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
Related
This question already has answers here:
Select rows which are not present in other table
(4 answers)
Closed 2 years ago.
I have not much experience with JOINS and the result I get with query below isn't correct.
I have a table called products and want to check if there are records in the table product_links.
I only want to get a list of items that doesn't have rows in product_links.
When I run the below query, I only get one line.
Anybody suggestions? Google couldn't help me or I'm searching with the wrong keywords.
SELECT a.id, a.SKU, a.title,
(SELECT COUNT(b.id) AS amount FROM product_links WHERE b.product=a.id) AS amount
FROM products AS a
LEFT JOIN product_links AS b ON b.product=a.id
I would recommend not exists:
select p.*
from products p
where not exists (select 1 from product_links pl where pl.product_id = p.id)
From your question i understand you need info of products which doesnt have any links.
Below is the query for that
SELECT * FROM products
WHERE id NOT IN (SELECT id FROM product_links);
This question already has answers here:
Joining three tables using MySQL
(11 answers)
How to join multiple tables in MySQL?
(4 answers)
Closed 3 years ago.
I create a simple data base on 3 tables: Author, that can has many Books, and Genre that also can has many Books.
I want to get all Genres of one Author using it authorId. I wrote next nested SQL select:
SELECT genre.id, genre.genre_id, genre.genre_name, genre.genre_descr FROM (SELECT book.genre_id FROM book WHERE book.author_id = 7654) AS b JOIN genre WHERE b.genre_id = genre.genre_id;
It works good for me, but is it possible some minimize this select? For example, get rid of nested select?
Yes, I know, that the good solution is to create reference many to many beetwen genre and author, but it will add some complications...
Thanks!
You are almost there. A simple join will do
SELECT DISTINCT g.* FROM genre g JOIN book b ON g.id = b.genre_id AND b.author_id = 7654
If you slow reads, then index on author_id should just work.
One possible solution without using join:
select * from genre where id in (select genre_id from book where author_id = 7654)
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
This question already has answers here:
Where clause for only one table in a left join
(4 answers)
Closed 5 years ago.
I'm dealing with two tables - users with about 20 million rows, and data with about 2 billion rows.
I need to select all users rows where active=1, and then join that with their corresponding data row where users.username=data.username AND data.date='2017-11-30'.
The catch is that many of these users won't have a data row where date='2017-11-30', but I still need their record to be returned, just without any info for that date.
What would be the most resource-efficient way to accomplish this? I got a start with this, but it doesn't look quite right:
SELECT users.username FROM users
INNER JOIN data ON data.username = users.username
WHERE
users.active = 1 AND data.date = ‘2017-11-30’
It sounds like you can just do a left join:
SELECT *
FROM users LEFT OUTER JOIN data
ON data.username = users.username AND data.date = '2017-11-30'
WHERE
users.active = 1
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.