASES; Step 9: Configuration for Remote Access Setting up the server for supporting remote accessibility is necessary if an individual is required to access MySQL remotely. Follow these steps: Access the mysql.cnf file and modify the particular file for MySQL: sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf Look for the line with the bind-address and change it to: bind-address = 0.0.0.0 Reload the MySQL service: sudo systemctl restart mysql Permit the user to have remote access: sudo mysql -u root -p GRANT ALL PRIVILEGES ON hostmandb.* TO 'minhal'@'localhost';FLUSH PRIVILEGES;EXIT; Step 10: Firewall Configuration If you have a firewall activated, you need to open the MySQL port 3306 to traffic. Set up the firewall following the below steps: Allow traffic through MySQL: sudo ufw allow mysql Now, activate the UFW on the system: sudo ufw enable Reload the firewall: sudo ufw reload Step 11: Restore and Backup Maintaining regular backups is crucial to avoiding data loss. The mysqldump utility is provided by MySQL for backup creation. To achieve this, consider these instructions: Backup a Single Database: This command employs mysqldump to create the backup of the hostmandb as a hostmandb_backup.sql file: sudo mysqldump -u root -p hostmandb> hostmandb_backup.sql Backup All Databases: For creating a backup of all databases as a file named all_databases_backup.sql with root privileges, utilize mysqldump: sudo mysqldump -u root -p --all-databases > all_databases_backup.sql Restore a Particular Database: Now, restore the hostmandb from the backup file hostmandb_backup.sql: sudo mysql -u root -p hostmandb < hostmandb_backup.sql Step 12: Optimize MySQL Operations (Optional) Depending on the workload and server resources, you can adjust settings to guarantee peak performance. These instructions will help you maximize MySQL's speed: Adjust InnoDB Buffer Pool Size: Caches for data and indexes are kept in the InnoDB buffer pool. Expanding its size can enhance its functionality. Edit the MySQL configuration file: sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf The below line should be added or changed: innodb_buffer_pool_size = 1G Its size should be adjusted according to the amount of memory on the server. Enable Query Cache: The query cache stores the outcome of SELECT queries. Enabling it can enhance operations for repetitive queries. Modify the .cnf file: sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf Add or edit the below lines: query_cache_type = 1query_cache_size = 64M Optimize Table Structure: Frequently optimize your customers table in hostmandb to recover wasted space and boost efficiency: USE hostmandb;OPTIMIZE TABLE customers; Analyze Operations: DB operations can be tracked and analyzed with tools like MySQL Workbench and mysqltuner. Using the command below, install mysqltuner: sudo apt install mysqltuner Run mysqltuner to get performance recommendations: sudo mysqltuner Conclusion Installing a MySQL environment is important in today's digital world. By following this instruction, you'll be able to safely install and connect to your MySQL database. This strategy not only increases security but also improves remote database maintenance efficiency. It helps to prevent breaches and ensures the confidentiality of your data. This article has given thorough instructions for the installation of MySQL's database environment on Debian. It is suggested that MySQL servers should be regularly monitored and optimized to guarantee optimum performance and dependability. In addition, Hostman offers pre-configured and ready-to-use cloud databases, including cloud MySQL. 

SQL triggers are a vital component of many database systems, allowing automated execution of specific actions when predefined events occur. Triggers act as responsive mechanisms within a database, ensuring consistency and enabling automation of repetitive tasks. These event-driven procedures are particularly effective for handling operations triggered by changes such as  INSERT,  UPDATE, or DELETE in a table. By using triggers, database administrators and developers can enforce rules, maintain logs, or even invoke complex processes with minimal manual intervention. Let’s begin by defining an example database for a small online store to understand how triggers work in practice: -- Let’s create a databse called SHOP ; CREATE DATABASE SHOP ; USE SHOP ; -- Now we create the Products table CREATE TABLE Products ( ProductID INT PRIMARY KEY, ProductName VARCHAR(100), Stock INT, Price DECIMAL(10, 2) ); -- Then the StockAudit table CREATE TABLE StockAudit ( AuditID INT AUTO_INCREMENT PRIMARY KEY, ProductID INT, ChangeType VARCHAR(10), QuantityChanged INT, ChangeTimestamp TIMESTAMP DEFAULT CURRENT_TIMESTAMP ); Classification of SQL Triggers SQL triggers can be classified based on their scope and timing. Row-level triggers are executed once for every row affected by a database operation, making them adequate for detailed tracking of data changes. For example, when updating