I am trying to select rows from a table which don't have a correspondence in the other table.
For this purpose, I'm currently using LEFT JOIN and WHERE joined_table.any_column IS NULL, but I don't think that's the fastest way.
SELECT * FROM main_table mt LEFT JOIN joined_table jt ON mt.foreign_id=jt.id WHERE jt.id IS NULL
This query works, but as I said, I'm looking for a faster alternative.
Your query is a standard query for this:
SELECT *
FROM main_table mt LEFT JOIN
joined_table jt
ON mt.foreign_id=jt.id
WHERE jt.id IS NULL;
You can try this as well:
SELECT mt.*
FROM main_table mt
WHERE not exists (select 1 from joined_table jt where mt.foreign_id = jt.id);
In some versions of MySQL, it might produce a better execution plan.
In my experience with MSSQL the syntax used (usually) produces the exact same query plan as the WHERE NOT EXISTS() syntax, however this is mysql, so I can't be sure about performance!!
That said, I'm a much bigger fan of using the WHERE NOT EXISTS() syntax for the following reasons :
it's easier to read. If you speak a bit of English anyone can deduce the meaning of the query
it's more foolproof, I've seen people test for NULL on a NULL-able field
it can't have side effects like 'doubled-records' due to the JOIN. If the referenced field is unique there is no problem, but again I've seen situations where people chose 'insufficient keys' causing the main-table to get multiple hits against the joined table... and off course they solved it again using DISTINCT (aarrgg!!! =)
As for performance, make sure to have a (unique) index on the referenced field(s) and if possible put a FK-relationship between both tables. Query-wise I doubt you can squeeze much more out of it.
My 2 cents.
The query that you are running is usually the fastest option, just make sure that you have an index forh both mt.foreign_id and jt.id.
You mentioned that this query is more complex, so it might be possible that the problem is in another part of the query. You should check the execution plan to see what is wrong and fix it.
Related
Is Query 1 more optimized say for example for a larger database than Query 2 even by slight or am I just doubling the work with an additional WHERE clause?
Query 1:
SELECT sample_data
FROM table1 INNER JOIN table2 ON table1.key = table2.key
WHERE table1.key = table2.key;
Query 2:
SELECT sample_data
FROM table1 INNER JOIN table2 ON table1.key = table2.key;
Because I read this article saying that using filters in JOIN clauses improve the performance..:
Is Query 1 more optimized say for example for a larger database than Query 2?
No, it is not more optimized. Query 2 is the correct way to handle the JOIN. Query 1 does the same thing, but with extra verbiage for the MySQL server software to scrub out as it figures out how to satisfy your query.
The advice at the Adobe documentation about filtering both tables in a join does not relate to the join's ON-condition. Their example says to do this...
SELECT whatever, whatever
FROM table1
JOIN table2 ON table2.table1_id = table1.table1_id
WHERE table1.date >= '2021-01-01'
AND table2.date >= '2021-01-01' /* THIS LINE IS WHAT THEY SUGGEST */
Their suggestion, from 2015, has to do with filtering non-join attributes from both tables. It's a suggestion to use to optimize a query if it just isn't fast enough for you. And, in my experience, it's not a very good suggestion. Ignore it, at least for now. More recent MySQL versions have gotten more efficient.
Let me add to this. SQL is a so-called "declarative" language. You declare what you want and the MySQL server figures out how to get it for you. SQL software is getting really good at doing that; keep in mind that MySQL is now a quarter century old. In that time its programmers have been continuously making it smarter at figuring out how to get stuff. You probably can't outsmart it. But you may need to add indexes when your tables get really big. https://use-the-index-luke.com/
Other languages are "procedural": you, as a programmer, spell out a procedure for getting what you want. You don't need to do that for SQL.
I like to put it this way:
ON is where you specify how the tables are related.
WHERE is for filtering.
That makes it easy for a human reading the query to understand it.
In reality (for MySQL), JOIN (aka INNER JOIN) treats ON and WHERE identically. That is, there is no performance difference. Your Query 1 unnecessarily specifies the "relation" twice.
Also, MySQL's Optimizer is smart enough to realize when two columns have the same value. For example,
SELECT ...
FROM a
JOIN bb ON a.foo = bb.foo
WHERE a.foo = 123
If the Optimizer decides that starting with the filter bb.foo = 123 is more optimal, it will do so. Note: This is not the same as the example you showed; it joins on one thing (id) but filters on another (date). The two queries there are not equivalent!
LEFT JOIN, necessarily treats ON and WHERE differently. (But that is another topic.)
I met the following MySQL code:
SELECT ServiceFee
FROM Shows
WHERE ID = (SELECT ShowID FROM Orders WHERE ID = ?)
It makes me wonder because the people who wrote this code usually use SQL joins. I would rewrite it
SELECT ServiceFee
FROM Shows
INNER JOIN Orders ON Shows.ID = Orders.ShowID
WHERE Orders.ID = ?
My question: Is there any reason why this code was written with a subquery and whether it is completely safe (producing the same result in all situations) to rewrite it with the join?
Are there any caveats?
"Is there any reason why this code was written with a subquery"
a very long time ago MySQL joins used to be slow
Without a context the two queries don't do the same. If Orders.ID is not unique, the first query might produce an error because the subquery can return multiple rows. If Orders.ID is unique (probably primary key) then there will not be any difference - neither in the result nor in the performance (with newer versions). So in this case it is just a question of preference. Usually i prefer joins. But in your example the subquery solution looks fine and readable. So i wouldn't rewrite it without a reason.
Nope, there are no caveats. As a matter of fact, the INNER JOIN query might run faster
There is a logical reason to prefer a Subquery over a Join:
Between Shows and Orders there's a 1-many relation, when you switch to a join you usually must add a DISTINCT to get rid of the duplicate rows returned.
In your case this is not needed, because you restrict to a single order, but when you remove the WHERE-condition or change it to return multiple rows you will notice that.
MySql query taking too long time to give result
Select pty_code from sis.tblBf a where a.BFno
not in (select b.BNo from sislatest.tbltransaction b)
note: i am comparing two different database and getting the difference.
Select A.pty_code from DataBase1.TableName A
where A.BFno NOT IN (SELECT B.BNo From DataBase2.TableName B)
Perhaps an indexing issue? This is where the mysql explain syntax comes in hand:
http://dev.mysql.com/doc/refman/5.0/en/using-explain.html
Other things that may help would be to 'optimize table', or even 'analyze table' to check for inconsistency.
Using a subselect, especially with a "not in" clause is going to be slow. I'd recommend rewriting to a left outer join with an "is null" in the where clause.
SELECT a.pty_code from sis.tblBf a LEFT JOIN sislatest.tbltransaction b on b.BNo = a.BFno WHERE b.BNo IS NULL
I think that would work but without the database to test against, I'm writing it blind.
If that's still slow, I'd take a look and make sure there are indices on b.BNo and A.BFNo.
I want to ask a question about database queries. In case of query such like where clause of the query is coming from the another query. For example
select ? from ? where ? = select ? from ?
This is the simple example so it is easy to write this. But for the more complex case, i want to know what is the best way in case of performance. Join? seperate queries? nested or another?
Thank you for answers.
Best Regards.
You should test it. These things depend a lot on the details of the query and of the indices it can use.
In my experience JOINs tend to be faster than nested queries in MySQL. In some cases MySQL isn't very smart and appears to run the subquery for every row produced by the outer query.
You can read more about these things in the official documentation:
Optimizing subqueries: http://dev.mysql.com/doc/refman/5.6/en/optimizing-subqueries.html
Rewriting subqueries as joins: http://dev.mysql.com/doc/refman/5.6/en/rewriting-subqueries.html
This is case dependent. In case you have a very less result in the inner query you should go for it. The flow works in the manner where in the inner query is executed first and the result set is being used in the outer query.
Meanwhile joins give you a Cartesian product which is again a heavy operation.
As Mitch and Joni stated, it depends. But generally a join will offer the best performance. You're trying to avoid running the nested query for each row of the outer query. A good query optimizer may do this for you anyway, by interpreting what you're trying to do and essentially "fixing" your mistake. But with the vast majority of queries, you should be writing it as a join in the first place. That way you're being explicit about what you're trying to do and you're fully understanding yourself what is being done, and what the most efficient way to do the work is.
I EXPECT the joins to be quicker, mainly because you have an equivalence and an explicit JOIN. Still use explain to see the differences in how the SQl engine will interpret them.
I would not expect these to be so different, where you can get real, large performance gains in using joins instead of subqueries is when you use correlated subqueries.
Since almost everyone is saying that joins will give the optimal performance I just logged in to say the exact opposite experience I had.
So some days back I was writing a query for 3-4 tables which had huge amount of data. I wrote a big sql query with joins and it was taking around 2-3 hours to execute it. Then I restructured it, created a nested select query, put as many where constraints as I can inside the nested one & made it as stricter as possible and then the performance improved by >90%, it now takes less than 4 mins to run.
This is just my experience and may be theoretically joins are better. I just felt to share my experience. Its better to try out different things, getting additional knowledge about the tables, it's indexes etc would help a lot.
Update:
And I just found out what I did is actually suggested in this optimization reference page of MySQL. http://dev.mysql.com/doc/refman/5.6/en/optimizing-subqueries.html
Pasting it here for quick reference:
Replace a join with a subquery. For example, try this:
SELECT DISTINCT column1 FROM t1 WHERE t1.column1 IN ( SELECT column1
FROM t2);
Instead of this:
SELECT DISTINCT t1.column1 FROM t1, t2 WHERE t1.column1 =
t2.column1;
Move clauses from outside to inside the subquery. For example, use
this query:
SELECT * FROM t1 WHERE s1 IN (SELECT s1 FROM t1 UNION ALL SELECT s1
FROM t2); Instead of this query:
SELECT * FROM t1 WHERE s1 IN (SELECT s1 FROM t1) OR s1 IN (SELECT s1
FROM t2); For another example, use this query:
SELECT (SELECT column1 + 5 FROM t1) FROM t2; Instead of this query:
SELECT (SELECT column1 FROM t1) + 5 FROM t2;
I came across writing the query in differnt ways like shown below
Type-I
SELECT JS.JobseekerID
, JS.FirstName
, JS.LastName
, JS.Currency
, JS.AccountRegDate
, JS.LastUpdated
, JS.NoticePeriod
, JS.Availability
, C.CountryName
, S.SalaryAmount
, DD.DisciplineName
, DT.DegreeLevel
FROM Jobseekers JS
INNER
JOIN Countries C
ON JS.CountryID = C.CountryID
INNER
JOIN SalaryBracket S
ON JS.MinSalaryID = S.SalaryID
INNER
JOIN DegreeDisciplines DD
ON JS.DegreeDisciplineID = DD.DisciplineID
INNER
JOIN DegreeType DT
ON JS.DegreeTypeID = DT.DegreeTypeID
WHERE
JS.ShowCV = 'Yes'
Type-II
SELECT JS.JobseekerID
, JS.FirstName
, JS.LastName
, JS.Currency
, JS.AccountRegDate
, JS.LastUpdated
, JS.NoticePeriod
, JS.Availability
, C.CountryName
, S.SalaryAmount
, DD.DisciplineName
, DT.DegreeLevel
FROM Jobseekers JS, Countries C, SalaryBracket S, DegreeDisciplines DD
, DegreeType DT
WHERE
JS.CountryID = C.CountryID
AND JS.MinSalaryID = S.SalaryID
AND JS.DegreeDisciplineID = DD.DisciplineID
AND JS.DegreeTypeID = DT.DegreeTypeID
AND JS.ShowCV = 'Yes'
I am using Mysql database
Both works really well, But I am wondering
which is best practice to use all time for any situation?
Performance wise which is better one?(Say the database as a millions records)
Any advantages of one over the other?
Is there any tool where I can check which is better query?
Thanks in advance
1- It's a no brainer, use the Type I
2- The type II join are also called 'implicit join', whereas the type I are called 'explicit join'. With modern DBMS, you will not have any performance problem with normal query. But I think with some big complex multi join query, the DBMS could have issue with the implicit join. Using explicit join only could improve your explain plan, so faster result !
3- So performance could be an issue, but most important maybe, the readability is improve for further maintenance. Explicit join explain exactly what you want to join on what field, whereas implicit join doesn't show if you make a join or a filter. The Where clause is for filter, not for join !
And a big big point for explicit join : outer join are really annoying with implicit join. It is so hard to read when you want multiple join with outer join that explicit join are THE solution.
4- Execution plan are what you need (See the doc)
Some duplicates :
Explicit vs implicit SQL joins
SQL join: where clause vs. on clause
INNER JOIN ON vs WHERE clause
in the most code i've seen, those querys are done like your Type-II - but i think Type-I is better because of readability (and more logic - a join is a join, so you should write it as a join (althoug the second one is just another writing style for inner joins)).
in performance, there shouldn't be a difference (if there is one, i think the Type-I would be a bit faster).
Look at "Explain"-syntax
http://dev.mysql.com/doc/refman/5.1/en/explain.html
My suggestion.
Update all your tables with some amount of records. Access the MySQL console and run SQL both command one by one. You can see the time execution time in the console.
For the two queries you mentioned (each with only inner joins) any modern database's query optimizer should produce exactly the same query plan, and thus the same performance.
For MySQL, if you prefix the query with EXPLAIN, it will spit out information about the query plan (instead of running the query). If the information from both queries is the same, them the query plan is the same, and the performance will be identical. From the MySQL Reference Manual:
EXPLAIN returns a row of information
for each table used in the SELECT
statement. The tables are listed in
the output in the order that MySQL
would read them while processing the
query. MySQL resolves all joins using
a nested-loop join method. This means
that MySQL reads a row from the first
table, and then finds a matching row
in the second table, the third table,
and so on. When all tables are
processed, MySQL outputs the selected
columns and backtracks through the
table list until a table is found for
which there are more matching rows.
The next row is read from this table
and the process continues with the
next table.
When the EXTENDED keyword is used,
EXPLAIN produces extra information
that can be viewed by issuing a SHOW
WARNINGS statement following the
EXPLAIN statement. This information
displays how the optimizer qualifies
table and column names in the SELECT
statement, what the SELECT looks like
after the application of rewriting and
optimization rules, and possibly other
notes about the optimization process.
As to which syntax is better? That's up to you, but once you move beyond inner joins to outer joins, you'll need to use the newer syntax, since there's no standard for describing outer joins using the older implicit join syntax.