Thursday, 1 November 2018

What is the difference between “INNER JOIN” and “OUTER JOIN”?

Also how do LEFT JOIN, RIGHT JOIN and FULL JOIN fit in?

 Answers


Assuming you're joining on columns with no duplicates, which is a very common case:
  • An inner join of A and B gives the result of A intersect B, i.e. the inner part of a Venn diagram intersection.
  • An outer join of A and B gives the results of A union B, i.e. the outer parts of a Venn diagram union.
Examples
Suppose you have two tables, with a single column each, and data as follows:
A    B
-    -
1    3
2    4
3    5
4    6
Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.
Inner join
An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.
select * from a INNER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a = b.b;

a | b
--+--
3 | 3
4 | 4
Left outer join
A left outer join will give all rows in A, plus any common rows in B.
select * from a LEFT OUTER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a = b.b(+);

a |  b
--+-----
1 | null
2 | null
3 |    3
4 |    4
Right outer join
A right outer join will give all rows in B, plus any common rows in A.
select * from a RIGHT OUTER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a(+) = b.b;

a    |  b
-----+----
3    |  3
4    |  4
null |  5
null |  6
Full outer join
A full outer join will give you the union of A and B, i.e. all the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.
select * from a FULL OUTER JOIN b on a.a = b.b;

 a   |  b
-----+-----
   1 | null
   2 | null
   3 |    3
   4 |    4
null |    6
null |    5


I recommend Jeff's blog article. The best description I've ever seen, plus there is a visualization, e.g.:
Inner Join:
Full Outer Join:


The following was taken from the article "MySQL - LEFT JOIN and RIGHT JOIN, INNER JOIN and OUTER JOIN" by Graham Ellis on his blog Horse's Mouth.
In a database such as MySQL, data is divided into a number of tables which are then connected (Joined) together by JOIN in SELECT commands to read records from multiple tables. Read this example to see how it works.
First, some sample data:
people
    mysql> select * from people;
    +------------+--------------+------+
    | name       | phone        | pid  |
    +------------+--------------+------+
    | Mr Brown   | 01225 708225 |    1 |
    | Miss Smith | 01225 899360 |    2 |
    | Mr Pullen  | 01380 724040 |    3 |
    +------------+--------------+------+
    3 rows in set (0.00 sec)

property
    mysql> select * from property;
    +------+------+----------------------+
    | pid  | spid | selling              |
    +------+------+----------------------+
    |    1 |    1 | Old House Farm       |
    |    3 |    2 | The Willows          |
    |    3 |    3 | Tall Trees           |
    |    3 |    4 | The Melksham Florist |
    |    4 |    5 | Dun Roamin           |
    +------+------+----------------------+
    5 rows in set (0.00 sec)
REGULAR JOIN
If we do a regular JOIN (with none of the keywords INNER, OUTER, LEFT or RIGHT), then we get all records that match in the appropriate way in the two tables, and records in both incoming tables that do not match are not reported:
mysql> select name, phone, selling 
from people join property 
on people.pid = property.pid;
+-----------+--------------+----------------------+
| name      | phone        | selling              |
+-----------+--------------+----------------------+
| Mr Brown  | 01225 708225 | Old House Farm       |
| Mr Pullen | 01380 724040 | The Willows          |
| Mr Pullen | 01380 724040 | Tall Trees           |
| Mr Pullen | 01380 724040 | The Melksham Florist |
+-----------+--------------+----------------------+
4 rows in set (0.01 sec)
LEFT JOIN
If we do a LEFT JOIN, we get all records that match in the same way and IN ADDITION we get an extra record for each unmatched record in the left table of the join - thus ensuring (in this example) that every PERSON gets a mention:
   mysql> select name, phone, selling 
    from people left join property 
    on people.pid = property.pid; 
    +------------+--------------+----------------------+
    | name       | phone        | selling              |
    +------------+--------------+----------------------+
    | Mr Brown   | 01225 708225 | Old House Farm       |
    | Miss Smith | 01225 899360 | NULL <<-- unmatch    |
    | Mr Pullen  | 01380 724040 | The Willows          |
    | Mr Pullen  | 01380 724040 | Tall Trees           |
    | Mr Pullen  | 01380 724040 | The Melksham Florist |
    +------------+--------------+----------------------+
    5 rows in set (0.00 sec)
RIGHT JOIN
If we do a RIGHT JOIN, we get all the records that match and IN ADDITION an extra record for each unmatched record in the right table of the join - in my example, that means that each property gets a mention even if we don't have seller details:
mysql> select name, phone, selling 
from people right join property 
on people.pid = property.pid;
+-----------+--------------+----------------------+
| name      | phone        | selling              |
+-----------+--------------+----------------------+
| Mr Brown  | 01225 708225 | Old House Farm       |
| Mr Pullen | 01380 724040 | The Willows          |
| Mr Pullen | 01380 724040 | Tall Trees           |
| Mr Pullen | 01380 724040 | The Melksham Florist |
| NULL      | NULL         | Dun Roamin           |
+-----------+--------------+----------------------+
5 rows in set (0.00 sec)
An INNER JOIN does a full join, just like the first example, and the word OUTER may be added after the word LEFT or RIGHT in the last two examples - it's provided for ODBC compatibility and doesn't add an extra capabilities.


