Is LEFT OUTER JOIN expressed as just LEFT JOIN in MySQL syntax? Or are LEFT OUTER JOIN and LEFT JOIN different things in MySQL?
Basically they are the same.
LEFT and RIGHT are outer type join. The keyword OUTER is optional, and what this means in this case is that whether you specify it or not makes no difference.
Additional Information,
At the top level there are mainly 3 types of joins:
INNER
OUTER (LEFT, RIGHT)
CROSS
Related
This question already has answers here:
why we have need of left and right join
(2 answers)
Closed 1 year ago.
I think either left/right join is sufficient, the other will derived if table names are interchanged, Anyone know why there is both left and right join?
Thanks
The order of join evaluation is generally left-to-right. So a join b join c executes as if you had parenthesized it (a join b) join c.
The optimizer can reorder inner joins because inner joins are commutative. Reordering outer joins is not commutative (i.e. a left join b is not the same as b left join a), so the optimizer can't swap them.
If you want to do a left outer join to the result of a joined pair of tables, use parentheses, like: a left join (b join c).
But you can also take advantage of the left-to-right order of evaluation by switching to a right outer join: b join c right outer join a -- without parentheses!
I have two tables, toynav_product_import - 18533 rows, catalog_product_entity - 42000 rows.
The below query, LEFT JOIN takes more than 2 minutes, while INNER JOIN runs in 0.009 seconds. The first table has the necessary index for the barcode field.
SELECT tpi.barcode FROM toynav_product_import tpi
INNER JOIN catalog_product_entity cpe ON tpi.barcode = cpe.sku
Please advise
toynav_product_import
catalog_product_entity
An outer join ( LEFT JOIN or RIGHT JOIN ) has to do all the work of an INNER JOIN plus the extra work of null-extending the results
And even if a LEFT JOIN were faster in specific situations, it is not functionally equivalent to an INNER JOIN, so you cannot simply go replacing all instances of one with the other!
Sorry cant post this as a comment
A LEFT JOIN is slower than the Inner Join. By definition, an outer join (LEFT JOIN or RIGHT JOIN) has to do all the work of an INNER JOIN plus the extra work of null-extending the results, Thats the reason. And as it also returns more number of Rows as compare to inner join, Thats why execution takes more time.
But by indexing the Foreign Keys properly, you can definitely increase the performance of the Joins.
It also depends on the Data, Its not always the case that Left join is slower, There are the cases when Left join is faster, But mostly Inner join is faster according to above described reasons.
Please refer to this link, the guy explained the difference very clearly.
I have 2 tables:
circuit(id_circuit, distance)
and
circuit_langue(id_circuit_language, #id_circuit, language, title).
if I do a join between circuit and circuit_langue, and it's possible that some objects from circuit don't have a circuit_langue,
what i have to do if I want to recuperate objects without circuit_langue ?
You most likely did INNER JOIN which shows only those records that have a matching row from both sides of the join (tables in this example).
You need a LEFT JOIN to view all circuits even if there is not circuit_langue row associated with it:
select *
from circuit c
left join circuit_langue cl on
c.id_circuit = cl.id_circuit
If you only need to display records that don't have a corresponding row in langue table you could add a WHERE condition to above query:
select *
from circuit c
left join circuit_langue cl on
c.id_circuit = cl.id_circuit
where cl.id_circuit is null
By default, the JOIN (INNER JOIN) recovers rows that match on both tables.
If you want to recover both the objects with and without a circuit_langue associated you can use a LEFT OUTER JOIN:
SELECT * FROM circuit c LEFT OUTER JOIN circuit_langue cl
ON c.id_circuit = cl.id_circuit
There are four main kinds of joins :
inner join (which is the default)
left outer join
right outer join
full outer join
You've used the INNER JOIN which only returns the matched values in both tables, while you actually need a LEFT OUTER JOIN which returns all rows from the left table, even if there are no matches in the right table.
For reference, the left table is the first one after the FROM keyword.
For further knowledge:
RIGHT OUTER JOIN : is exactly the opposite of LEFT OUTER JOIN.
