SQL JOIN and different types of JOINs

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What is a SQL JOIN and what are different types

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I have created an illustration that explains better than words  in my opinion

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What is SQL JOIN    SQL JOIN is a method to retrieve data from two or more database tables   What are the different SQL JOINs    There are a total of five JOINs  They are      1  JOIN or INNER JOIN   2  OUTER JOIN       2 1 LEFT OUTER JOIN or LEFT JOIN      2 2 RIGHT OUTER JOIN or RIGHT JOIN      2 3 FULL OUTER JOIN or FULL JOIN    3  NATURAL JOIN   4  CROSS JOIN   5  SELF JOIN   1  JOIN or INNER JOIN    In this kind of a JOIN  we get all records that match the condition in both tables  and records in both tables that do not match are not reported   In other words  INNER JOIN is based on the single fact that  ONLY the matching entries in BOTH the tables SHOULD be listed   Note that a JOIN without any other JOIN keywords  like INNER  OUTER  LEFT  etc  is an INNER JOIN  In other words  JOIN is a Syntactic sugar for INNER JOIN  see  Difference between JOIN and INNER JOIN    2  OUTER JOIN    OUTER JOIN retrieves  Either         the matched rows from one table and all rows in the other table      Or         all rows in all tables  it doesn t matter whether or not there is a match    There are three kinds of Outer Join    2 1 LEFT OUTER JOIN or LEFT JOIN  This join returns all the rows from the left table in conjunction with the matching rows from the      right table  If there are no columns matching in the right table  it returns NULL values   2 2 RIGHT OUTER JOIN or RIGHT JOIN  This JOIN returns all the rows from the right table in conjunction with the matching rows from the      left table  If there are no columns matching in the left table  it returns NULL values   2 3 FULL OUTER JOIN or FULL JOIN   This JOIN combines LEFT OUTER JOIN and RIGHT OUTER JOIN  It returns rows from either table when the conditions are met and returns NULL value when there is no match   In other words  OUTER JOIN is based on the fact that  ONLY the matching entries in ONE OF the tables  RIGHT or LEFT  or BOTH of the tables FULL  SHOULD be listed   Note that  OUTER JOIN  is a loosened form of  INNER JOIN     3  NATURAL JOIN    It is based on the two conditions     the JOIN is made on all the columns with the same name for equality  Removes duplicate columns from the result    This seems to be more of theoretical in nature and as a result  probably  most DBMS  don t even bother supporting this   4  CROSS JOIN    It is the Cartesian product of the two tables involved  The result of a CROSS JOIN will not make sense in most of the situations  Moreover  we won t need this at all  or needs the least  to be precise    5  SELF JOIN    It is not a different form of JOIN  rather it is a JOIN  INNER  OUTER  etc  of a table to itself   JOINs based on Operators  Depending on the operator used for a JOIN clause  there can be two types of JOINs  They are   Equi JOIN Theta JOIN   1  Equi JOIN    For whatever JOIN type  INNER  OUTER  etc   if we use ONLY the equality operator      then we say that the JOIN is an EQUI JOIN   2  Theta JOIN    This is same as EQUI JOIN but it allows all other operators like     lt      etc      Many consider both EQUI JOIN and Theta JOIN similar to INNER  OUTER   etc JOINs  But I strongly believe that its a mistake and makes the   ideas vague  Because INNER JOIN  OUTER JOIN etc are all connected with   the tables and their data whereas EQUI JOIN and THETA JOIN are only   connected with the operators we use in the former       Again  there are many who consider NATURAL JOIN as some sort of    peculiar  EQUI JOIN  In fact  it is true  because of the first   condition I mentioned for NATURAL JOIN  However  we don t have to   restrict that simply to NATURAL JOINs alone  INNER JOINs  OUTER JOINs   etc could be an EQUI JOIN too