In simple words:
An inner join retrieve the matched rows only.
Whereas an outer join retrieve the matched rows from one table and all rows in other table ....the result depends on which one you are using:
  • Left: Matched rows in the right table and all rows in the left table
  • Right: Matched rows in the left table and all rows in the right table or
  • Full: All rows in all tables. It doesn't matter if there is a match or not


A inner join only shows rows if there is a matching record on the other (right) side of the join.
A (left) outer join shows rows for each record on the left hand side, even if there are no matching rows on the other (right) side of the join. If there is no matching row, the columns for the other (right) side would show NULLs.


In simple words :
Inner join -> Take ONLY common records from parent and child tables WHERE primary key of Parent table matches Foreign key in Child table.
Left join ->
pseudo code
1.Take All records from left Table
2.for(each record in right table,) {
    if(Records from left & right table matching on primary & foreign key){
       use their values as it is as result of join at the right side for 2nd table.
    } else {
       put value NULL values in that particular record as result of join at the right side for 2nd table.
    }
  }
Right join : Exactly opposite of left join . Put name of table in LEFT JOIN at right side in Right join , you get same output as LEFT JOIN.
Outer join : Show all records in Both tables No matter what. If records in Left table are not matching to right table based on Primary , Forieign key , use NULL value as result of join .
Example :
Lets assume now for 2 tables
1.employees , 2.phone_numbers_employees
employees : id , name 

phone_numbers_employees : id , phone_num , emp_id
Here , employees table is Master table , phone_numbers_employees is child table(it contains emp_id as foreign key which connects employee.id so its child table.)
Inner joins
Take the records of 2 tables ONLY IF Primary key of employees table(its id) matches Foreign key of Child table phone_numbers_employees(emp_id).
So query would be :
SELECT e.id , e.name , p.phone_num FROM employees AS e INNER JOIN phone_numbers_employees AS p ON e.id = p.emp_id;
Here take only matching rows on primary key = foreign key as explained above.Here non matching rows on primary key = foreign key are skipped as result of join.
Left joins :
Left join retains all rows of the left table, regardless of whether there is a row that matches on the right table.
SELECT e.id , e.name , p.phone_num FROM employees AS e LEFT JOIN phone_numbers_employees AS p ON e.id = p.emp_id;
Outer joins :
SELECT e.id , e.name , p.phone_num FROM employees AS e OUTER JOIN phone_numbers_employees AS p ON e.id = p.emp_id;
Diagramatically it looks like :




The answer is in the meaning of each one, so in the results.
Note :
In SQLite there is no RIGHT OUTER JOIN or FULL OUTER JOIN.
And also in MySQL there is no FULL OUTER JOIN.
My answer is based on above Note.
When you have two tables like these:
--[table1]               --[table2]
id | name                id | name
---+-------              ---+-------
1  | a1                  1  | a2
2  | b1                  3  | b2
CROSS JOIN / OUTER JOIN :
You can have all of those tables data with CROSS JOIN or just with , like this:
SELECT * FROM table1, table2
--[OR]
SELECT * FROM table1 CROSS JOIN table2

