MySQL: No Database Selected - Select the Default DB

Introduction

In MySQL, when a connection is established with the server, it is essential to select a database before executing any queries. If a database is not selected, the error message "No database selected" will be thrown. This article will explain why this error occurs and how to select a default database in MySQL.

Error Message: No database selected

The error "No database selected" occurs when a query is executed without specifying a database. Consider the following example:

SELECT * FROM customers;

This query will result in the error message "No database selected" because there is no database specified to perform the query.

Selecting the Default Database

To avoid the "No database selected" error, you need to select a default database. A default database is a database that is automatically used for queries if no database is specified explicitly.

To select the default database, you can use the USE statement followed by the database name. Here's an example:

USE mydatabase;

In the above example, the database named "mydatabase" is selected as the default database. Once the default database is set, you can execute queries without explicitly specifying the database.

Example Usage

Let's consider a scenario where we have a database named "mydatabase" containing a table called "employees." To select the default database and retrieve all employees' information, you can follow these steps:

  1. Establish a connection to the MySQL server.
  2. Select the default database using the USE statement.
USE mydatabase;
  1. Execute the query to retrieve all employees' information.
SELECT * FROM employees;

Flowchart

The following is a flowchart representing the process of selecting the default database in MySQL:

flowchart TD
    A[Establish Connection]
    B[Select Default Database]
    C[Execute Queries]

    A --> B
    B --> C

Conclusion

In MySQL, it is essential to select a database before executing any queries. If a database is not selected, the error message "No database selected" will be thrown. To avoid this error, you can use the USE statement to select a default database. Once the default database is set, you can execute queries without explicitly specifying the database. Following the steps mentioned in this article, you can easily select a default database and perform queries seamlessly.