Natural Joins

A natural join is just a join with an implicit join predicate. Natural joins can be inner, left outer, right outer, or full outer joins and take the following form:

SELECT column‑list FROM left-join-table
NATURAL [ INNER | LEFT OUTER | RIGHT OUTER | FULL OUTER ] JOIN right-join-table

Natural joins are, by default, natural inner joins; however, there can also be natural left/right/full outer joins. The primary difference between an inner and natural join is that inner joins have an explicit join condition, whereas the natural join’s conditions are formed by matching all pairs of columns in the tables that have the same name and compatible data types, making natural joins equi-joins because join condition are equal between common columns. (If the data types are incompatible, Vertica returns an error.)

The following query is a simple natural join between tables T1 and T2 when the T2 column val is greater than 5:

=> SELECT * FROM T1 NATURAL JOIN T2 WHERE T2.val > 5;

The store_sales_fact table and the product_dimension table have two columns that share the same name and data type: product_key and product_version. The following example creates a natural join between those two tables at their shared columns:

=> SELECT product_description, sales_quantity FROM store.store_sales_fact
   NATURAL JOIN public.product_dimension;

The following three queries return the same result expressed as a basic query, an inner join, and a natural join. at the table expressions are equivalent only if the common attribute in the store_sales_fact table and the store_dimension table is store_key. If both tables have a column named store_key, then the natural join would also have a store_sales_fact.store_key = store_dimension.store_key join condition. Since the results are the same in all three instances, they are shown in the first (basic) query only:

=> SELECT store_name FROM store.store_sales_fact, store.store_dimension
   WHERE store.store_sales_fact.store_key = store.store_dimension.store_key
   AND store.store_dimension.store_state = 'MA' ORDER BY store_name;
 store_name 
------------
 Store11
 Store128
 Store178
 Store66
 Store8
 Store90
(6 rows)

The query written as an inner join:

=> SELECT store_name FROM store.store_sales_fact
   INNER JOIN store.store_dimension
   ON (store.store_sales_fact.store_key = store.store_dimension.store_key)
   WHERE store.store_dimension.store_state = 'MA' ORDER BY store_name;

In the case of the natural join, the join predicate appears implicitly by comparing all of the columns in both tables that are joined by the same column name. The result set contains only one column representing the pair of equally-named columns.

=> SELECT store_name FROM store.store_sales_fact
   NATURAL JOIN store.store_dimension
   WHERE store.store_dimension.store_state = 'MA' ORDER BY store_name;