Difference between natural join and inner join

Question

What is the difference between a natural join and an inner join

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A Natural Join is where 2 tables are joined on the basis of all common columns.

common column : is a column which has same name in both tables + has compatible datatypes in both the tables. You can use only = operator

A Inner Join is where 2 tables are joined on the basis of common columns mentioned in the ON clause.

common column : is a column which has compatible datatypes in both the tables but need not have the same name. You can use only any comparision operator like =, <=, >=, <, >, <>

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A NATURAL join is just short syntax for a specific INNER join -- or  quot equi-join quot  -- and  once the syntax is unwrapped  both represent the same Relational Algebra operation  It s not a  quot different kind quot  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  quot duplicates quot  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

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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

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Inner join  join two table where column name is same   Natural join  join two table where column name and data types are same

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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

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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

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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

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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

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difference is that int the inner equi default join and natural join that in the natuarl join common column win will be display in single time but  inner equi default simple join the common column will be display double time

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Inner join and natural join are almost same but there is a slight difference between them  The difference is in natural join no need to specify condition but in inner join condition is obligatory  If we do specify the condition in inner join   it resultant tables is like a cartesian product

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mysql gt  SELECT    FROM tb1    ---- ------    id   num     ---- ------     6     60      7     70      8     80      1      1      2      2      3      3    ---- ------  6 rows in set  0 00 sec   mysql gt  SELECT    FROM tb2    ---- ------    id   num     ---- ------     4     40      5     50      9     90      1      1      2      2      3      3    ---- ------  6 rows in set  0 00 sec    INNER JOIN     mysql gt  SELECT    FROM tb1 JOIN tb2     ---- ------ ---- ------    id   num    id   num     ---- ------ ---- ------     6     60    4     40      7     70    4     40      8     80    4     40      1      1    4     40      2      2    4     40      3      3    4     40      6     60    5     50      7     70    5     50      8     80    5     50          more       return 36 rows in set  0 01 sec   AND NATURAL JOIN        mysql gt  SELECT    FROM tb1 NATURAL JOIN tb2        ---- ------        id   num         ---- ------         1      1          2      2          3      3        ---- ------      3 rows in set  0 01 sec

[sql] Difference between natural join and inner join

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