I am wondering whether the natural join operation in relational algebra is associative. I mean, is the following equation true?
(S1 NATURAL JOIN S2) NATURAL JOIN S3 = S1 NATURAL JOIN (S2 NATURAL JOIN S3)
It looks associative to me intuitively, but i am not sure.
Thanks
An inner join only returns rows that match the on condition. It does not have special treatment for either the right or the left hand table. So an inner join is associative.
A natural join is an inner join on all columns with the same name. Since an inner join is associative, so is a natural join.
Related
From definitions i've read on internet, in equi join the join condition is equality (=) while inner join can have other operators such as less than (<) or greater than (>) as well.
a non-equi join is a type of join whose join condition uses conditional operators other than equals.
Does that mean non-equi joins and inner joins are same?
Those are two different things, "equi-" and "non-equi" joins are independent of the logical join type.
"Equi-join" is when all columns in ON clause are matched on equality, for example ON t1.c1 = t2.c1 AND t1.c2 = t2.c2.
"Non-equi-join" is when one or more columns are using an inequality comparison (e.g. < less than, > more than, <> not equal etc.), for example ON t1.c1 = t2.c1 AND t1.c2 > t2.c2.
Both "equi-join" and "non-equi-join" may be used together with any logical join, e.g. INNER JOIN, OUTER JOIN, LEFT JOIN, RIGHT JOIN, etc.
I found the following diagram from Complex SQL.com site to be the easiest way to understand it:
You can read further details for example in An Illustrated Guide to the SQL Non Equi Join or SQL Joins on Beginner SQL Tutorial.
I'm using Oracle 11g for the study. Now, I don't understand the below example because, SQL result is same. I want to listen about join differences above example
SELECT P.player_name,
P.back_no,
T.region_name,
T.team_name
FROM player P
inner join team T
ON P.team_id = T.team_id
AND P.position = 'GK'
ORDER BY P.back_no ASC;
SELECT P.player_name,
P.back_no,
T.region_name,
T.team_name
FROM player P
inner join team T
ON P.team_id = T.team_id
WHERE P.position = 'GK'
ORDER BY P.back_no ASC;
I think the below link might help you to understand the difference. Read Here.
Almost every join is an equijoin, because the condition for matching rows is based on the equality of two values—one from each of the tables being joined. So that's what makes it an equijoin: the ON condition is equality. This includes inner joins and all three types of outer joins.
Inner joins, on the other hand, can be based on equality to match rows, or on some other condition entirely. If it's not an equijoin, then it's usually called a theta join, although to be precise, an equijoin is just one of the possible theta joins; other theta joins use less than, less than or equal, etc., as the comparison operator. As long as the comparison evaluates to TRUE, the matched rows qualify for the join.
You can find the examples in the above mentioned link.
Ok, I currently tasked with maintaining a legacy SQL Server 2005 database recently migrated to 2008 [R2] and there are some sprocs with something like the following:
SELECT
#val = c.TableVal
FROM
dbo.TaqbleA a
LEFT JOIN dbo.TableB b
ON a.TableId = b.TableId
,dbo.TableC c
WHERE
a.TableId = c.TableId
Fist off I know that the join conditions as seen with table A and B are the normal and how I'd rewrite this but I'm wondering if Table C is treated as a LEFT JOIN as it is listed after the LEFT JOIN or if it is treated as an INNER JOIN? My gut says it is treated as an INNER JOIN, but I'm not certain. Which is it?
Furthermore, I know I'm not the only one who has ever seen something like this and it would be good to have this answer immortalized in the StackExchange...
Thanks.
It is effectively an INNER JOIN or CROSS JOIN; the comma has lower precedence than the explicit JOIN, so FROM ... LEFT JOIN ... ON ..., ... means FROM (... LEFT JOIN ... ON ...), ..., with the LEFT JOIN pertaining only to the first two tables.
Consider the example from MSDN documentation:
SELECT p.Name, pr.ProductReviewID
FROM Production.Product p
LEFT OUTER JOIN Production.ProductReview pr
ON p.ProductID = pr.ProductID
In this example, it is clear that the table on the left is "Production" and that is where all rows will be returned from, and then only those that match in ProductReview.
But now consider the following hypothetical query with 3 tables A,B,C
select * from A
inner Join B on A.field1 = B.field1
left outer join C on C.field2 = b.Field2
Which is the left table in this query (from which all records will be returned, regardless of a match to C)? Is it A or B? Or is it the result of the join from A & B?
My confusion arises from the following MSDN documentation, which states that "Outer joins can be specified in the FROM clause only" which would mean that the left table in my hypothetical query is A, but then I dont have an ON clause that specifies the join condition - in which case is my hypothetical query a bad one?
