I have a noob question but rather a troublesome one for me. I am using SELECT on three tables the middle one of which is realtional (Holds relations - ID of user against ID of Place), the first is a table of users, the last of places. I have written this perfectly woking query
$query = "SELECT users.Username,usrxplc.User,places.Name
FROM users,usrxplc,places
WHERE usrxplc.Place=places.ID AND usrxplc.User=users.ID"
That spits out all places associated with all users. Fine, but I would like to limit it only to a certain user. Seems simple, but I am stuck.
You use a WHERE clause to filter the results, so just add a clause for users.ID:
select users.Username,
usrxplc.User,
places.name
from users,
usrxplc,
places
where usrxplc.Place = places.ID
and usrxplc.User = users.ID
and users.ID = 123
Just felt the need to post the alternative - instead of selecting and all tables you can use INNER JOIN to join one table onto another
SELECT
users.Username,
places.Name
FROM users
INNER JOIN usrxplc ON usrxplc.User=users.ID
INNER JOIN places ON places.ID = usrxplc.Place
WHERE users.ID = 111
It's functionally the same as the other answer, however when you get onto more complex queries and tables you will find that using JOINs allows for greater optimisation as you are able to further limit the rows each individual JOIN gets, for example the following is also valid, where the User row is limited before joining onto other tables
SELECT
users.Username,
places.Name
FROM places
INNER JOIN usrxplc ON usrxplc.Place = places.ID
INNER JOIN users ON users.ID = usrxplc.User AND users.ID = 111
In more complicated queries, or if these tables were to be far larger, this would in turn offer a more optimal query generally speaking
Related
A user has a student (one to one) and a student can have many languages and hobbies (both times many to many).
If I run this query,
SELECT email, hobbies.name, languages.name
FROM users
INNER JOIN students
ON users.id = students.user_id
LEFT OUTER JOIN languages_student
ON students.id = languages_student.student_id
INNER JOIN languages
ON languages_student.language_id = languages.id
LEFT OUTER JOIN hobbies_student
on students.id = hobbies_student.student_id
INNER JOIN hobbies
ON hobbies_student.hobbie_id = hobbies.id
WHERE users.id = 6
I get this result set:
If I add another language to a student, I get six rows in the result set. Is there a way of combining the second and third columns in order to get something more compact and not redundant? Can each hobby appear just once and get a NULL in languages when they run out?
There are a couple of approaches to being able to aggregate this information. One is to do this in your application logic based on the type of result set you currently show. This might be done be reading the rows from the result set into an appropriate data structure you can then use in your application to display this information.
The second approach is to use GROUP_CONCAT() to concatenate values within same group (in this case email name) into a single row. That might lead to a results set like this:
shields.katlynn#swaniaski.biz Endurance Sports,Golf Balochi,Assamesse
This might mean that in your application, you would need to split apart the data in each row to get to individual values.
An example of how you might write the query to get the above result would be:
SELECT
email,
GROUP_CONCAT(DISTINCT hobbies.name) AS hobbies,
GROUP_CONCAT(DISTINCT languages.name) AS languages
FROM users
INNER JOIN students
ON users.id = students.user_id
LEFT OUTER JOIN languages_student
ON students.id = languages_student.student_id
INNER JOIN languages
ON languages_student.language_id = languages.id
LEFT OUTER JOIN hobbies_student
on students.id = hobbies_student.student_id
INNER JOIN hobbies
ON hobbies_student.hobbie_id = hobbies.id
WHERE users.id = 6
GROUP BY email
In either case, you will need some sort of post-retrieval data processing in your application.
I have an elementary question about SQL query with joining the same table twice. It sounds very simple, but I have some troubles with it. I hope, anyone can help me with this issue :)
I have two little tables: "peoples" (columns: id, name, ...) and "likes" (id, who, whom). People may set the "likes" to each other. The relationship is many to many.
I want get the table with peoples likes: count of received "likes", delivered and count of mutual likes.
All is correctly, when I use only one join. But for two joins (or more) MySQL combine all rows (as expected) and I get wrong values in counts. I don't know, how I must use count/sum/group-by operators in this case:( I would like to do this without subqueries in one query.
I used a query like this:
SELECT *, count(l1.whom), count(l2.whom)
FROM people p
LEFT JOIN likes l1 ON l1.who = p.id
LEFT JOIN likes l2 ON l2.whom = p.id
GROUP BY p.id;
SELECT p.id, name,
count(lwho.who) delivered_likes,
count(lwhom.whom) received_likes,
count(lmut.who) mutual_likes
FROM people AS p
LEFT JOIN likes AS lwho ON p.id = lwho.who
LEFT JOIN likes AS lwhom ON lwhom.id = lwho.id
LEFT JOIN likes AS lmut ON lwhom.who = lmut.whom AND lwhom.whom = lmut.who
GROUP BY p.id;
But it's calculated the counts of likes incorrect.
It's issue just for training and performance is not important, but I guess, that three joins in my last query is too much. Can I do it using 2 joins?
Thanks in advance for help.
I surmise that there is a 1:N relationship between people and likes.
One problem with your second query, as far as I can tell, is that the lwhom correlation of likes is joined to lwho via id=id. Basically lwhom is lwho. I'd recommend changing the ON clause for this correlation from lwhom.id = lwho.id to p.id = lwhom.whom.
The counts will still be affected by the JOINs, however. Supposing that you have an ID column in the likes table, though, you could then have each COUNT tally the distinct Like IDs per person – if not, consider just using COUNT(DISTINCT correlation.*) instead.
