FROM clause

The FROM clause specifies the source of the data on which the query should operate. Logically, the FROM clause is where the query starts execution. It can contain a single table, a combination of multiple joined tables, or another SELECT query inside a subquery node.

The FROM clause derives a table from one or more tables in a comma-separated table reference list.

Here is the basic syntax of the FROM clause:

FROM table_reference [, table_reference [, ...]]

A table_reference can be a table name, a derived table such as a subquery, a JOIN construct, or complex combinations.

If multiple sources are specified, the result is all the sources' Cartesian product (i.e., cross join). The result of the FROM list is an intermediate virtual table that can then be subject to transformations by the WHERE, GROUP BY, and HAVING clauses and is finally the result of the overall table expression.

Joined tables

A joined table is a table derived from two other (real or derived) tables according to the rules of the particular join type. Inner, outer, and cross-joins are available.

Syntax:

t1 join_type t2 [ join_condition ]

Subqueries

Subqueries specifying a derived table must be enclosed in parentheses and must be assigned a table alias name.

Syntax:

FROM (SELECT * FROM table1) AS alias_name

This example is equivalent to FROM table1 AS alias_name.

Table functions

Table functions produce a set of rows made up of either base data types (scalar types) or composite data types (table rows). They are used like a table, view, or subquery in the FROM clause of a query. Columns returned by table functions can be included in SELECT, JOIN, or WHERE clauses in the same manner as table columns, view, or subquery columns.