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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!
Related
This question already has answers here:
Difference between JOIN and INNER JOIN
(6 answers)
SQL JOIN and different types of JOINs
(6 answers)
Closed 4 years ago.
SELECT ID, Name, Marks, Grade
FROM Students AS s
JOIN Grades AS g ON s.Marks BETWEEN g.Min_Mark AND g.Max_Mark;
I browsed numbers of resources about left join, right join, inner join and full outer join, but I have no idea what is this "join" means. Is this some shortcut? if not, what is this?
Thanks.
Just read the documentation from MySQL. It says.
In MySQL, JOIN, CROSS JOIN, and INNER JOIN are syntactic equivalents (they can replace each other). In standard SQL, they are not equivalent. INNER JOIN is used with an ON clause, CROSS JOIN is used otherwise.
To answer your question: join is the same like a inner join
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.
This question already has answers here:
Is RIGHT JOIN Ever Required?
(7 answers)
Closed 6 years ago.
Suppose we have an EMPLOYEE and a DEPENDENT table. Each employee has an identifier Ssn and possibly a dependent with identifier Essn. Are the following two queries equivalent?
SELECT DISTINCT E.Lname, D.Dependent_name
FROM ((EMPLOYEE E) LEFT OUTER JOIN (DEPENDENT D) ON E.Fname=D.Dependent_name);
SELECT DISTINCT E.Lname, D.Dependent_name
FROM ((DEPENDENT D) RIGHT OUTER JOIN (EMPLOYEE E) ON E.Fname=D.Dependent_name);
In general, is it true that
(TABLE A) LEFT OUTER JOIN (TABLE B)
is the same as
(TABLE B) RIGHT OUTER JOIN (TABLE A)
?
Yes, it is true, however, I strongly recommend not to use RIGHT JOIN.
Since our natural reading order is from left to right and top to bottom, RIGHT JOIN comes very unnatural in this manner.
Also, If you'll have a mix of LEFT JOIN and RIGHT JOIN it would be very, very hard to follow through.
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"?
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