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
Related
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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!
This question already has answers here:
How to emulate full outer join in this query?
(1 answer)
How can I do a FULL OUTER JOIN in MySQL?
(15 answers)
Why does MySQL report a syntax error on FULL OUTER JOIN?
(4 answers)
Closed 3 years ago.
I've seen a few methods online to do this, but i couldn't manage to apply them in my case. I have 4 tables:
Suppliers(id_sup, name, city)
Companies(id_co, name, city)
Producuts(id_p, name, city)
Deliveries(id_sup, id_co, id_p)
What I need to do is do a query, where I get printed every city, only once, and for each city show the number of suppliers/companies/products that are there.
I managed (somehow) to do this in Oracle SQL:
SELECT coalesce(s.city,c.city,p.city) city,
count(distinct s.id_sup) Suppliers,
count(distinct c.id_co) Companies,
count(distinct p.id_p) Products
FROM suppliers p
FULL OUTER JOIN companies c on s.city = c.city FULL OUTER JOIN products p on s.city=p.city
GROUP BY coalesce(s.city,c.city,p.city);
But I just don't know how to do this in Mysql. Any ideas?
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.
This question already has answers here:
INNER JOIN ON vs WHERE clause
(12 answers)
Closed 7 years ago.
I have two queries:
$query = mysql_query("SELECT ord.orderID, customers.CustomerName,
FROM ord, customers
WHERE customers.CustomerSalary=ord.orderID");
and
$query = mysql_query("SELECT ord.orderID, customers.CustomerName,
FROM ord
INNER JOIN customers
ON customers.CustomerSalary=ord.orderID");
This queries return the same result. What is different between them.
$query = mysql_query("SELECT ord.orderID, customers.CustomerName,
FROM ord
INNER JOIN customers
ON customers.CustomerSalary=ord.orderID");
This query is optimised than the other one as Inner join get applied first and then where clause will be applied.
Other benifit I see that is you can change Inner Outer join.
Clean code.
Both are same. The first query is basically inner join on the back-end but the way you have written second query, you can use left join, right join etc with that query.
Both will perform Cartesian Product on the back-end and filter out the results based on the check.
You will need to use the second query format in order to use left join, right join etc.
Another approach commonly used is known as nested queries. They are pretty faster than joins since there is no Cartesian product on the back-end.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
INNER JOIN ON vs WHERE clause
INNER JOIN:
SELECT *
FROM PETS
INNER JOIN OWNER ON PETS.OWNER_ID = 1
AND OWNER.ID = 1
MULTIPLE TABLE SELECT:
SELECT *
FROM PETS, OWNER
WHERE PETS.OWNER_ID = 1
AND OWNER.ID = 1
Is one better than the other?
Faster than the other?
They seem to produce exactly the same results.
Is there any difference at all?
I am trying to learn the best methods. In using the join, I noticed that the exact same thing can be achieved with the multiple table call
read this answer about the difference between a cross join and an inner join Performance of inner join compared to cross join