FROM clause

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.

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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 ]

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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.

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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.

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LATERAL subqueries

Subqueries appearing in FROM can be preceded by the keyword LATERAL. This allows them to reference columns provided by preceding FROM items. Without LATERAL, each subquery is evaluated independently and so cannot cross-reference any other FROM item.

To create a LATERAL subquery, use the LATERAL keyword directly before the inner subquery’s SELECT statement.

The following query includes two LATERAL subqueries. The first LATERAL subquery calculates the maximum sale amount and caches the result in a derived table max_sale. The second LATERAL subquery finds the customer name based on the maximum sale amount from the derived table, and stores the result in another derived table max_sale_customer.

SELECT
  salesperson.name,
  max_sale.amount,
  max_sale_customer.customer_name
FROM
  salesperson,
  -- Calculate the maximum sale and cache it in a derived table
  LATERAL
  (SELECT MAX(amount) AS amount
    FROM all_sales
    WHERE all_sales.salesperson_id = salesperson.id)
  AS max_sale,
  -- find customer, reusing cached maximum sale amount
  LATERAL
  (SELECT customer_name
    FROM all_sales
    WHERE all_sales.salesperson_id = salesperson.id
    AND all_sales.amount =
        -- the cached maximum sale amount
        max_sale.amount)
  AS max_sale_customer;

You can apply a LEFT join to a LATERAL subquery to ensure that source rows appear in the result, even if the subquery yields no rows for them.

For example, the above query can be rewritten to LEFT join a LATERAL subquery:

SELECT
  salesperson.name,
  max_sale.amount,
  max_sale.customer_name
FROM
  salesperson left join
  -- find maximum size and customer at same time
  LATERAL
  (SELECT amount, customer_name
    FROM all_sales
    WHERE all_sales.salesperson_id = salesperson.id
    ORDER BY amount DESC LIMIT 1)
  AS max_sale on true;