Difference between natural join and inner join

Question

One significant difference between INNER JOIN and NATURAL JOIN is the number of columns returned.

Consider:

TableA                           TableB
+------------+----------+        +--------------------+    
|Column1     | Column2  |        |Column1  |  Column3 |
+-----------------------+        +--------------------+
| 1          |  2       |        | 1       |   3      |
+------------+----------+        +---------+----------+

The INNER JOIN of TableA and TableB on Column1 will return

SELECT * FROM TableA AS a INNER JOIN TableB AS b USING (Column1);
SELECT * FROM TableA AS a INNER JOIN TableB AS b ON a.Column1 = b.Column1;

+------------+-----------+---------------------+    
| a.Column1  | a.Column2 | b.Column1| b.Column3|
+------------------------+---------------------+
| 1          |  2        | 1        |   3      |
+------------+-----------+----------+----------+

The NATURAL JOIN of TableA and TableB on Column1 will return:

SELECT * FROM TableA NATURAL JOIN TableB
+------------+----------+----------+    
|Column1     | Column2  | Column3  |
+-----------------------+----------+
| 1          |  2       |   3      |
+------------+----------+----------+

The repeated column is avoided.

(AFAICT from the standard grammar, you can't specify the joining columns in a natural join; the join is strictly name-based. See also Wikipedia.)

(There's a cheat in the inner join output; the a. and b. parts would not be in the column names; you'd just have column1, column2, column1, column3 as the headings.)

• An inner join is one where the matching row in the joined table is required for a row from the first table to be returned
• An outer join is one where the matching row in the joined table is not required for a row from the first table to be returned
• A natural join is a join (you can have either natural left or natural right) that assumes the join criteria to be where same-named columns in both table match

I would avoid using natural joins like the plague, because natural joins are:

• not standard sql [SQL 92] and therefore not portable, not particularly readable (by most SQL coders) and possibly not supported by various tools/libraries
• not informative; you can't tell what columns are being joined on without referring to the schema
• your join conditions are invisibly vulnerable to schema changes - if there are multiple natural join columns and one such column is removed from a table, the query will still execute, but probably not correctly and this change in behaviour will be silent
• hardly worth the effort; you're only saving about 10 seconds of typing

A natural join is just a shortcut to avoid typing, with a presumption that the join is simple and matches fields of the same name.

SELECT
  *
FROM
  table1
NATURAL JOIN
  table2
    -- implicitly uses `room_number` to join

Is the same as...

SELECT
  *
FROM
  table1
INNER JOIN
  table2
    ON table1.room_number = table2.room_number

What you can't do with the shortcut format, however, is more complex joins...

SELECT
  *
FROM
  table1
INNER JOIN
  table2
    ON (table1.room_number = table2.room_number)
    OR (table1.room_number IS NULL AND table2.room_number IS NULL)

SQL is not faithful to the relational model in many ways. The result of a SQL query is not a relation because it may have columns with duplicate names, 'anonymous' (unnamed) columns, duplicate rows, nulls, etc. SQL doesn't treat tables as relations because it relies on column ordering etc.

The idea behind NATURAL JOIN in SQL is to make it easier to be more faithful to the relational model. The result of the NATURAL JOIN of two tables will have columns de-duplicated by name, hence no anonymous columns. Similarly, UNION CORRESPONDING and EXCEPT CORRESPONDING are provided to address SQL's dependence on column ordering in the legacy UNION syntax.

However, as with all programming techniques it requires discipline to be useful. One requirement for a successful NATURAL JOIN is consistently named columns, because joins are implied on columns with the same names (it is a shame that the syntax for renaming columns in SQL is verbose but the side effect is to encourage discipline when naming columns in base tables and VIEWs :)

Note a SQL NATURAL JOIN is an equi-join**, however this is no bar to usefulness. Consider that if NATURAL JOIN was the only join type supported in SQL it would still be relationally complete.

While it is indeed true that any NATURAL JOIN may be written using INNER JOIN and projection (SELECT), it is also true that any INNER JOIN may be written using product (CROSS JOIN) and restriction (WHERE); further note that a NATURAL JOIN between tables with no column names in common will give the same result as CROSS JOIN. So if you are only interested in results that are relations (and why ever not?!) then NATURAL JOIN is the only join type you need. Sure, it is true that from a language design perspective shorthands such as INNER JOIN and CROSS JOIN have their value, but also consider that almost any SQL query can be written in 10 syntactically different, but semantically equivalent, ways and this is what makes SQL optimizers so very hard to develop.

Here are some example queries (using the usual parts and suppliers database) that are semantically equivalent:

SELECT *
  FROM S NATURAL JOIN SP;

-- Must disambiguate and 'project away' duplicate SNO attribute
SELECT S.SNO, SNAME, STATUS, CITY, PNO, QTY
  FROM S INNER JOIN SP 
          USING (SNO);                        

-- Alternative projection
SELECT S.*, PNO, QTY
  FROM S INNER JOIN SP 
          ON S.SNO = SP.SNO;

-- Same columns, different order == equivalent?!
SELECT SP.*, S.SNAME, S.STATUS, S.CITY
  FROM S INNER JOIN SP 
      ON S.SNO = SP.SNO;

-- 'Old school'
SELECT S.*, PNO, QTY
  FROM S, SP 
 WHERE S.SNO = SP.SNO;

---

** Relational natural join is not an equijoin, it is a projection of one. – philipxy

A NATURAL join is just short syntax for a specific INNER join -- or "equi-join" -- and, once the syntax is unwrapped, both represent the same Relational Algebra operation. It's not a "different kind" of join, as with the case of OUTER (LEFT/RIGHT) or CROSS joins.

See the equi-join section on Wikipedia:

A natural join offers a further specialization of equi-joins. The join predicate arises implicitly by comparing all columns in both tables that have the same column-names in the joined tables. The resulting joined table contains only one column for each pair of equally-named columns.

Most experts agree that NATURAL JOINs are dangerous and therefore strongly discourage their use. The danger comes from inadvertently adding a new column, named the same as another column ...

That is, all NATURAL joins may be written as INNER joins (but the converse is not true). To do so, just create the predicate explicitly -- e.g. USING or ON -- and, as Jonathan Leffler pointed out, select the desired result-set columns to avoid "duplicates" if desired.

Happy coding.

---

(The NATURAL keyword can also be applied to LEFT and RIGHT joins, and the same applies. A NATURAL LEFT/RIGHT join is just a short syntax for a specific LEFT/RIGHT join.)

Natural Join: It is combination or combined result of all the columns in the two tables. It will return all rows of the first table with respect to the second table.

Inner Join: This join will work unless if any of the column name shall be sxame in two tables