A detailed discussion of query and assignment operations in MySQL

MySQL is a popular relational database management system that is widely used in web applications and server-side development. In MySQL, querying and assignment are very basic operations, but they may not be easy to understand for novices. This article will discuss query and assignment operations in MySQL in detail.

Query

Query is one of the most basic operations in MySQL. In MySQL, queries are implemented using SELECT statements. The SELECT statement is used to retrieve data from one or more tables. The following is a simple SELECT statement example:

SELECT * FROM users;

This SELECT statement will retrieve all data located in the users table. The * symbol is a wildcard character that means to retrieve all columns. You can also use column names to select the columns to retrieve. For example, the following SELECT statement will retrieve only the ID, name, and email columns:

SELECT id, name, email FROM users;

Additionally, you can use the WHERE clause to filter the retrieved data. WHERE allows you to filter the retrieved data. For example, the following SELECT statement will retrieve all users who are 18 years or older:

SELECT * FROM users WHERE age >= 18;

Assignment

The assignment operation is another basic operation in MySQL. In MySQL, you can use the UPDATE statement to update data in one or more tables. The following is a simple UPDATE statement example:

UPDATE users SET name='John' WHERE id=1;

This UPDATE statement will update the name of the record with ID 1 in the user table to "John". The SET keyword in the statement is used to specify the columns to be updated and their updated values. The WHERE clause is used to select the records to be updated.

In addition to assigning values ​​to data in a single table, you can also use the JOIN statement to update data in multiple tables. The JOIN statement is used to combine multiple tables into one queryable table. The following is an example of updating using a JOIN statement:

UPDATE users u JOIN orders o ON u.id=o.user_id
SET u.total_orders=u.total_orders+1
WHERE o.status='completed';

This UPDATE statement will update the total_orders column in the user table, which only updates the number of orders for users who have completed their orders. The JOIN statement is used to merge the Users table and the Orders table into one queryable table so that the number of orders can be added to the Users table.

Conclusion

Querying and assignment are one of the most basic operations in MySQL. Using the SELECT statement, you can retrieve data from one or more tables and filter the retrieved data using the WHERE clause. Using the UPDATE statement, you can update data in one or more tables. In MySQL, you can also use the JOIN statement to combine multiple tables into one queryable table. Understanding these basic operations is very important to use MySQL correctly.

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