MySQL

In MySQL, the SELECT * statement is a key part of Data Query Language (DQL) used to fetch all columns from one or more tables. This statement is essential for retrieving data in bulk without specifying individual columns. Although it simplifies querying, especially when dealing with many columns, it is recommended to use it wisely in production environments due to potential performance issues when tables grow large.

Here’s a breakdown of how SELECT * works and some useful tips for new students:

1. Basic Syntax:

   • The SELECT * statement retrieves all columns from a table without the need to specify each one.
   • Syntax:

   SELECT * FROM employees;

2. Joining Multiple Tables:

   • You can also use SELECT * with joins to retrieve columns from multiple tables.
   • For example, if you want to retrieve data from both the employees and departments tables, the query might look like this:

   SELECT * 
   FROM employees 
   JOIN departments 
   ON employees.department_id = departments.department_id;

3. Using WHERE Clause for Filtering:

   • Although SELECT * retrieves all columns, you can combine it with a WHERE clause to filter the rows returned.

   SELECT * 
   FROM employees 
   WHERE department_id = 2;

4. Limiting the Results:

   • You can limit the number of rows returned using the LIMIT clause.

   SELECT * 
   FROM employees 
   LIMIT 10;

5. Avoid Overuse in Production:

   • While SELECT * is useful in early stages of development or for exploratory queries, avoid using it in production queries as it can fetch unnecessary data. It’s better to specify the required columns to optimize performance.

These examples show common use cases for SELECT *, but remember that understanding the structure of the data and only retrieving necessary columns will help improve both performance and readability of your queries.

To gain complete access, login with gmail or outlook, no need of signup, click here

SELECT * FROM org_employee;

SELECT * FROM org_client;

SELECT * FROM act_payment;

SELECT client_id, first_name, last_name, gender, city
FROM org_client;

SELECT city, gender, first_name, last_name, email, client_id
FROM org_client;

SELECT city, gender, first_name, last_name, email FROM org_client;

SELECT city
    gender,
    first_name,
    last_name,
    email
FROM org_client;

SELECT city, gender, first_name, last_name, email 
FROM org_client
ORDER BY first_name;

SELECT email
FROM org_client;

Example 1:

Let's understand the process of extracting data from a specified table. In this case, we aim to retrieve all rows and columns from the employee table.

Example 1 - Raw data from employee table

Example 1 - Query

SELECT * FROM org_employee;

Example 1 - Query data mapping

In above image, the color green signifies that the output for our query will include all rows and columns being selected.

Example 1 - Query Output

Example 2:

Let's understand the process of extracting desired columns from a specified table. In this case, we aim to retrieve 4 columns from the employee table.

Example 2 - Raw data from employee table

Example 2 - Query

SELECT employee_id, first_name, last_name, email FROM org_employee;

Example 2 - Query data mapping

In above image, the color green signifies that the output for our query will include data from desired columns(employee_id, first_name, last_name and email).

Example 2 - Query Output