FULL OUTER JOIN :returns rows when there is a match in either one of the tables.
how to convert this left outer join query to subset query
select
j.*, concat(d.Name, ', ', d.Gelar) as DSN,
prg.Nama_Indonesia as PRG,
kl.Kelas as kls,
kl.Sesi as ssi
from
jadwal j left outer join
dosen d on j.IDDosen=d.ID left outer join
kelas kl on j.Kelas=kl.ID left outer join
program prg on j.Program=prg.Kode left outer join
jabatanorganisasi jo on d.JabatanOrganisasi=jo.Kode left outer join
tahun t on j.tahun=t.id
order by
d.Name, prg.Nama_Indonesia, kl.Sesi, kl.Kelas;
please help
give me axample
For the first query you will be having rows, since all of the joins are LEFT JOIN, so atleast all entries from table jadwal will be there in output.
But on the other query you are doing selection using conditions(works like INNER JOIN), if the condition not satisfies there wont be any result-set.Thats why you are not getting any outputs.
There is no data with these conditions.Please check data
where
j.IDDosen=d.ID and
j.kelas=kl.ID and
j.program=prg.kode and
j.tahun=t.id AND
d.JabatanOrganisasi=jo.kode
Hope this helps
In the first query, you are using left joins (which means if there's no match between from table and the joining tables it would retrieve all the results from the from table), and in the second query you are using inner joins.
You can take a look on this What is the difference between "INNER JOIN" and "OUTER JOIN"?
This question already has answers here:
Difference between RIGHT & LEFT JOIN vs RIGHT & LEFT OUTER JOIN in SQL [duplicate]
(4 answers)
Closed 9 years ago.
I have seen joins called LEFT OUTER JOIN or RIGHT OUTER JOIN. In some places I have seen LEFT JOIN or RIGHT JOIN. I am confused by this.
I posted a question 2 days ago, but I am unable to understand the links the solutions provide.
Are these types of joins both the same, or is there some difference between the two?
There are no difference between both.
Refer visual represenation of joins
The first link they quoted gives you:
INNER JOIN: returns rows when there is a match in both tables.
LEFT JOIN / LEFT OUTER JOIN: returns all rows from the left table, even if there are no matches in the right table.
RIGHT JOIN / RIGHT OUTER JOIN: returns all rows from the right table, even if there are no matches in the left table.
FULL JOIN / FULL OUTER JOIN / OUTER JOIN: returns rows when there is a match in one of the tables.
SELF JOIN: is used to join a table to itself, as if the table were two tables, temporarily renaming at least one table in the SQL statement.
CARTESIAN JOIN: returns the cartesian product of the sets of records from the two or more joined tables.
The self join is actually not a special join. It just reflects the fact that you can join a table with itself. Doing so you must alias it in order to address the fact that it appears more than once in the same statement.
The cartesian join can be considered as an inner join without a restricting condition. Or you may view an inner join as a cartesian join with an added restriction (the join condition).
If you look at the manual page for JOIN you will see the following lines:
join_table:
table_reference [INNER | CROSS] JOIN table_factor [join_condition]
| table_reference STRAIGHT_JOIN table_factor
| table_reference STRAIGHT_JOIN table_factor ON conditional_expr
| table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
| table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor
The bold line clearly shows that the keyword OUTER is optional.
In MySQL syntax, LEFT OUTER JOIN and LEFT JOIN are identical: (from http://dev.mysql.com/doc/refman/5.0/en/join.html)
| table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
| table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor
Note that the OUTER keyword is optional for LEFT/RIGHT JOIN.
LEFT and RIGHT joins are both outer joins. I believe there are some flavors of SQL that may require the OUTER keyword, but MySQL does not. That is to say that at times LEFT JOIN may not be valid.
I am working in SQL Server and as per my usage and experience there is absolutely no difference between LEFT JOIN and LEFT OUTER JOIN. The same is true for RIGHT JOIN and RIGHT OUTER JOIN. When you use LEFT JOIN keyword in SQL Server, it means LEFT OUTER JOIN only.
So as per my opinion its the generic rule which is same for all database engines.
see here and here