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From

The FROM keyword specifies the table from which the data should be retrieved. It defines the source of your query and works together with SELECT to determine where the data comes from.

1. Basic Usage​

The simplest form of the FROM clause is to specify a single table name: 

SELECT column1, column2 
FROM table_name;
  • Here, table_name is the name of the database table.
  • For example: 
    SELECT name, age 
    FROM employees;
    This retrieves the name and age columns from the employees table.

2. Using Aliases with Tables​

Table aliases can make your queries more readable, especially when using long table names or joining multiple tables. 

SELECT e.name, d.department_name 
FROM employees e, departments d;
  • Here, e and d are aliases for the employees and departments tables, allowing you to reference them more easily.

3. Joining Multiple Tables​

The FROM clause can include multiple tables with different types of joins to combine data.

  • Example with INNER JOIN: 
    SELECT e.name, d.department_name 
    FROM employees e 
    INNER JOIN departments d 
    ON e.department_id = d.id;
    This fetches employee names along with their department names by linking the two tables on a common column (department_id).

4. Using Subqueries in FROM​

You can use subqueries in the FROM clause to treat the result of a query as a temporary table. 

SELECT name, total_sales 
FROM (
  SELECT employee_id, SUM(sales) AS total_sales 
  FROM sales 
  GROUP BY employee_id
) AS sales_summary;
  • Here, the subquery calculates total sales for each employee, and the main query retrieves the employee name and total sales.

5. Combining with Joins and Filters​

When combined with WHERE, GROUP BY, and other clauses, the FROM clause serves as the foundation for querying data. For example: 

SELECT e.name, COUNT(p.project_id) AS total_projects 
FROM employees e 
LEFT JOIN projects p 
ON e.id = p.employee_id 
WHERE e.status = 'active' 
GROUP BY e.name;

This query retrieves active employees and the total number of projects they are involved in.