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.
Related
Below two queries result the same result set. In first I have only used INNER JOIN and in second query mix of joins like LEFT and RIGHT JOIN. I personally prefer INNER JOIN when there is no specific task/requirement of other joins. But I just want to know that is there any difference between the below two queries in terms of performance or execution time. Is it ok to use inner join than the mix of joins?
1.
SELECT film.title, category.name, film.rating, language.name
FROM film INNER JOIN film_category ON film_category.film_id = film.film_id
INNER JOIN category ON category.category_id = film_category.category_id
INNER JOIN language ON language.language_id = film.language_id
WHERE category.name = "Sci-Fi" AND film.rating = "NC-17";
SELECT film.title, film.release_year, film.rating,category.name, language.name
FROM film LEFT JOIN language ON language.language_id=film.language_id
RIGHT JOIN film_category ON film_category.film_id = film.film_id
LEFT JOIN category ON category.category_id=film_category.category_id
WHERE film.rating="NC-17" AND category.name="Sci-Fi";
Please see this INNER JOIN vs LEFT JOIN performance in SQL Server.
However, choosing the proper join type is depending on the usecase and result set which you need to extract.
Please do not mix the different types except in this way: INNER, INNER, ... LEFT, LEFT, ... Any other combination has ambiguities about what gets done first. If you must mix them up, use parentheses to indicate which JOIN must be done before the others.
As for whether INNER/LEFT/RIGHT are identical, let me explain with one example:
SELECT ...
FROM a
LEFT JOIN b ON ... -- really INNER
WHERE b.x = 17
That WHERE effectively turns the LEFT JOIN into INNER JOIN. The Optimizer will do such. I, as a human, will stumble over the query until I realize that. So, humor me by calling it INNER JOIN.
Phrased another way, use LEFT only when the "right" table's columns are optional, but you want NULLs when they are missing. Of course, you may want the NULLs so you can say "find rows of a that are not in b:
SELECT ...
FROM a
LEFT JOIN b ON ...
WHERE b.id IS NULL -- common use for LEFT
While I have your attention, here are some notes/rules:
The keywords INNER, CROSS, and OUTER are ignored by MySQL. The ON and WHERE clauses will determine which type of JOIN is really intended.
Have you ever seen an owl turn its head nearly all the way around? That's what happens to my head when I see a RIGHT JOIN. Please convert it to a LEFT JOIN.
Though the Optimizer does not require this distinction, please use ON to specify how the tables are related and use WHERE for filtering. (With INNER, they are equivalent; with LEFT, you may get different results.)
Sometimes EXISTS( SELECT ... ) is better than a LEFT JOIN.
Optimizations vary depending on the existence of GROUP BY, ORDER BY, and LIMIT. But that is a loooong discussion.
Back to your question of which is faster, etc. Well, if the Optimizer is going to turn one into another, then those two have identical performance.
My question is that when I have a SELECT with more than one JOIN. Where am I supposed to put the ON clause?
For example:
when it's an inner join after an inner join
when it's an inner join after a left join
when it's a left join after an inner join
In the first example, I've seen people put the ON clause right after each joins.
In the second example, I've seen people put all the ON clause after the last JOIN. So right now I'm a little bit confused on where to put it and does it give me the same answer even if it is put in different places.
You should interleave the on clauses, regardless of the type of join. So:
from a join
b
on . . . left join
c
on . . .
And so on as you add more tables.
MySQL makes the on clause optional, which confuses things. However, standard SQL does allow:
from a join
b join
c
on b.? = c.?
on a.? = b.?
However, this is generally discouraged. People find that hard to follow.
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"?
I have two tables:
Shop_Products
Shop_Products_Egenskaber_Overruling
I want to select all records in Shop_Products_Egenskaber_Overruling which has a related record in
Shop_Products. This Means a record with an equal ProductNum.
This Works for me with the statement below, but I don't think a CROSS JOIN is the best approach for large record sets. When using the statement in web controls, it becomes pretty slow, even with only 1000 records. Is there a better way to accomplish this?
SELECT Shop_Products.*, Shop_Products_Egenskaber_Overruling.*
FROM Shop_Products CROSS JOIN
Shop_Products_Egenskaber_Overruling
WHERE Shop_Products.ProductNum = Shop_Products_Egenskaber_Overruling.ProductNum
Any optimizing suggestions?
Best regards.
You can do it that way but not sure it will ensure an optimization
SELECT Shop_Products.*, Shop_Products_Egenskaber_Overruling.*
FROM Shop_Products
INNER JOIN Shop_Products_Egenskaber_Overruling on Shop_Products.ProductNum = Shop_Products_Egenskaber_Overruling.ProductNum
You are actually looking for an INNER JOIN.
SELECT
SO.*,
SPEO.*
FROM SHOP_PRODUCTS SP
INNER JOIN Shop_Products_Egenskaber_Overruling SPEO
ON SP.ProductNum = SPEO.ProductNum
This will have improved performance over your CROSS-JOIN, because the condition to look for records with equal ProductNum is implicit in the JOIN condition and the WHERE clause is eliminated.
WHERE clauses always execute AFTER a JOIN. In your case, all possible combinations are created by the CROSS JOIN and then filtered by the conditions in the WHERE clause.
By using an INNER JOIN you are doing the filtering in the first step.
Cross join is slower, because it produce all combinations, which filtred after by where predicate. So you can use INNER JOIN for better performance. But I think It would be useful if you check execution plan of this query anyway, because in Oracle there is no difference between where and inner join solutions Inner join vs Where
Try using INNER JOIN
SELECT Produkter.*, Egenskaber.*
FROM Shop_Products Produkter
INNER JOIN Shop_Products_Egenskaber_Overruling Egenskaber ON Produkter.ProductNum=Egenskaber.ProductNum
Jag namngav aven dem pa Norska..