Digressions aside, the following should hopefully work:
SELECT p.id, name,
count(distinct lwho.id) delivered_likes,
count(distinct lwhom.id) received_likes,
count(distinct lmut.id) mutual_likes
FROM people AS p
LEFT JOIN likes AS lwho ON p.id = lwho.who
LEFT JOIN likes AS lwhom ON p.id = lwhom.whom
LEFT JOIN likes AS lmut ON lwhom.who = lmut.whom AND lwhom.whom = lmut.who
GROUP BY p.id,p.name;
I have an SQL Fiddle here.
I have 3 tables :
Users, which contains relevant user data
ComputerUsers, which ties together Users with information from a fourth (and for my purposes here irrelevant) table, Computers.
And finally, Software, which contains a list of software present on various computers.
The only thing tying a user together with a piece of software is
Users.User_ID <-> ComputerUsers.User_ID/ComputerUsers.Computer_ID<->Software.Computer_ID
I know that I can use
SELECT * FROM Users INNER JOIN ComputerUsers ON Users.User_ID = ComputerUsers.User_ID INNER JOIN Software ON ComputerUsers.Computer_ID = Software.Computer_ID
to tie together the user with all of their software, especially if I filter it like
Where Users.User_ID = 'Some_Value'
My gripe is that when I run this SQL, I get a result set that contains :
All of the columns from the Users table, both columns from the Computer_Users table, and all columns from the software table.
I'm sure there's a better way to do this but I'm a rookie with MySQL, SQL, and database stuff in general.
How would I go about accomplishing joining together the Users table with the Software table while omitting the columns from the Computer_Users table?
You can do this to get columns from just two tables:
SELECT u.*, s.*
FROM Users u INNER JOIN
ComputerUsers cu
ON u.User_ID = cu.User_ID INNER JOIN
Software s
ON cu.Computer_ID = s.Computer_ID;
A cleverer way is to use the using clause:
SELECT *
FROM Users u INNER JOIN
ComputerUsers cu
USING (User_ID) INNER JOIN
Software s
USING (Computer_ID);
Columns used for the join conditions do not get repeated when you use USING.
You need to list the columns you want in the SELECT portion of the query, rather than using *. You can do this by table:
SELECT Users.*, Software.*
FROM ...
Or by column:
SELECT Users.UserName, Users.Login, Software.Title, Software.Version ...
FROM ...
In this sql:
SELECT s.*,
u.id,
u.name
FROM shops s
LEFT JOIN users u ON u.id = s.user_id
OR u.id = s.owner_user_id
WHERE s.status = 1
For some reason this query takes an amazing time. although id is the primary key. it seems especially after I added this part OR u.id=s.owner_user_id the query became slow. owner_user_id often is 0 only handful of times. But why would it take so long apparently scanning the whole table? The database table users is very long and big. I didn't design it. this is for a client who subsequent programmers added too many fields. the table is 22k rows and dozens of fields.
*the names of the fields for demonstration only. actual names are different, so don't ask me why I'm looking for owner_user_id (; I did solve the slowness by remove the "OR ..." part and instead searching for the id in the loop if it is not 0. but I would like to know why this is happening and how to speedup that query as is.
You may be able to speed it up by using IN instead of the OR but that is minor.
SELECT u.id,
u.name
FROM shops s
LEFT JOIN users u ON u.id IN ( s.user_id, s.owner_user_id )
WHERE s.status = 1
Firstly, are there any indexes on this table? Mainly one on the user.id field or the s.user_id or s.owner_user_id?
However, I must ask why you need to use a LEFT JOIN instead of a regular join. The LEFT JOIN causes the matching of every row with every other one. And since I'm assuming the value / id should either be in the user_id or the owner_user_id field, and that there will always be a match, if that is the case then the use of a JOIN should speed the query up a bit.
And as Mitch said, 22k rows is tiny.
How are you going to know which user record is which? Here's how I'd do it
SELECT s.*,
u.name AS user_name,
o.name AS owner_name
FROM shops s
LEFT JOIN users u ON s.user_id = u.id
LEFT JOIN users o ON s.owner_user_id = o.id
WHERE s.status = 1
I've omitted the IDs from the user table in the SELECT as these will be part of s.* anyway.
I'm curious about the left joins too. If shops.user_id and shops.owner_user_id are required foreign keys, use inner joins instead.
I've spent a bit of time researching this on here and the mysql site but I'm a bit confused on two things: which sort of join to use and how (or if) to use an alias.
The query:
SELECT forum_threads.id, forum_threads.forum_id, forum_threads.sticky,
forum_threads.vis_rank, forum_threads.locked, forum_threads.lock_rank,
forum_threads.author_id, forum_threads.thread_title, forum_threads.post_time,
forum_threads.views, forum_threads.replies, users.username AS author_username
FROM forum_threads LEFT JOIN users ON forum_threads.author_id = users.id
WHERE forum_threads.forum_id=XXX
Now that query currently finds all threads from the given forum and joins the threads author id to the username table. I also have lastpostid which I'd also like to include in that query and join again on the users table so I can get the username for the last poster too.
I tried adding:
LEFT JOIN users ON threads.lastpostid = users.username
but that just results in an alias error as users isn't unique.
I also tried using both an alias on the main query and on the second join but it keeps giving me missing field errors.
Could someone give me a point in right direction please?
Yes, you need a different alias each time. Every time you refer to the table in the query you should use the approprate alias.
SELECT
forum_threads.id,
-- etc...,
forum_threads.replies,
u1.username AS author_username
u2.username AS last_post_username
FROM forum_threads
LEFT JOIN users u1 ON forum_threads.author_id = u1.id
LEFT JOIN users u2 ON threads.lastpostid = u2.username
WHERE forum_threads.forum_id=XXX