--[Results:]
id | name | id | name 
---+------+----+------
1  | a1   | 1  | a2
1  | a1   | 3  | b2
2  | b1   | 1  | a2
2  | b1   | 3  | b2
INNER JOIN :
When you want to add a filter to above results based on a relation like table1.id = table2.id you can use INNER JOIN:
SELECT * FROM table1, table2 WHERE table1.id = table2.id
--[OR]
SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id

--[Results:]
id | name | id | name 
---+------+----+------
1  | a1   | 1  | a2
LEFT [OUTER] JOIN :
When you want to have all rows of one of tables in the above result -with same relation- you can use LEFT JOIN:
(For RIGHT JOIN just change place of tables)
SELECT * FROM table1, table2 WHERE table1.id = table2.id 
UNION ALL
SELECT *, Null, Null FROM table1 WHERE Not table1.id In (SELECT id FROM table2)
--[OR]
SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id

--[Results:]
id | name | id   | name 
---+------+------+------
1  | a1   | 1    | a2
2  | b1   | Null | Null
FULL OUTER JOIN :
When you also want to have all rows of the other table in your results you can use FULL OUTER JOIN:
SELECT * FROM table1, table2 WHERE table1.id = table2.id
UNION ALL
SELECT *, Null, Null FROM table1 WHERE Not table1.id In (SELECT id FROM table2)
UNION ALL
SELECT Null, Null, * FROM table2 WHERE Not table2.id In (SELECT id FROM table1)
--[OR] (recommended for SQLite)
SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id
UNION ALL
SELECT * FROM table2 LEFT JOIN table1 ON table2.id = table1.id
WHERE table1.id IS NULL
--[OR]
SELECT * FROM table1 FULL OUTER JOIN table2 On table1.id = table2.id

--[Results:]
id   | name | id   | name 
-----+------+------+------
1    | a1   | 1    | a2
2    | b1   | Null | Null
Null | Null | 3    | b2
Well, as your need you choose each one that covers your need ;).


The difference is in the way tables are joined if there are no common records.
  • JOIN is same as INNER JOIN and means to only show records common to both tables. Whether the records are common is determined by the fields in join clause. For example:
    FROM t1
JOIN t2 on t1.ID = t2.ID
    means show only records where the same ID value exists in both tables.
  • LEFT JOIN is same as LEFT OUTER JOIN and means to show all records from left table (i.e. the one that precedes in SQL statement) regardless of the existance of matching records in the right table.
  • RIGHT JOIN is same as RIGHT OUTER JOIN and means opposite of LEFT JOIN, i.e. shows all records from the second (right) table and only matching records from first (left) table.


I don't see much details about performance and optimizer in the other answers.
Sometimes it is good to know that only INNER JOIN is associative which means the optimizer has the most option to play with it. It can reorder the join order to make it faster keeping the same result. The optimizer can use the most join modes.
Generally it is a good practice to try to use INNER JOIN instead of the different kind of joins. (Of course if it is possible considering the expected result set.)
There are a couple of good examples and explanation here about this strange associative behavior:


Having criticized the much-loved red-shaded Venn diagram, I thought it only fair to post my own attempt.
Although @Martin Smith's answer is the best of this bunch by a long way, his only shows the key column from each table, whereas I think ideally non-key columns should also be shown.
The best I could do in the half hour allowed, I still don't think it adequately shows that the nulls are there due to absence of key values in TableB or that OUTER JOIN is actually a union rather than a join:


Simplest Definitions
Inner Join: Returns matched records from both tables.
Full Outer Join: Returns matched and unmatched records from both tables with null for unmatched records from Both Tables.
Left Outer Join: Returns matched and unmatched records only from table on Left Side.
Right Outer Join: Returns matched and unmatched records only from table on Right Side.
In-Short
Matched + Left Unmatched + Right Unmatched = Full Outer Join
Matched + Left Unmatched = Left Outer Join
Matched + Right Unmatched = Right Outer Join
Matched = Inner Join