inventory quantities for multiple products, a row-level trigger can record changes for each product individually. Conversely, statement-level triggers run once for an entire operation, regardless of how many rows are affected. These are useful for performing global checks or logging summary information. Triggers can also be categorized by their execution timing relative to the triggering event. Before triggers are executed prior to the event, often to validate or modify data before it is written to the database. After triggers execute after the event, making them ideal for tasks such as auditing or enforcing referential integrity. This is an example of a row-level AFTER INSERT trigger which logs new product additions: -- The DELIMITER command is used to change the statement delimiter from ; to // while defining the trigger DELIMITER // CREATE TRIGGER LogNewProduct AFTER INSERT ON Products FOR EACH ROW BEGIN INSERT INTO StockAudit (ProductID, ChangeType, QuantityChanged) VALUES (NEW.ProductID, 'ADD', NEW.Stock); END; // DELIMITER ; How Triggers Operate in a Database Triggers are defined by specifying the event they respond to, the table they act upon, and the SQL statements they execute. When a trigger’s event occurs, the database automatically invokes it, running the associated logic seamlessly. This behavior eliminates the necessity for external application code to maintain consistency. For instance, consider a scenario where we need to prevent negative stock levels in our inventory. We can achieve this with a BEFORE UPDATE trigger that validates the updated stock value: DELIMITER // -- Trigger to prevent negative stock values CREATE TRIGGER PreventNegativeStock BEFORE UPDATE ON Products FOR EACH ROW BEGIN -- Check if the new stock value is less than 0 IF NEW.Stock < 0 THEN -- Raise an error if the stock value is negative SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Stock cannot be negative'; END IF; END; // DELIMITER ; This guarantees that no changes violating the business rules are applied to the database. Practical Advantages of Using Triggers Triggers offer numerous advantages, such as enforcing business logic directly within the database layer. This ensures that data integrity is preserved across all applications accessing the database, reducing the need for repetitive coding. By centralizing critical logic, triggers simplify maintenance and enhance consistency. For example, a trigger can automate logging of stock adjustments, saving developers from implementing this functionality in multiple application layers. Consider this AFTER UPDATE trigger: DELIMITER // -- Trigger to log stock adjustments after an update on the Products table CREATE TRIGGER LogStockAdjustment AFTER UPDATE ON Products FOR EACH ROW BEGIN -- Insert a record into the StockAudit table with the product ID, change type, and quantity changed INSERT INTO StockAudit (ProductID, ChangeType, QuantityChanged) VALUES (OLD.ProductID, 'ADJUST', NEW.Stock - OLD.Stock); END; // DELIMITER ; This trigger automatically records every stock change, streamlining audit processes and ensuring compliance. Challenges and Considerations While triggers are powerful, they are not without challenges. Debugging triggers can be tricky since they operate at the database level and their effects may not be immediately visible. For example, a misconfigured trigger might inadvertently cause cascading changes or conflicts with other triggers, complicating issue resolution. Performance is another critical consideration. Triggers that are not well designed can slow down database operations, especially if they include resource-intensive logic or are triggered frequently. For instance, a trigger performing complex calculations on large datasets can bottleneck critical operations like order processing or stock updates. To mitigate these challenges, it is advisable to: Keep trigger logic concise and efficient. Use triggers sparingly and only for tasks best handled within the database. Test triggers extensively in controlled environments before deployment. Real-World Example: Cascading Triggers Cascading triggers can ensure data integrity across related tables. Consider a database with Orders and OrderDetails tables. When an order is deleted, it is essential to remove all associated details: DELIMITER // -- Trigger to cascade delete order details after a delete on the Orders table CREATE TRIGGER CascadeDeleteOrderDetails AFTER DELETE ON Orders FOR EACH ROW BEGIN -- Delete the corresponding records from the OrderDetails table DELETE FROM OrderDetails WHERE OrderID = OLD.OrderID; END; // DELIMITER ; This ensures that orphaned records are automatically removed, maintaining database consistency without manual intervention. However, cascading triggers require careful documentation to avoid unintended interactions. Optimizing Trigger Performance To prevent performance bottlenecks, triggers should handle minimal logic and avoid intensive operations. For tasks requiring significant