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Interestingly most other answers suffer from these two problems    They focus on basic forms of join only They  ab use Venn diagrams  which are an inaccurate tool for visualising joins  they re much better for unions     I ve recently written an article on the topic  A Probably Incomplete  Comprehensive Guide to the Many Different Ways to JOIN Tables in SQL  which I ll summarise here   First and foremost  JOINs are cartesian products  This is why Venn diagrams explain them so inaccurately  because a JOIN creates a cartesian product between the two joined tables  Wikipedia illustrates it nicely     The SQL syntax for cartesian products is CROSS JOIN  For example   SELECT    -- This just generates all the days in January 2017 FROM generate series     2017-01-01   TIMESTAMP     2017-01-01   TIMESTAMP   INTERVAL  1 month -1 day     INTERVAL  1 day    AS days day   -- Here  we re combining all days with all departments CROSS JOIN departments   Which combines all rows from one table with all rows from the other table   Source    --------     ------------    day          department    --------     ------------    Jan 01       Dept 1         Jan 02       Dept 2                      Dept 3         Jan 30      ------------    Jan 31    --------    Result    -------- ------------    day      department    -------- ------------    Jan 01   Dept 1         Jan 01   Dept 2         Jan 01   Dept 3         Jan 02   Dept 1         Jan 02   Dept 2         Jan 02   Dept 3                                 Jan 31   Dept 1         Jan 31   Dept 2         Jan 31   Dept 3        -------- ------------    If we just write a comma separated list of tables  we ll get the same   -- CROSS JOINing two tables  SELECT   FROM table1  table2   INNER JOIN  Theta-JOIN   An INNER JOIN is just a filtered CROSS JOIN where the filter predicate is called Theta in relational algebra   For instance   SELECT    -- Same as before FROM generate series     2017-01-01   TIMESTAMP     2017-01-01   TIMESTAMP   INTERVAL  1 month -1 day     INTERVAL  1 day    AS days day   -- Now  exclude all days departments combinations for -- days before the department was created JOIN departments AS d ON day  gt   d created at   Note that the keyword INNER is optional  except in MS Access     look at the article for result examples   EQUI JOIN  A special kind of Theta-JOIN is equi JOIN  which we use most  The predicate joins the primary key of one table with the foreign key of another table  If we use the Sakila database for illustration  we can write   SELECT   FROM actor AS a JOIN film actor AS fa ON a actor id   fa actor id JOIN film AS f ON f film id   fa film id   This combines all actors with their films   Or also  on some databases   SELECT   FROM actor JOIN film actor USING  actor id  JOIN film USING  film id    The USING   syntax allows for specifying a column that must be present on either side of a JOIN operation s tables and creates an equality predicate on those two columns   NATURAL JOIN  Other answers have listed this  JOIN type  separately  but that doesn t make sense  It s just a syntax sugar form for equi JOIN  which is a special case of Theta-JOIN or INNER JOIN  NATURAL JOIN simply collects all columns that are common to both tables being joined and joins USING   those columns  Which is hardly ever useful  because of accidental matches  like LAST UPDATE columns in the Sakila database    Here s the syntax   SELECT   FROM actor NATURAL JOIN film actor NATURAL JOIN film   OUTER JOIN  Now  OUTER JOIN is a bit different from INNER JOIN as it creates a UNION of several cartesian products  We can write   -- Convenient syntax  SELECT   FROM a LEFT JOIN b ON  lt predicate gt   -- Cumbersome  equivalent syntax  SELECT a    b   FROM a JOIN b ON  lt predicate gt  UNION ALL SELECT a    NULL  NULL       NULL FROM a WHERE NOT EXISTS     SELECT   FROM b WHERE  lt predicate gt      No one wants to write the latter  so we write OUTER JOIN  which is usually better optimised by databases    Like INNER  the keyword OUTER is optional  here   OUTER JOIN comes in three flavours    LEFT   OUTER   JOIN  The left table of the JOIN expression is added to the union as shown above  RIGHT   OUTER   JOIN  The right table of the JOIN expression is added to the union as shown above  FULL   OUTER   JOIN  Both tables of the JOIN expression are added to the union as shown above    All of these can be combined with the keyword USING   or with NATURAL  I ve actually had a real world use-case for a NATURAL FULL JOIN recently   Alternative syntaxes  There are some historic  deprecated syntaxes in Oracle and SQL Server  which supported OUTER JOIN already before the SQL standard had a syntax for this   -- Oracle SELECT   FROM actor a  film actor fa  film f WHERE a actor id   fa actor id    AND fa film id   f film id     -- SQL Server SELECT   FROM actor a  film actor fa  film f WHERE a actor id    fa actor id AND fa film id    f film id   Having said so  don t use this syntax  I just list this here so you can recognise