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.
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.
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 am trying to write an SQL query which will select records of a student within 3 tables that have the same column_I'd.
This is what I wrote but the the records selected are not accurate:
select
Nov_DEC_billing.*,
Nov_DEC_students_portfolio.*,
admission_form.academic_year
from
Nov_DEC_billing,
Nov_DEC_student_portfolio,
admission_form
where
Nov_DEC_billing.ID = Nov_DEC_student_portfolio.ID=admission_form.ID
AND
admission_form.Program ='Nov/dec'
I get a records selected alright but its not accurate. Please what's the right way to join 3 tables that share the same column_id.???
Use JOIN in your query
SELECT b.*, p.*, a.academic_year
FROM Nov_DEC_billing b
JOIN Nov_DEC_student_portfolio p ON p.id = b.id
JOIN admission_form a ON a.id = b.id
WHERE a.Program='Nov/dec'
You need to join tables something like this:
SELECT Nov_DEC_billing.*,
Nov_DEC_students_portfolio.*,
admission_form.academic_year
FROM Nov_DEC_billing AS ndb,
LEFT JOIN Nov_DEC_student_portfolio AS ndsp ON ndsp.ID=ndb.ID,
LEFT JOIN admission_form AS af ON af.ID=ndb.ID
WHERE af.Program='Nov/dec'
You should join all the tables to a single one.
What you will is join all the tables to a single one and then select from it.
Since you have 2 tables you should first join 2 and then join another one on the result.
See here left join example for the exact syntax.
Nov_DEC_billing.ID=Nov_DEC_student_portfolio.ID=admission_form.ID
doesn't do what you expect. It takes the first part, Nov_DEC_billing.ID=Nov_DEC_student_portfolio.ID and evaluates it. If the values match, that part becomes a 1, if they don't match, it becomes 0. Then the 0 or the 1 is compared against admission_form.ID. That is very likely to give strange results.
So you'd have to split that into:
Nov_DEC_billing.ID=Nov_DEC_student_portfolio.ID
AND Nov_DEC_student_portfolio.ID=admission_form.ID
Or just use explicit join syntax, as the others already advised (and which I do too). That forces you to split this anyway.
I am trying to append 'lookup data' to a main record text/description field so it looks something like this:
This is the Description Text
LookUpName1:
LookupValue1
LookupValueN
This worked fine with Inner Join like so
Select J.id, Concat('<b>LookUpName</b>:<br>',group_concat(F.LookUpValue SEPARATOR '<br>'))
from MainTable J Inner Join
LookUpTable L Inner Join
LookUpValuesTable F
On J.ID = L.JobID and F.ID = L.FilterID
Group by J.ID
However my goal is to add append multiple Lookup Tables and if I add them to this as Inner Joins I naturally just get those record where both/all the LookupTables have records.
On the other hand when I tried Join or Left Join I got an error on the Group by J.ID.
My goal is to append any of the existing Lookup Table values to all of the Description. Right now all I can achieve is returning appended descriptions which have ALL of the Lookup table values.
Your query would work if the on clauses were in the "right" place:
select J.id,
Concat('<b>LookUpName</b>:<br>', group_concat(F.LookUpValue separator '<br>'))
from MainTable J left join
LookUpTable L
on J.ID = L.JobID left join
LookUpValuesTable F
on F.ID = L.FilterID
group by J.ID;
The problem with your query is a MySQL (mis)feature. The on clause is optional for an inner join. Don't ask me why the MySQL designers thought inner join and cross join should be syntactically equivalent. Every other database requires an on clause for an inner join. It is easy enough to express a cross join using on 1=1.
However, the on clause is required for the left join, so when you switch to a left join, the compiler has a problem with the unorthodox syntax. The real problem is a missing on clause; this just happens to show up as "I wasn't expecting a group by yet." Using more traditional syntax with each join followed by an on should fix the problem.
Sitting here at work writing SQL for the first time in a while. I know all the vendiagrams of all the joins, but I was wondering what type of join I just wrote. Is this even a typical join since I'm not using a 'JOIN' keyword?
select * from table1 t, table2 a where a.column1 = -10300 AND t.column1 = a.column2;
Thanks,
k9
This performs the same as an inner join because of the "t.column1 = a.column2". If that was left out, it would be a cross apply.
Here's some more info: http://explainextended.com/2009/07/16/inner-join-vs-cross-apply/
This is what would be considered an older style inner join.
In MySQL writing JOIN unqualified implies INNER JOIN. In other words the INNER in INNER JOIN is optional.
Official documentation about JOIN here
This is an old oracle-style inner join. It will give you all rows from both tables where the column matches, which is pretty much what an inner join is.
FYI, this used to be standard practice in Oracle SQL before everything went to ANSI-standard SQL.
Its Inner join as you used Inner Join logic in Where.