processing, consider using scheduled jobs or batch processes instead. For example, instead of recalculating inventory levels on every update, a nightly job could summarize stock levels for reporting purposes. Here’s a simplified trigger that avoids complex calculations: DELIMITER // -- Trigger to log stock changes after an update on the Products table CREATE TRIGGER SimpleStockLog AFTER UPDATE ON Products FOR EACH ROW BEGIN -- Check if the new stock value is different from the old stock value IF NEW.Stock <> OLD.Stock THEN -- Insert a record into the StockAudit table with the product ID, change type, and quantity changed INSERT INTO StockAudit (ProductID, ChangeType, QuantityChanged) VALUES (NEW.ProductID, 'UPDATE', NEW.Stock - OLD.Stock); END IF; END; // DELIMITER ; Conditional Logic and Business Rules Conditional logic within triggers enables dynamic enforcement of business rules. For example, a trigger can adjust discounts based on stock availability: DELIMITER // -- Trigger to adjust discount based on stock levels after an update on the Products table TRIGGER AdjustDiscount AFTER UPDATE ON Products FOR EACH ROW BEGIN -- Check if the new stock value is greater than 100 IF NEW.Stock > 100 THEN -- Set the discount to 10 if the stock is greater than 100 UPDATE Products SET Discount = 10 WHERE ProductID = NEW.ProductID; ELSE -- Set the discount to 0 if the stock is 100 or less UPDATE Products SET Discount = 0 WHERE ProductID = NEW.ProductID; END IF; END; // DELIMITER ; This dynamic adjustment ensures that promotions align with inventory levels. Conclusion SQL triggers are indispensable for automating tasks, enforcing rules, and maintaining data integrity within a database. While they offer significant benefits, their design and implementation require careful consideration to avoid performance issues and unintended consequences. By adhering to best practices, such as keeping triggers simple, testing thoroughly, and documenting dependencies, developers can harness their full potential. Properly implemented triggers can elevate database management, making operations more efficient and reliable. Hostman provides pre-configured and ready-to-use cloud databases, including cloud MySQL.

Updating data in databases is a critical task when working with MySQL. It involves modifying the values of existing records in a table. Updates can range from modifying fields in a group of rows (or even all rows in a table) to adjusting a specific field in a single row. Understanding the syntax for updating data is essential for effectively working with both local and cloud databases. The key command for modifying records in a MySQL database table is UPDATE. Updates occur sequentially, from the first row to the last. Depending on the type of update, there are two syntax options for the UPDATE statement in MySQL. Syntax for Updating a Single Table UPDATE [LOW_PRIORITY] [IGNORE] table_reference SET assignment_list WHERE where_condition ORDER BY ... LIMIT row_count; Parameters: Required: SET assignment_list: Specifies which columns to modify and how (assignment_list is the list of columns and their new values). Optional: LOW_PRIORITY: If specified, the UPDATE is delayed until no other user is reading data from the table. IGNORE: Ensures the UPDATE continues even if errors occur. Rows with duplicate values in unique key columns are not updated. WHERE where_condition: Specifies the conditions for selecting rows to update. If omitted, all rows in the table will be updated. ORDER BY: Determines the order in which rows are updated. LIMIT row_count: Limits the number of rows updated (row_count specifies the number of rows). This count applies to rows matching the WHERE condition, regardless of whether they are actually modified. Syntax for Updating Multiple Tables UPDATE [LOW_PRIORITY] [IGNORE] table_references SET assignment_list WHERE where_condition; Parameters: table_references: Specifies the tables to update. Changes are applied as defined in assignment_list. ORDER BY and LIMIT are not allowed when updating multiple tables. Other optional parameters (LOW_PRIORITY, IGNORE, WHERE) behave the same as for a single-table update. Note that when updating multiple tables, there is no guarantee that updates will occur in a specific order. Creating a Test Database Let’s create a database for a bookstore that sells rare and antique books from around the world. The table will have four tables: author, genre, book, and sales. CREATE TABLE author ( id INT PRIMARY KEY AUTO_INCREMENT, author_name VARCHAR(50) NOT NULL ); INSERT INTO author (author_name) VALUES ('Leo Tolstoy'), ('Franz Kafka'), ('Nikolai Gogol'), ('William Shakespeare'), ('Homer'); CREATE TABLE genre ( id INT PRIMARY KEY AUTO_INCREMENT, genre_name VARCHAR(30) NOT NULL ); INSERT INTO genre (genre_name) VALUES ('Realist novel'), ('Dystopian novel'), ('Picaresque novel'), ('Epic poetry'); CREATE TABLE book ( book_id INT PRIMARY KEY AUTO_INCREMENT, title VARCHAR(50), author_id INT NOT NULL, genre_id INT, price DECIMAL(8,2) NOT