it from old blog posts   legacy code   Partitioned OUTER JOIN  Few people know this  but the SQL standard specifies partitioned OUTER JOIN  and Oracle implements it   You can write things like this   WITH    -- Using CONNECT BY to generate all dates in January   days day  AS       SELECT DATE  2017-01-01    LEVEL - 1     FROM dual     CONNECT BY LEVEL  lt   31         -- Our departments   departments department  created at  AS       SELECT  Dept 1   DATE  2017-01-10  FROM dual UNION ALL     SELECT  Dept 2   DATE  2017-01-11  FROM dual UNION ALL     SELECT  Dept 3   DATE  2017-01-12  FROM dual UNION ALL     SELECT  Dept 4   DATE  2017-04-01  FROM dual UNION ALL     SELECT  Dept 5   DATE  2017-04-02  FROM dual     SELECT   FROM days  LEFT JOIN departments    PARTITION BY  department  -- This is where the magic happens   ON day  gt   created at   Parts of the result    -------- ------------ ------------    day      department   created at    -------- ------------ ------------    Jan 01   Dept 1                    -- Didn t match  but still get row   Jan 02   Dept 1                    -- Didn t match  but still get row            Dept 1                    -- Didn t match  but still get row   Jan 09   Dept 1                    -- Didn t match  but still get row   Jan 10   Dept 1       Jan 10       -- Matches  so get join result   Jan 11   Dept 1       Jan 10       -- Matches  so get join result   Jan 12   Dept 1       Jan 10       -- Matches  so get join result            Dept 1       Jan 10       -- Matches  so get join result   Jan 31   Dept 1       Jan 10       -- Matches  so get join result   The point here is that all rows from the partitioned side of the join will wind up in the result regardless if the JOIN matched anything on the  other side of the JOIN   Long story short  This is to fill up sparse data in reports  Very useful   SEMI JOIN  Seriously  No other answer got this  Of course not  because it doesn t have a native syntax in SQL  unfortunately  just like ANTI JOIN below   But we can use IN   and EXISTS    e g  to find all actors who have played in films   SELECT   FROM actor a WHERE EXISTS     SELECT   FROM film actor fa   WHERE a actor id   fa actor id     The WHERE a actor id   fa actor id predicate acts as the semi join predicate  If you don t believe it  check out execution plans  e g  in Oracle  You ll see that the database executes a SEMI JOIN operation  not the EXISTS   predicate     ANTI JOIN  This is just the opposite of SEMI JOIN  be careful not to use NOT IN though  as it has an important caveat   Here are all the actors without films   SELECT   FROM actor a WHERE NOT EXISTS     SELECT   FROM film actor fa   WHERE a actor id   fa actor id     Some folks  especially MySQL people  also write ANTI JOIN like this   SELECT   FROM actor a LEFT JOIN film actor fa USING  actor id  WHERE film id IS NULL   I think the historic reason is performance   LATERAL JOIN  OMG  this one is too cool  I m the only one to mention it  Here s a cool query   SELECT a first name  a last name  f   FROM actor AS a LEFT OUTER JOIN LATERAL     SELECT f title  SUM amount  AS revenue   FROM film AS f   JOIN film actor AS fa USING  film id    JOIN inventory AS i USING  film id    JOIN rental AS r USING  inventory id    JOIN payment AS p USING  rental id    WHERE fa actor id   a actor id -- JOIN predicate with the outer query    GROUP BY f film id   ORDER BY revenue DESC   LIMIT 5   AS f ON true   It will find the TOP 5 revenue producing films per actor  Every time you need a TOP-N-per-something query  LATERAL JOIN will be your friend  If you re a SQL Server person  then you know this JOIN type under the name APPLY  SELECT a first name  a last name  f   FROM actor AS a OUTER APPLY     SELECT f title  SUM amount  AS revenue   FROM film AS f   JOIN film actor AS fa ON f film id   fa film id   JOIN inventory AS i ON f film id   i film id   JOIN rental AS r ON i inventory id   r inventory id   JOIN payment AS p ON r rental id   p rental id   WHERE fa actor id   a actor id -- JOIN predicate with the outer query    GROUP BY f film id   ORDER BY revenue DESC   LIMIT 5   AS f   OK  perhaps that s cheating  because a LATERAL JOIN or APPLY expression is really a  correlated subquery  that produces several rows  But if we allow for  correlated subqueries   we can also talk about     MULTISET  This is only really implemented by Oracle and Informix  to my knowledge   but it can be emulated in PostgreSQL using arrays and or XML and in SQL Server using XML   MULTISET produces a correlated subquery and nests the resulting set of rows in the outer query  The below query selects all actors and for each actor collects their films in a nested collection   SELECT a    MULTISET     SELECT f     FROM film AS f   JOIN film actor AS fa USING  film id    WHERE a actor id   fa actor id   AS films FROM actor   As you have seen  there are more types of JOIN than just the  boring  INNER  OUTER  and CROSS JOIN that are usually mentioned  More details in my article  And please  stop using Venn diagrams to illustrate them

