I need to do a query on the same field on a MySQL table but with different parameters. This is my query. The issue that comes up is that MySQL is saying that users_name is ambiguous.
SELECT projects_id,
projects_users_id,projects_companies_id,projects_name,
projects_description,project_registered_date,
projects_users_id_register,
users.users_name as userInCharge,companies.companies_name as company,
users.users_name as user_register FROM projects
LEFT JOIN users as userInCharge ON (users_id = projects_users_id)
LEFT JOIN users as register ON (users_id=projects_users_id_register)
LEFT JOIN companies ON (companies_id = projects_companies_id) ORDER BY projects_id DESC
My goal is to get the ID of both the person in charge of the project and the person who registered the project. How do I do this?
Your ambiguity is arising because of these LEFT JOIN clauses:
LEFT JOIN users as userInCharge ON (users_id = projects_users_id)
LEFT JOIN users as register ON (users_id=projects_users_id_register)
You need to specify where users_id is coming from (yes - I know they're from the same table, but SQL is a bit slow sometimes) Try:
LEFT JOIN users as userInCharge ON (userInCharge.users_id = projects_users_id)
LEFT JOIN users as register ON (register.users_id=projects_users_id_register)
Use the table alias and not users.users_name.
And also in the joins.
You JOIN with users table as userInCharge and register so use these names when You specify columns
userInCharge.users_name as userInCharge
register.users_name as user_register
instead of
users.users_name as userInCharge
users.users_name as user_register
Related
I have a products table where I include 3 columns, created_user_id, updated_user_id and in_charge_user_id, all of which are related to my user table, where I store the id and name of the users.
I want to build an efficient query to obtain the names of the corresponding user_id's.
The query that I build so far is the following:
SELECT products.*,
(SELECT name FROM user WHERE user_id = products.created_user_id) as created_user,
(SELECT name FROM user WHERE user_id = products.updated_user_id) as updated_user,
(SELECT name FROM user WHERE user_id = products.in_charge_user_id) as in_charge_user
FROM products
The problem with this query is that if I have 30,000 records, I am executing 3 more queries per row.
What would be a more efficient way of achieving this? I am using mysql.
For each type of user id (created, updated, in_charge) you would JOIN the users table once:
SELECT
products.*,
u1.username AS created_username,
u2.username AS updated_username,,
u3.username AS in_charge_username,
FROM products
JOIN user u1 ON products.created_user_id = u1.user_id
JOIN user u2 ON products.updated_user_id = u2.user_id
LEFT JOIN user u3 ON products.in_charge_user_id = u3.user_id
This is the best practice method to obtain the data.
It is similiar to your query with sub-selects but a more modern approach which I think the database can optimize and utilize better.
Important:
You need foreign key index on all the user_id fields in both tables!
Then the query will be very fast no matter how many rows are in the table. This requires an engine which supports foreign keys, like InnoDB.
LEFT JOIN or INNER JOIN ?
As the other answers suggest a LEFT JOIN, I would not do a left join.
If you have an user id in the products table, there MUST be a linked user_id in the user table, except for the in_charge_user which is only present some times. If not, the data would be semantically corrupt. The foreign keys assure that you always have a linked user_id and a user_id can only be deleted when there is no linked product left.
JOIN is equivalent to INNER JOIN.
You can use LEFT JOIN instead of subselects.
Your query should be like:
SELECT
P.*,
[CU].[name],
[UU].[name],
[CU].[name]
FROM products AS [P]
LEFT JOIN user AS [CU] ON [CU].[user_id] = [P].[created_user_id]
LEFT JOIN user AS [UU] ON [UU].[user_id] = [P].[updated_user_id]
LEFT JOIN user AS [CU] ON [CU].[user_id] = [P].[in_charge_user_id]
First, your query should be fine. You only need an index on user(user_id) or better yet user(user_id, name) for performance. I imagine that the first exists.
