Fromclauses

Fromclauses, in the context of SQL, refer to the portion of a query that identifies the data sources used by the statement. A query’s data sources include base tables, views, derived tables (subqueries in the FROM clause), and common table expressions defined with WITH that appear in the FROM list. In standard SQL, a FROM clause follows the SELECT clause (and precedes the WHERE, GROUP BY, and ORDER BY clauses).

Structure and aliases: The FROM clause begins with FROM and is followed by one or more table references. Modern SQL favors explicit JOIN syntax over the older comma-separated list. Each reference can be a table, a view, or a derived table produced by a subquery; derived tables and subqueries must have an alias. A CTE introduced by WITH can also be referenced from the FROM clause.

Joins and predicates: Joins inside the FROM clause specify how rows from different sources are combined. Typical join types include INNER JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN, FULL OUTER JOIN, and CROSS JOIN. Each join typically carries an ON or USING predicate that constrains how rows are matched.

Special features and dialect differences: Some dialects provide extensions such as LATERAL (PostgreSQL), CROSS APPLY and OUTER APPLY (SQL Server), or NATURAL JOIN (supported in several systems). Aliasing rules require every table or derived table to have an alias, and inline subqueries used in the FROM clause must be named.

Semantics and optimization: The FROM clause defines the initial data set that downstream clauses operate on. The syntactic order of joins does not necessarily reflect execution order; the optimizer chooses an efficient strategy. Qualification rules determine which columns are accessible from each source.

Example: SELECT a.name, b.sales FROM customers AS a INNER JOIN orders AS b ON a.id = b.customer_id; This demonstrates referencing multiple sources and join predicates in the FROM clause.