User · Answer

An illustration from W3schools

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Definition      JOINS are way to query the data that combined together from multiple tables simultaneously   Types of JOINS     Concern to RDBMS there are 5-types of joins     Equi-Join  Combines common records from two tables based on equality condition  Technically  Join made by using equality-operator     to compare values of Primary Key of one table and Foreign Key values of another table  hence result set includes common matched  records from both tables  For implementation see INNER-JOIN  Natural-Join   It is enhanced version of Equi-Join  in which SELECT operation omits duplicate column  For implementation see INNER-JOIN Non-Equi-Join  It is reverse of Equi-join where joining condition is uses other than equal operator    e g       lt            lt  or BETWEEN etc  For implementation see INNER-JOIN  Self-Join   A customized behavior of join where a table combined with itself  This is typically needed for querying self-referencing tables  or Unary relationship entity   For implementation see INNER-JOINs  Cartesian Product  It cross combines all records of both tables without any condition  Technically  it returns the result set of a query without WHERE-Clause    As per SQL concern and advancement  there are 3-types of joins and all RDBMS joins can be achieved using these types of joins    INNER-JOIN  It merges or combines  matched rows from two tables  The matching is done based on common columns of tables and their comparing operation  If equality based condition then  EQUI-JOIN performed  otherwise Non-EQUI-Join  OUTER-JOIN  It merges or combines  matched rows from two tables and unmatched rows with NULL values  However  can customized selection of un-matched rows e g  selecting unmatched row from first table or second table by sub-types  LEFT OUTER JOIN and RIGHT OUTER JOIN    2 1  LEFT Outer JOIN  a k a  LEFT-JOIN   Returns matched rows from two tables and unmatched from the LEFT table i e  first table  only   2 2  RIGHT Outer JOIN  a k a  RIGHT-JOIN   Returns matched rows from two tables and unmatched from the RIGHT table only   2 3  FULL OUTER JOIN  a k a OUTER JOIN   Returns matched and unmatched from both tables  CROSS-JOIN  This join does not merges combines instead it performs Cartesian product     Note  Self-JOIN can be achieved by either INNER-JOIN  OUTER-JOIN and CROSS-JOIN based on requirement but the table must join with itself   For more information   Examples   1 1  INNER-JOIN  Equi-join implementation  SELECT    FROM Table1 A   INNER JOIN Table2 B ON A  lt Primary-Key gt   B  lt Foreign-Key gt     1 2  INNER-JOIN  Natural-JOIN implementation  Select A    B Col1  B Col2          --But no B ForeignKeyColumn in Select  FROM Table1 A  INNER JOIN Table2 B On A Pk   B Fk    1 3  INNER-JOIN with NON-Equi-join implementation  Select    FROM Table1 A INNER JOIN Table2 B On A Pk  lt   B Fk    1 4  INNER-JOIN with SELF-JOIN  Select    FROM Table1 A1 INNER JOIN Table1 A2 On A1 Pk   A2 Fk    2 1  OUTER JOIN  full outer join   Select    FROM Table1 A FULL OUTER JOIN Table2 B On A Pk   B Fk    2 2  LEFT JOIN   Select    FROM Table1 A LEFT OUTER JOIN Table2 B On A Pk   B Fk    2 3  RIGHT JOIN   Select    FROM Table1 A RIGHT OUTER JOIN Table2 B On A Pk   B Fk    3 1  CROSS JOIN   Select    FROM TableA CROSS JOIN TableB    3 2  CROSS JOIN-Self JOIN   Select    FROM Table1 A1 CROSS JOIN Table1 A2      OR    Select    FROM Table1 A1 Table1 A2

User · Answer

I m going to push my pet peeve  the USING keyword   If both tables on both sides of the JOIN have their foreign keys properly named  ie  same name  not just  id  then this can be used   SELECT     FROM customers JOIN orders USING  customer id    I find this very practical  readable  and not used often enough

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