Since there is an INNER JOIN between A and B, only rows from B that match A will qualify for the LEFT JOIN to C.
I'm not 100% sure I understand you question, but assuming I am understanding it correctly:
Your "left" table in your hypothetical query is B, since your ON condition specifies the B.Field2.
The terms 'left" and "right" are not sufficiently specific in this context. Instead, you should use the terms "preserved" and "unpreserved". In that light, tables A and B are preserved and table C is unpreserved.
The reference in the MSDN documentation is meant to imply you cannot use joins (outer or otherwise) in the Select, Where, Group By, Having or Order By clauses outside of a subquery (where they are still in a From clause).
From your joins
A inner Join B on A.field1 = B
left outer join C on C.field2 = b.Field2
You need to have records from table A and B.
The left join only has data from table C field field2 matching the B table, but note that table A field2 does not have to match.
To see your data for table C run the following:
select c.*
from A inner Join B on A.field1 = B.field1 left outer join C on C.field2 = b.Field2
They use the term FROM clause in a general (broad) sense meaning the whole section of the query that starts from the keyword FROM and includes all the joins there are.
Here's a fuller context (note the previous sentence):
Inner joins can be specified in either the FROM or WHERE clauses. Outer joins can be specified in the FROM clause only.
See? They mean you cannot specify an outer join in the WHERE clause as is the case with inner joins. You can only do that in the FROM clause (that is, after however many other joins too). The result will be applied to the result of the previous joins.
What is the difference between LEFT JOIN and LEFT OUTER JOIN?
As per the documentation: FROM (Transact-SQL):
<join_type> ::=
[ { INNER | { { LEFT | RIGHT | FULL } [ OUTER ] } } [ <join_hint> ] ]
JOIN
The keyword OUTER is marked as optional (enclosed in square brackets). In this specific case, whether you specify OUTER or not makes no difference. Note that while the other elements of the join clause is also marked as optional, leaving them out will make a difference.
For instance, the entire type-part of the JOIN clause is optional, in which case the default is INNER if you just specify JOIN. In other words, this is legal:
SELECT *
FROM A JOIN B ON A.X = B.Y
Here's a list of equivalent syntaxes:
A LEFT JOIN B A LEFT OUTER JOIN B
A RIGHT JOIN B A RIGHT OUTER JOIN B
A FULL JOIN B A FULL OUTER JOIN B
A INNER JOIN B A JOIN B
Also take a look at the answer I left on this other SO question: SQL left join vs multiple tables on FROM line?.
To answer your question there is no difference between LEFT JOIN
and LEFT OUTER JOIN, they are exactly same that said...
At the top level there are mainly 3 types of joins:
INNER
OUTER
CROSS
INNER JOIN - fetches data if present in both the tables.
OUTER JOIN are of 3 types:
LEFT OUTER JOIN - fetches data if present in the left table.
RIGHT OUTER JOIN - fetches data if present in the right table.
FULL OUTER JOIN - fetches data if present in either of the two tables.
CROSS JOIN, as the name suggests, does [n X m] that joins everything to everything.
Similar to scenario where we simply lists the tables for joining (in the FROM clause of the SELECT statement), using commas to separate them.
Points to be noted:
If you just mention JOIN then by default it is a INNER JOIN.
An OUTER join has to be LEFT | RIGHT | FULL you can not simply say OUTER JOIN.
You can drop OUTER keyword and just say LEFT JOIN or RIGHT JOIN or FULL JOIN.
For those who want to visualise these in a better way, please go to this link:
A Visual Explanation of SQL Joins
What is the difference between left join and left outer join?
Nothing. LEFT JOIN and LEFT OUTER JOIN are equivalent.
Left Join and Left Outer Join are one and the same. The former is the shorthand for the latter. The same can be said about the Right Join and Right Outer Join relationship. The demonstration will illustrate the equality. Working examples of each query have been provided via SQL Fiddle. This tool will allow for hands on manipulation of the query.
Given
Left Join and Left Outer Join
Results
Right Join and Right Outer Join
Results
I'm a PostgreSQL DBA, as far as I could understand the difference between outer or not outer joins difference is a topic that has considerable discussion all around the internet. Until today I never saw a difference between those two; So I went further and I try to find the difference between those.
At the end I read the whole documentation about it and I found the answer for this,
So if you look on documentation (at least in PostgreSQL) you can find this phrase:
"The words INNER and OUTER are optional in all forms. INNER is the default; LEFT, RIGHT, and FULL imply an outer join."