Second, you can write this using LEFT JOIN:
SELECT p.*, uc.name as created_user,
uu.name as updated_user, uin.name as in_charge_user
FROM products p LEFT JOIN
user uc
ON uc.user_id = p.created_user_id LEFT JOIN
user uu
ON uu.user_id = p.updated_user_id LEFT JOIN
user uin
ON uin.user_id = p.in_charge_user_id;
With one of the above indexes, the two methods should have very similar performance.
Also note the use of LEFT JOIN. This handles the case where one or more of the user ids is missing.
Try this below query
SELECT products.*, c.name as created_user,u.name as updated_user,i.name as in_charge_user
FROM products left join user c on(products.created_user_id=c.user_id ) left join user u on(products.updated_user_id=u.user_id ) left join user u on(products.in_charge_user_id=i.user_id )
Also as Gordon Linoff mentioned create index on user table will fetch your data faster.
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 ...
I have a printed table here, and I issue a query to attempt to join the tables where the Tech_id, clients_id, job_id, part_id should populate with corresponding key in their tables / column too.
Here is my query:
SELECT * FROM work_orders, technicians as tech, parts_list as parts, job_types as job, clients as client
LEFT JOIN technicians ON tech_id = technicians.tech_name
LEFT JOIN parts_list ON part_id = parts_list.Part_Name
LEFT JOIN job_types ON job_id = job_types.Job_Name
LEFT JOIN clients ON clients_id = clients.client_name
I've messed around with multiple different variations, this one seem to be syntax correct, but now I'm getting: Column 'clients_id' in on clause is ambiguous
I'm sure that it will happen for not only clients but maybe others. I want to be able to print the table as in the picture above, but with the clients listed. Is it possible to be done via one query as well? thanks.
You have two problems.
First (this might not be your problem, but that's a "good practice"), you shouldn't use SELECT *, as you could indeed have a field with same name in different tables.
This is one (of the many) good reason to avoid * in a Select clause.
Then, your main problem is that you select tables in your from clause, and then again by joining.
Problematic line :
FROM work_orders, technicians as tech, parts_list as parts, job_types as job, clients as client
So (I don't know your table structure, so they may be errors, but you've got the idea)
SELECT
w.client_id,
t.tech_name
--etc
FROM work_orders w
LEFT JOIN technicians t ON c.tech_id = t.tech_name
LEFT JOIN parts_list p ON c.part_id = p.Part_Name
LEFT JOIN job_types j ON w.job_id = j.Job_Name
LEFT JOIN clients c ON w.clients_id = c.client_name
This means that clients_id exists in multiple tables. You need to specify which one you want. So if you for example want the clients_id of the clients table, do SELECT clients.clients_id
If all the fiels listed in your question are in the clients table you could do:
SELECT clients.* FROM work_orders, technicians as tech, parts_list as parts, job_types as job, clients as client
LEFT JOIN technicians ON tech_id = technicians.tech_name
LEFT JOIN parts_list ON part_id = parts_list.Part_Name
LEFT JOIN job_types ON job_id = job_types.Job_Name
LEFT JOIN clients ON clients_id = clients.client_name
I have a table called bans where I have the follow fields:
room_id, banned_user_id, banned_by_id, reason, ts_start, ts_end
The users data come from the table called users, now I wanted to query the bans to retrive the name of who was banned and by who along with reason, time the ban was placed and time it ends.
So I have this query:
SELECT u.username, us.username, b.reason, b.ts_start, b.ts_end
FROM `bans` b
LEFT JOIN users us ON b.banned_by_uid = us.uid
LEFT JOIN users u ON b.banned_uid = u.uid
WHERE room_id = 3
My question here is wether my query is ok by using the LEFT JOIN for the 2 data I have to grab from the table users or there is a different approach for this kinda of scenario ?
Your query is perfectly acceptable. Each join to users is on a specific ID, which translates into a simple lookup, with minimal overhead.
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