SQL MDN Docs FROM

FROM keyword in SQL

The FROM keyword in SQL is used to specify the source table (sometimes multiple tables) from which to retrieve data. It’s an essential part of pretty much all queries, as it determines where the database system should look for the specified columns and records.

Syntax

The basic syntax for the FROM clause is straightforward. You simply follow the SELECT keyword and the columns you wish to retrieve with the FROM keyword, and then specify the table name:

SELECT id, email, first_name
FROM users

Joining Tables

One of the powerful features of SQL is the ability to join tables. The FROM clause can be used in conjunction with various types of joins (such as INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN) to combine rows from two or more tables based on a related column between them.

Specifying Multiple Tables

You can also specify multiple tables in the FROM clause. This will create a Cartesian product of the tables specified (i.e., every combination of rows from the first table with every row of the second table).

SELECT *
FROM table1, table2

However, this approach is less common as it often produces a very large number of rows and is typically not what you want. Joins are a more efficient way to retrieve data from multiple tables.

Let’s create a couple of small CTE-s to demonstarte the effect of the FROM clause with multiple tables:

WITH letters AS (
  SELECT 'a' AS letter
  UNION
  SELECT 'b'
  UNION
  SELECT 'c'
), numbers AS (
  SELECT '1' AS number
  UNION
  SELECT '2'
  UNION
  SELECT '3'
)

SELECT *
FROM letters, numbers

Using FROM with subqueries

The FROM clause can also include a subquery, effectively treating the result set of the subquery as a temporary table that can be selected from.

SELECT *
FROM (
  SELECT *
  FROM users
  WHERE
    status = 'customer'
) AS customers

Unless you’re using an old MySQL version, the Common Table Expressions (CTE-s) are a way superior way of strucutring such subqueies.

Query without FROM

It’s possible to write an entire query without a FORM clause. This is very handy when you want to play with SQL functions without using data from any table: