select * from table t inner join table_3 t3 on (t3.t_id=t.id) where t3.k_id IN(2,3,5);
select * from table t inner join table_3 t3 on (t3.t_id=t.id) where t3.k_id IN(select id from table_2);
How do these two statements differ as performance in big tables? In 2nd statement, is the inner "select" queried again and again or is it queried once only? Thanks
No, these two queries are quite different. Compare the two query execution plans with EXPLAIN, it'll show a DEPENDENT SUBQUERY select type for the second query. You can however optimize your query easily turning the dependent subquery intto a static subquery. It'll be something like
select *
from table t
inner join table_3 t3 on (t3.t_id=t.id)
where exists (select 1 from table2 where t3.k_id = table2.id);
Didn't try this out so please verify that both queries are equivalent.
Related
I am not sure if this is possible. But is it possible to do a join on 2 tables, but return the data for only one of the tables. I want to join the two tables based on a condition, but I only want the data for one of the tables. Is this possible with SQL, if so how? After reading the docs, it seems that when you do a join you get the data for both tables. Thanks for any help!
You get data from both tables because join is based on "Cartesian Product" + "Selection". But after the join, you can do a "Projection" with desired columns.
SQL has an easy syntax for this:
Select t1.* --taking data just from one table
from one_table t1
inner join other_table t2
on t1.pk = t2.fk
You can chose the table through the alias: t1.* or t2.*. The symbol * means "all fields".
Also you can include where clause, order by or other join types like outer join or cross join.
A typical SQL query has multiple clauses.
The SELECT clause mentions the columns you want in your result set.
The FROM clause, which includes JOIN operations, mentions the tables from which you want to retrieve those columns.
The WHERE clause filters the result set.
The ORDER BY clause specifies the order in which the rows in your result set are presented.
There are a few other clauses like GROUP BY and LIMIT. You can read about those.
To do what you ask, select the columns you want, then mention the tables you want. Something like this.
SELECT t1.id, t1.name, t1.address
FROM t1
JOIN t2 ON t2.t1_id = t1.id
This gives you data from t1 from rows that match t2.
Pro tip: Avoid the use of SELECT *. Instead, mention the columns you want.
This would typically be done using exists (or in) if you prefer:
select t1.*
from table1 t1
where exists (select 1 from table2 t2 on t2.x = t1.y);
Although you can use join, it runs the risk of multiplying the number of rows in the result set -- if there are duplicate matches in table2. There is no danger of such duplicates using exists (or in). I also find the logic to be more natural.
If you join on 2 tables.
You can use SELECT to select the data you want
If you want to get a table of data, you can do this,just select one table date
SELECT b.title
FROM blog b
JOIN type t ON b.type_id=t.id;
If you want to get the data from two tables, you can do this,select two table date.
SELECT b.title,t.type_name
FROM blog b
JOIN type t ON b.type_id=t.id;
Suppose I have small table(t1) and large table(t2).I have indexed column1 and column2 of t2. If I want to INNER JOIN t1 and (select * from t2 where column1=x) then is the indexing on t2 be helpful even after the (select * from t2 where column1=x) during the inner join with t1?
If My query is (select * from t2 where column1=x) then obviously indexing is helpful. What happens when my complete query is run? will it first run (select * from t2 where column1=x)(here indexing will be used) and then INNER JOIN with t1 without using indexing?
Almost always it is better to JOIN two tables instead of JOINing to a "derived" table.
Probably inefficient:
FROM t1
JOIN ( SELECT ... FROM t2 ... ) AS t3 ON ...
Probably better:
FROM t1
JOIN t2 ON ...
One likely exception is when the derived table (t3) is much smaller than the table (t2) it comes from. This may happen when there is a GROUP BY, DISTINCT, and/or LIMIT inside t3.
If you want to discuss further, please provide the fully spelled out SELECT and SHOW CREATE TABLE for the two tables. An important discussion point is what indexes exist (or are missing).
I have something in a query that I have to edit, that I don't understand.
There are 4 tables that are joined: tickets, tasks, tickets_users, users. The whole query is not important, but you have an example at the end of the post. What bugs me is this kind of code used many times in relation to other tables:
(SELECT name
FROM users
WHERE users.id=tickets_users.users_id
) AS RequesterName,
Is this a subquery with the tables users and tickets_users joined? What is this?
WHERE users.id=tickets_users.users_id
If this was a join I would have expected to see:
ON users.id = tickets_users.users_id
And how is this different from a typical join? Just use the same column definition: users.name and just join with the users table.
Can anyone enlighten me on the advanced SQL querying prowess of the original author?
The query looks like this:
SELECT
description,
(SELECT name
FROM users
WHERE users.id = tickets_users.users_id) AS RequesterName,
(SELECT description
FROM tickets
WHERE tickets.id = ticket_tasks.tickets_id) AS TicketDescription,
ticket_tasks.content AS TaskDescription
FROM
ticket_tasks
RIGHT JOIN
tickets ON ticket_tasks.tickets_id = tickets.id
INNER JOIN
tickets_users ON tickets_users.tickets_id = tickettasks.tickets_id
Thanks,
This is what is called a correlated subquery. To describe it in simple terms its doing a select inside a select.
However doing this more than once in ANY query is not recommended AT ALL.. the performance issue with this will be huge.
A correlated subquery will return a row by row comparison for each row of the select... if that doesnt make sense then think of it this way...
SELECT
id,
(SELECT id FROM tableA AS ta WHERE ta.id > t.id)
FROM
tableB AS t;
This will do for each row in tableB, every row in tableA will be selected and compared to tableB id.
NOTE:
If you have 100 rows in all 4 tables and you do a correlated subquery for each one then you are doing 100*100*100*100 row comparisons. thats 100,000,000 (one hundred million) comparisons!
A correlated subquery is NOT a join, but rather a subquery..
SELECT *
FROM
(SELECT id FROM t -- this is a subquery
) AS temp
However, JOINs are different... generally you can do it one of these two ways
This is the faster way
SELECT *
FROM t
JOIN t1 ON t1.id = t.id
This is the slower way
SELECT *
FROM t, t1
WHERE t1.id = t.id
what the second join is doing is making the Cartesian Product of the two tables and then filtering out the extra stuff in the WHERE clause as opposed to the first JOIN that filters as it joins.
For the different types of joins theres a few and all are useful in their prospective actions..
INNER JOIN (same as JOIN)
LEFT JOIN
RIGHT JOIN
LEFT OUTER JOIN
RIGHT OUTER JOIN
In mysql FULL JOIN or FULL OUTER JOIN does not exist.. so in order to do a FULL join you need to combine a LEFT and RIGHT join. See this link for a better understanding of what joins do with Venn diagrams LINK
REMEMBER this is for SQL so it includes the FULL joins as well. those don't work in MySQL.
Ok, I am using Mysql DB. I have 2 simple tables.
Table1
ID-Text
12-txt1
13-txt2
42-txt3
.....
Table2
ID-Type-Text
13- 1 - MuTxt1
42- 1 - MuTxt2
12- 2 - Xnnn
Now I want to join these 2 tables to get all data for Type=1 in table 2
SQL1:
Select * from
Table1 t1
Join
(select * from Table2 where Type=1) t2
on t1.ID=t2.ID
SQL2:
Select * from
Table1 t1
Join
Table2 t2
on t1.ID=t2.ID
where t2.Type=1
These 2 queries give the same result, but which one is faster?
I don't know how Mysql does the Join (or How the Join works in Mysql) & that why I wonder this!!
Exxtra info, Now if i don't want type=1 but want t2.text='MuTxt1', so Sql2 will become
Select * from
Table1 t1
Join
Table2 t2
on t1.ID=t2.ID
where t2.text='MuTxt1'
I feel like this query is slower??
Sometimes the MySQL query optimizer does a pretty decent job and sometimes it sucks. Having said that, there are exception to my answer where the optimizer optimizes something else better.
Sub-Queries are generally expensive as MySQL will need to execute and store results seperately. Normally if you could use a sub-query or a join, the join is faster. Especially when using sub-query as part of your where clause and don't put a limit to it.
Select *
from Table1 t1
Join Table2 t2 on t1.ID=t2.ID
where t2.Type=1
and
Select *
from Table1 t1
Join Table2 t2
where t1.ID =t2.ID AND t2.Type=1
should perform equally well, while
Select *
from Table1 t1
Join (select *
from Table2
where Type=1) t2
on t1.ID=t2.ID
most likely is a lot slower as MySQL stores the result of select * from Table2 where Type=1 into a temporary table.
Generally joins work by building a table comprised of all combinations of rows from both table and afterwards removing lines which do not match the conditions. MySQL of course will try to use indexes containing the columns compared in the on clause and specified in the where clause.
If you are interested in which indexes are used, write EXPLAIN in front of your query and execute.
As per my view 2nd query is more better than first query in terms of code readability and performance. You can include filter condition in Join clause also like
Select * from
Table1 t1
Join
Table2 t2 on t1.ID=t2.ID and t2.Type=1
You can compare execution time for all queries in SQL fiddle here :
Query 1
Query 2
My Query
I think this question is hard to answer since we don't exactly know the internals of the query parser in the database. Usually these kind of constructions are evaluated by the database in a similar way (it can see that the first and second query are identical so parses it correctly, or not).
I would write the second one since it is more clear what is happening.
I am trying to join two table using left join, that is table1 left join table2.
I would only like part of the rows from A to be joined with B. Is it recommended that i use a sub query to filter rows from table1 or avoid them in where clause to improve my query performance?
select t1.a
,t1.b
,t2.c
from (select *
from table1
where a='x'
) t1 LEFT JOIN table2 t2 on t1.d=t2.d
or
select t1.a
,t1.b
,t2.c
from table1 t1 LEFT JOIN table2 t2 on t1.d=t2.d
where t1.a='x'
Check the query plan but I doubt it would make any difference.
It very depends on the structure and content of your database. The best way is to look into the query plan and compare it for both versions of your query.
You can find this documentation useful: MySQL Query Execution Plan