INNER JOIN
An inner join produces a result set that is limited to the rows where there is a match in both tables for what we're looking for. If you don't know which kind of join you need, this will usually be your best bet.
LEFT OUTER JOIN
A left outer join, or left join, results in a set where all of the rows from the first, or left hand side, table are preserved. The rows from the second, or right hand side table only show up if they have a match with the rows from the first table. Where there are values from the left table but not from the right, the table will read null, which means that the value has not been set.
RIGHT OUTER JOIN
A right outer join, or right join, is the same as a left join, except the roles are reversed. All of the rows from the right hand side table show up in the result, but the rows from the table on the left are only there if they match the table on the right. Empty spaces are null, just like with the the left join.
FULL OUTER JOIN
A full outer join, or just outer join, produces a result set with all of the rows of both tables, regardless of whether there are any matches. Similarly to the left and right joins, we call the empty spaces null.
For More Reference


What is the difference between “INNER JOIN” and “OUTER JOIN”?
They are the most commonly used existential operators in SQL, where INNER JOIN is used for 'exists' and LEFT OUTER JOIN is used for 'does not exist'.
Consider these queries:
users who have posted and have votes
users who have posted but have no badges
People who look for set-based solutions (an industry term) would recognise the respective queries as:
users who have posted INTERSECT users who have votes
users who have posted MINUS users who have badges
Translating these into standard SQL:
SELECT UserId FROM Posts
INTERSECT 
SELECT UserId FROM Votes;

SELECT UserId FROM Posts
EXCEPT 
SELECT UserId FROM Badges;
Others will think along similar lines of set inclusion:
users who have posted and IN the set of users who have votes
users who have posted and NOT IN the set of users who have badges
Translating these into standard SQL:
SELECT UserId 
  FROM Posts
 WHERE UserId IN ( SELECT UserId FROM Votes );

SELECT UserId 
  FROM Posts
 WHERE UserId NOT IN ( SELECT UserId FROM Badges );
Some will think in terms of 'existance' within sets e.g.
users who have posted and EXIST in the set of users who have votes
users who have posted and do NOT EXIST in the set of users who have badges
Translating these into standard SQL (note we now need to use range variables i.e. p, v, b):
SELECT p.UserId 
  FROM Posts p
 WHERE EXISTS ( SELECT *
                  FROM Votes v
                 WHERE v.UserId = p.UserId );

SELECT p.UserId 
  FROM Posts p
 WHERE NOT EXISTS ( SELECT *
                      FROM Badges b
                     WHERE b.UserId = p.UserId );
However, I've found that the "industry standard" approach is to exclusively use joins. I don't know what the thinking is here (Law of the Instrument? Premature optimization?), so I'll go straight to the syntax:
SELECT p.UserId 
  FROM Posts p
       INNER JOIN Votes v ON v.UserId = p.UserId;

SELECT p.UserId 
  FROM Posts p
       LEFT JOIN Badges b ON b.UserId = p.UserId
 WHERE b.UserId IS NULL;
Things to note:
  • The only projection is from Users but we still need all those range variables (p, v, b) for search conditions.
  • The UserId IS NULL search condition 'belongs' to the the OUTER JOIN but is disconnected in the query.
  • LEFT is the industry standard: professionals will rewrite a query to avoid using RIGHT!
  • The OUTER keyword from LEFT OUTER JOIN is omitted.
Closing remarks:
Sometimes joins are used in queries solely to determine whether values exist or do not exists in another set. Learn to look carefully at the attributes being projected (the columns in the SELECT clause): if there are none from the joined table then they are simply being used as existential operators. Additionally for outer join, look for instances of <key_column> IS NULL in the WHERE clause.


1.Inner Join: Also called as Join. It returns the rows present in both the Left table, and right table only if there is a match. Otherwise, it returns zero records.
Example:
SELECT
  e1.emp_name,
  e2.emp_salary    
FROM emp1 e1
INNER JOIN emp2 e2
  ON e1.emp_id = e2.emp_id
2.Full Outer Join: Also called as Full Join. It returns all the rows present in both the Left table, and right table.
Example:
SELECT
  e1.emp_name,
  e2.emp_salary    
FROM emp1 e1
FULL OUTER JOIN emp2 e2
  ON e1.emp_id = e2.emp_id
3.Left Outer join: Or simply called as Left Join. It returns all the rows present in the Left table and matching rows from the right table (if any).
4.Right Outer Join: Also called as Right Join. It returns matching rows from the left table (if any), and all the rows present in the Right table.
Advantages of Joins
  1. Executes faster.

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