FROM

The FROM clause in SQL specifies the tables from which the retrieval should be made. It is an integral part of SELECT statements and variants of SELECT like SELECT INTO and SELECT WHERE. FROM can be used to join tables as well.

Typically, FROM is followed by space delimited list of tables in which the SELECT operation is to be executed. If you need to pull data from multiple tables, you would separate each table with a comma.

Here are some examples:

Example 1 - Simple Usage

If you've a table called employees, you can select all employees' data like this:

SELECT * 
FROM employees;

In this example, * means "all columns". So, SELECT * FROM employees; will retrieve all data from the employees table.

Example 2 - FROM with Multiple Tables

If you've multiple tables, say employees and departments, and you want to select data from both, you can do the following:

SELECT employees.name, departments.department 
FROM employees, departments 
WHERE employees.dept_id = departments.dept_id;

In this example, the FROM clause is following by two tables: employees and departments. employees.name and departments.department indicate that we're selecting the name column from the employees table and the department column from the departments table.

Remember, always respect the order of operations in SQL. The FROM clause works only after tables are identified.

In complex SQL queries where you might need to pull data from multiple tables, aliases are used to temporarily rename the tables within the individual SQL statement.

Example 3 - FROM with Aliases

Below is an example of a FROM clause with aliases:

SELECT e.name, d.department 
FROM employees AS e, departments AS d
WHERE e.dept_id = d.dept_id;

In this example, employees and departments tables are termed as e and d respectively.

That's it! Remember that FROM is not limited only to SELECT. It is applicable to UPDATE and DELETE operations as well.