Consider 2 tables: USERS and TASKS for a task delivery app
USERS
userID | email | name
TASKS
userID | designator_userUNIQUE | destination_userUNIQUE
To get the destination_user full name from the users table I am using the following query that works fine:
SELECT *
FROM tasks
INNER JOIN users ON users.userID = tasks.destination_userUNIQUE
WHERE tasks.client='$clientUNIQUE'
ORDER BY tasks.date DESC, tasks.time DESC
However, if I want to get the full name from the designator_userUNIQUE using the below query, it simply generates an sql error (mysql_numrows() expects parameter 1 to be resource).
SELECT *
FROM tasks
INNER JOIN users ON users.userID = tasks.destination_userUNIQUE
INNER JOIN users ON users.name = tasks.designator_user UNIQUE
WHERE tasks.client='$clientUNIQUE'
ORDER BY tasks.date DESC, tasks.time DESC
Any ideas?
You need to give different aliases to each instance of the users table that you join with, to prevent ambiguity.
SELECT *
FROM tasks AS t
INNER JOIN users AS dest ON dest.userID = t.destination_userUNIQUE
INNER JOIN users AS desig ON desig.userID = t.designator_userUNIQUE
WHERE t.client='$clientUNIQUE'
ORDER BY t.date DESC, t.time DESC
Related
I want to improve my search performance. I want to search the user against the services and specialty. user in listed as per the filter applied either by specialty or by services or can be both.There are 5 tables i want to get the data from all the table
I have below table structure
1) users :- id ,first_name,last_name
2) users_services :- id ,user_id,service_id
3) users_speciality :- id,user_id,specility_id
4) mst_services :- id,name
5) mst_speciality :- id,name
I have used this query to get the result and it works fine.
select u.id,first_name,last_name,location,services.name as service_name,speciality.name as specility_name from users as u
inner join users_services on u.id =users_services.user_id
Inner join mst_services as services on services.id=users_services.service_id
inner join users_speciality on u.id =users_speciality.user_id
Inner join mst_speciality as speciality on speciality.id=users_speciality.service_id WHERE speciality.name ="specificity one"
as per the normalization it seems correct.But when data is more than 1,00,000 that time joining too many table causes may create problem.
what should i do for filter the user according to services and specialty ?
Have you tried like this ?
SELECT users.id,first_name,last_name, mst_services.name as service, mst_speciality.name AS speciality
FROM users
LEFT JOIN users_services ON users.id = users_services.user_id
LEFT JOIN users_speciality ON users.id = users_speciality.user_id
LEFT JOIN mst_services ON service_id = mst_services.id
LEFT JOIN mst_speciality ON speciality_id = mst_speciality.id
WHERE users.id IN
(SELECT user_id FROM users_services
WHERE service_id = (SELECT id FROM mst_services WHERE name = "service one")
UNION ALL
SELECT user_id FROM users_speciality
WHERE speciality_id = (SELECT id FROM mst_speciality WHERE name = "spercial one"))
ORDER BY first_name,last_name,service,speciality
I have 2 tables: user and review, a one-to-many relationship.
When I execute the following query:
SELECT
user_id,
count(*) totalReviews,
USER . NAME
FROM
review,
USER
WHERE
USER .id = review.user_id
GROUP BY
user_id
I get:
1 2 marius
2 2 daniela
3 1 alin
What I want to do now is to display first 2 users because they have given the most reviews(2).
I tried adding having, if I hardcode having totalReviews=2 it works, but if I write having total = max(total) I get 0 results, while if I'm trying with,
SELECT
*
FROM
(
SELECT
user_id,
count(*) total,
USER . NAME
FROM
review,
USER
WHERE
USER .id = review.user_id
GROUP BY
user_id
) A
WHERE
total = (SELECT max(total) FROM A) `
I get an error (table A doesn't exist)
You would do this with ORDER BY and LIMIT:
SELECT u.id, count(*) as totalReviews, u.name
FROM review r JOIN
user u
ON u.id = r.user_id
GROUP BY u.id, u.name
ORDER BY totalReviews DESC
LIMIT 2;
Notes:
Never use commas in the FROM clause. Always use proper, explicit JOIN syntax.
Table aliases make the query easier to write and read.
EDIT:
If occurs to me that you want all users with the maximum number of reviews, not exactly 2. Here is one method:
SELECT u.id, COUNT(*) as totalReviews, u.name
FROM review r JOIN
user u
ON u.id = r.user_id
GROUP BY u.id, u.name
HAVING totalReviews = (SELECT COUNT(*)
FROM review r2
GROUP BY r2.user_id
ORDER BY COUNT(*) DESC
LIMIT 1
);
Note that the subquery in the HAVING clause is simpler than the outer query. There is no need to bring in the user name.
I have two tables
users : id,name
health : id,status,test_date,user_id
health table containing user health history
now i want to get the last health test and user info of a specific user
I tried this query
SELECT users.*, health.* FROM users INNER JOIN health ON users.id=health.user_id
having (max(health.id)) order by users.id desc limit 50
but i failed
Try this:
SELECT users.*, health.* FROM users
INNER JOIN health
ON health.id = (SELECT id FROM health WHERE health.id = users.id ORDER BY id DESC LIMIT 1)
This is an other option:
SELECT U.*, H.*
FROM users AS U
INNER JOIN (
SELECT user_id, MAX(id) AS id
FROM health
GROUP BY user_id
) AS D
ON D.user_id = U.user_id
INNER JOIN health AS H
ON H.id = D.id
NOTE: "D" will result in a DERIVED table and therefore in a faster execution in MySQL 5.7+ or MariaDB 10+.
Reference:
Optimizing Derived Tables and View References
MySQL 5.7: Improved Performance of Queries with Derived Tables
I have following table in my database. I am trying to retrieve data from all three tables.
Table Structure:
User_New
User_ID
Name
Password
Email
User_Group_New
User_ID
Group_ID
Group_New
Group_ID
Group_Name
Issue: I am getting Duplicate records.
Query:
SELECT *
FROM `User_New`
INNER JOIN User_Group_New ON User_Group_New.User_ID = User_New.User_ID
INNER JOIN Group_New ON Group_New.Group_ID = User_Group_New.Group_ID
LIMIT 0, 30
SELECT DISTINCT *
FROM `User_New`
INNER JOIN User_Group_New ON User_Group_New.User_ID = User_New.User_ID
INNER JOIN Group_New ON Group_New.Group_ID = User_Group_New.Group_ID
LIMIT 0, 30
May that solve yours.
I'm trying to get latest created datetime for unique user_id. I've got below query but it does not seem to be working....It does not get the latest created time.
SELECT * FROM `chats`
WHERE receiver_id = 1
GROUP BY user_id
ORDER BY created DESC
Is there a reason why?
UPDATE:
Actually I found my answer myself. Please look below. I had to use INNER JOIN for nested searched and filtered result table then find using where clause of that result then left join on that table to get the data I needed!
SELECT c.*, users.username
FROM chats c
INNER JOIN(
SELECT MAX(created) AS Date, user_id, receiver_id, chat, type, id
FROM chats
WHERE receiver_id = 1
GROUP BY user_id ) cc ON cc.Date = c.created AND cc.user_id = c.user_id
LEFT JOIN users ON users.id = c.user_id
WHERE c.receiver_id = 1
You should use a MAX aggregate function -
SELECT user_id, MAX(created) latest_datetime FROM chats
GROUP BY user_id;
Try this query, it will show all users and latest created datetime.