In another words,
LEFT JOIN and LEFT OUTER JOIN ARE THE SAME
RIGHT JOIN and RIGHT OUTER JOIN ARE THE SAME
I hope it can be a contribute for those who are still trying to find the answer.
I find it easier to think of Joins in the following order:
CROSS JOIN - a Cartesian product of both tables. ALL joins begin here
INNER JOIN - a CROSS JOIN with a filter added.
OUTER JOIN - an INNER JOIN with missing elements (from either LEFT or RIGHT table)
added afterward.
Until I figured out this (relatively) simple model, JOINS were always a bit more of a black art. Now they make perfect sense.
Hope this helps more than it confuses.
To answer your question
In Sql Server joins syntax OUTER is optional
It is mentioned in msdn article : https://msdn.microsoft.com/en-us/library/ms177634(v=sql.130).aspx
So following list shows join equivalent syntaxes with and without OUTER
LEFT OUTER JOIN => LEFT JOIN
RIGHT OUTER JOIN => RIGHT JOIN
FULL OUTER JOIN => FULL JOIN
Other equivalent syntaxes
INNER JOIN => JOIN
CROSS JOIN => ,
Strongly Recommend Dotnet Mob Artice : Joins in Sql Server
Why are LEFT/RIGHT and LEFT OUTER/RIGHT OUTER the same? Let's explain why this vocabulary.
Understand that LEFT and RIGHT joins are specific cases of the OUTER join, and therefore couldn't be anything else than OUTER LEFT/OUTER RIGHT. The OUTER join is also called FULL OUTER as opposed to LEFT and RIGHT joins that are PARTIAL results of the OUTER join. Indeed:
Table A | Table B Table A | Table B Table A | Table B Table A | Table B
1 | 5 1 | 1 1 | 1 1 | 1
2 | 1 2 | 2 2 | 2 2 | 2
3 | 6 3 | null 3 | null - | -
4 | 2 4 | null 4 | null - | -
null | 5 - | - null | 5
null | 6 - | - null | 6
OUTER JOIN (FULL) LEFT OUTER (partial) RIGHT OUTER (partial)
It is now clear why those operations have aliases, as well as it is clear only 3 cases exist: INNER, OUTER, CROSS. With two sub-cases for the OUTER.
The vocabulary, the way teachers explain this, as well as some answers above, often make it looks like there are lots of different types of join. But it's actually very simple.
There are only 3 joins:
A) Cross Join = Cartesian (E.g: Table A, Table B)
B) Inner Join = JOIN (E.g: Table A Join/Inner Join Table
B)
C) Outer join:
There are three type of outer join
Left Outer Join = Left Join
Right Outer Join = Right Join
Full Outer Join = Full Join
There are mainly three types of JOIN
Inner: fetches data, that are present in both tables
Only JOIN means INNER JOIN
Outer: are of three types
LEFT OUTER - - fetches data present only in left table & matching condition
RIGHT OUTER - - fetches data present only in right table & matching condition
FULL OUTER - - fetches data present any or both table
(LEFT or RIGHT or FULL) OUTER JOIN can be written w/o writing "OUTER"
Cross Join: joins everything to everything
Syntactic sugar, makes it more obvious to the casual reader that the join isn't an inner one.
Just in the context of this question, I want to post the 2 'APPLY' operators as well:
JOINS:
INNER JOIN = JOIN
OUTER JOIN
LEFT OUTER JOIN = LEFT JOIN
RIGHT OUTER JOIN = RIGHT JOIN
FULL OUTER JOIN = FULL JOIN
CROSS JOIN
SELF-JOIN: This is not exactly a separate type of join. This is basically joining a table to itself using one of the above joins. But I felt it is worth mentioning in the context JOIN discussions as you will hear this term from many in the SQL Developer community.
APPLY:
CROSS APPLY -- Similar to INNER JOIN (But has added advantage of being able to compute something in the Right table for each row of the Left table and would return only the matching rows)
OUTER APPLY -- Similar to LEFT OUTER JOIN (But has added advantage of being able to compute something in the Right table for each row of the Left table and would return all the rows from the Left table irrespective of a match on the Right table)
https://www.mssqltips.com/sqlservertip/1958/sql-server-cross-apply-and-outer-apply/
https://sqlhints.com/2016/10/23/outer-apply-in-sql-server/
Real life example, when to use OUTER / CROSS APPLY in SQL
I find APPLY operator very beneficial as they give better performance than having to do the same computation in a subquery. They are also replacement of many Analytical functions in older versions of SQL Server. That is why I believe that after being comfortable with JOINS, one SQL developer should try to learn the APPLY operators next.