NULL, amount INT DEFAULT 0, FOREIGN KEY (author_id) REFERENCES author (id), FOREIGN KEY (genre_id) REFERENCES genre (id) ); INSERT INTO book (title, author_id, genre_id, price, amount) VALUES ('Anna Karenina', 1, 1, 650.00, 15), ('The Castle', 2, 2, 570.20, 6), ('Dead Souls', 3, 3, 480.00, 2), ('Iliad', 5, 4, 518.99, 4), ('Odyssey', 5, 4, 518.99, 7); CREATE TABLE sales ( id INT PRIMARY KEY AUTO_INCREMENT, book_id INT NOT NULL, count INT NOT NULL, cost DECIMAL(8,2) NOT NULL, FOREIGN KEY (book_id) REFERENCES book (book_id) ); We will get the following tables. Table: book Here, we have the columns: book_id: Unique book identifier. title: The book's title. author_id: Author identifier (foreign key to author). genre_id: Genre identifier (foreign key to genre). price: Price of the book per unit. amount: Number of books in stock. The genre table will have the following content:  id genre_name 1 Realist novel 2 Dystopian novel 3 Picaresque novel 4 Epic poetry And our author table will look like this: id author_name 1 Leo Tolstoy 2 Franz Kafka 3 Nikolai Gogol 4 William Shakespeare 5 Homer Table: sales The columns here are: id: Unique identifier for the transaction. book_id: Unique book identifier (foreign key to book). count: Number of books sold. cost: Total cost of the purchased books. Data Update Operations Now, having created a sample database, we will demonstrate the execution of various data update operations using the UPDATE statement and other commands in MySQL. 1. Updating All Rows If we omit the WHERE clause in an UPDATE statement, all rows in the table will be updated. For example, suppose there is a promotion in a bookstore where all books are priced at a fixed rate of 500. The query would look like this: UPDATE bookSET price = 500; Resulting Table: If we try to assign a value that is already in a column, MySQL will not update it. If we want to assign a NULL value to a column defined as NOT NULL will, the query will return an error:  Column 'name_column' cannot be null Using the IGNORE parameter forces the value to default: 0 for numeric types, "" for string types, default dates (e.g., 0000 for YEAR, 0000-00-00 00:00:00 for DATETIME). 2. Updating Rows with a Condition Updating all rows is rare; typically, updates are performed based on specific conditions. For instance, to apply a discount on books with fewer than 5 copies in stock: UPDATE book SET price = 300WHERE amount < 5; Resulting Table: 3. Updating Values Using Expressions Columns can be updated using expressions instead of static values. For example, we can apply a 15% discount on Russian classics (author IDs 1 and 3): UPDATE book SET price = price * 0.85WHERE author_id IN (1, 3); Resulting Table: Updates are executed from left to right. For example, the query below increments the amount by 1 and then doubles it: UPDATE book SET amount = amount + 1, amount = amount * 2; Resulting Table: 4. Updating with DEFAULT We can update column values to their default values (DEFAULT), which are defined during the creation or modification of the table. To find out the default values used in our table, we can execute the following query: DESC book; Table Structure: Next, we reset the values of the amount column to its default value. Since the default value for amount is 0, all rows will now have amount set to 0: UPDATE book SET amount = DEFAULT; Resulting Table: 5. Updating Multiple Columns We can update multiple columns in a single query. For example, let's change the price and amount values for rows where book_id < 4: UPDATE book SET price = price * 0.9, amount = amount - 1 WHERE book_id < 4; Resulting Table: 6. Using LIMIT The LIMIT clause allows us to restrict the number of rows to be updated. For instance, we update only the first row where genre_id = 4: UPDATE book SET price = 100 WHERE genre_id = 4 LIMIT 1; The table contains two rows with genre_id equal to 4, but since we specified LIMIT 1, only one will be updated. Resulting Table: Note: The LIMIT N parameter does not guarantee that exactly N rows will be updated—it processes the first N rows matching the WHERE condition, regardless of whether the rows are updated or not. 7. Updating Multiple Tables In MySQL, it is possible to update multiple tables simultaneously. For example, update the amount in the book table and set the author_name to "-" in the author table for specific conditions: UPDATE book, author SET amount = amount + 3, author.author_name = '-' WHERE book.book_id = 4 AND author.id = 4; Resulting book Table: While the author table will look like this: id author_name 1 Leo Tolstoy 2 Franz Kafka 3 Nikolai Gogol 4 William Shakespeare 5 Homer 8. Updating Tables with a Join (INNER JOIN) While performing updates, we can also join tables using the INNER JOIN command. UPDATE book b INNER JOIN author a ON b.author_id = a.idSET b.title = CONCAT(b.title, ' (', a.author_name,')'); Specifying INNER is not mandatory, as this type of join is used by default. We can rewrite the query as follows and get the same result: UPDATE book, author a SET b.title = CONCAT(b.title, ' (', a.author_name,')')WHERE b.author_id = a.id; 9. Updating Tables with a Join (LEFT JOIN) We can also use a LEFT JOIN. In this case, we must specify LEFT JOIN in the query.  For example, we can update the stock quantity of books after a purchase. Let's add two rows to the sales table: INSERT INTO sales (book_id, count, cost) VALUES (1, 3, (SELECT price FROM book WHERE book_id = 1)*3), (3, 1, (SELECT price FROM book WHERE book_id = 3)*1); The store sold 3 copies of 'Anna Karenina' and 1 copy of 'Dead Souls'. Let's execute the query: UPDATE book LEFT JOIN sales on book.book_id = sales.book_idSET amount = amount - countWHERE sales.book_id is not NULL; We can see that now we have less copies of these books: If we try not to use LEFT JOIN, we will encounter the error: Out of range value for column 'amount' at row 3 This happens because amount cannot be negative. Alternatively, if we add IGNORE, the result will be: As we can see, we reduced the quantity in all rows by three books, which is not what we wanted. 10. Updating with CASE, IF, IFNULL, COALESCE When updating a table, it is also possible to use conditional operators such as CASE, IF, and others.The CASE function evaluates a set of conditions and, depending on the result, returns one of the possible outcomes. The syntax for using CASE and WHEN in an UPDATE statement in MySQL is as follows: UPDATE book SET price = CASE genre_id WHEN 1 THEN 100 WHEN 2 THEN 150 ELSE price END; In this case, if the book has genre 1, we set the price to 100, and if the genre is 2, the price is set to 150. The IF function returns one of two values depending on the result of a conditional expression. If the book has genre 4, we decrease its price by 200; otherwise, we leave the price unchanged. UPDATE bookSET price = IF (genre_id = 4, price-200, price); The result: The IFNULL function checks the value of an expression – if it is NULL, a specified value is returned; otherwise, the expression itself is returned. Let's assume that one of the amount values is NULL: Let's check all the values in the amount column, and if any NULL values are found, we will replace them with 0: UPDATE bookSET amount = IFNULL(amount, 0); Resulting Table: The COALESCE function is quite similar to IFNULL. The main difference is that this function can accept multiple values (two or more). Like IFNULL, it returns the first value that is not NULL. To see how this works, let's create a table like this: id col1 col2 col3 col4 1   val12 val13 val14 2     val23 val24 3       val34 And run the query: UPDATE test_tableSET col4 = COALESCE(col1, col2, col3, 'no value'); We will get: id col1 col2 col3 col4 1   val12 val13 val12 2     val23 val23 3       no value 11. Updating with Sorting Sorting can help when updating a field with a unique key. If we want to shift our id values by 1, updating the first row would result in two rows having id = 2, which will cause an error. However, if we add ORDER BY and start updating from the end, the query will execute successfully: UPDATE bookSET book_id=book_id+1  We will get: 12. Updating Based on Data from Other Tables In MySQL, when working with UPDATE, it is possible to use nested SELECT and FROM commands in the WHERE condition. In the following example, we first retrieve the id of the 'Epic poetry' genre, then use the retrieved value to select the rows for updating the table. UPDATE book SET amount = 0 WHERE genre_id = ( SELECT id FROM genre Where genre_name = 'Epic poetry' ); Alternatively, we can select the values that need to be changed using the query: UPDATE book SET price = ( SELECT MIN (cost) FROM sales) WHERE amount < 5; We are updating the price values of all books whose stock quantity is less than 5, setting the price to the minimum selling amount. The minimum selling amount is 480: It is not possible to update the table by selecting values from the same table in a subquery. However, there is a trick we can use – we can join the table with itself: UPDATE book AS book_1 INNER JOIN( SELECT genre_id, MIN(amount) AS min_amount FROM book GROUP BY genre_id ) AS book_2 ON book_1.genre_id = book_2.genre_id SET book_1.amount = book_2.min_amount; In this case, the subquery creates a temporary table for the join and closes it before the UPDATE begins execution. The subquery finds the minimum quantity of books for each genre, which is then used to update the amount column. In our table, only genre 4 has more than one row. The values in both rows should be replaced with the minimum value for that genre, which is 4. We will get: Another option is using SELECT FROM SELECT: UPDATE book AS book_1 SET book_1.price = (SELECT MIN(price) AS min_price FROM ( SELECT price FROM book) as book_2); In this case, a temporary table is also created. However, only a single value is assigned to all rows. Conclusion We have covered the features of using the UPDATE statement in MySQL in as much detail as possible and covered simple scenarios with practical examples.