I have table a which stores the user id and the ids of his origin and destination. On table b I have the location id and the specific name of the place. What I'm trying to do is join the tables but the name column from table b will have to be used twice since I'm trying to get 2 addresses. I'm trying to read up on MySQL but just keep doing it wrong. Any help would be appreciated.
table a
------------------------
| uid | to | from |
------------------------
| 1 | 1 | 2 |
------------------------
table b
---------------
| lid | name |
---------------
| 1 | one |
---------------
| 2 | two |
---------------
/what I'm trying to achieve/
------------------------------------------
|a.uid | a.to | b.name | a.from | b.name |
------------------------------------------
| 1 | 1 | one | 2 | two |
------------------------------------------
You will have to join table b twice, and every time using different table name (b1, b2) using as
select *
from a join b as b1 on a.to = b1.lid
join b as b2 on a.from = b2.lid
so the result would be
--------------------------------------------
|a.uid | a.to | b1.name | a.from | b2.name |
--------------------------------------------
| 1 | 1 | one | 2 | two |
--------------------------------------------
but what you probably want is to prevent name clash - if you e.g. call it from PHP - so then also rename the columns:
select a.*, b1.name as toName, b2.name as fromName
... (rest of the query as above)
If it's limited to just being twice just join in table b twice.
But it looks like you could have any number of numbers between a.from and a.to and in that case I would suggest that you do this in two or more queries.
One to get the row from a and than one to get all rows in b that is between a.from and a.to.
Related
I am trying to get values matching the value from the second column. For example, I want to know who is the sender for Bill Gates by only using IDs.
I have two tables,
*users* table
| user_ID | Full_name |
| -------- | -------------- |
| 1 | Steve Jobs |
| 2 | Bill Gates |
| 3 | Elon Musk |
*relationships* table (with both column foreign keys)
| user_sender | user_receiver |
| ------------ | -------------- |
| 1 | 2 |
| 3 | 1 |
| 3 | 2 |
I want to select based on "user_receiver" column the matching values in the column "user_sender"
For example, I want to know who is user_sender for 2
OUTPUT:
| | |
| ------------ | -------------- |
| 1 | 2 |
| 3 | 2 |
You need to join the tables and select the rows you want
you have access to all columns of both tables by addressing them with their alias
SELECT u.user_ID , u.Full_name,r.user_receiver
FROM users u JOIN
relationships r ON u.user_ID = r.user_sender
WHERE r.user_receiver = 2
If you want to look based on the name, then join the relationships to users.
SELECT
rel.user_sender
, rel.user_receiver
-- , sender.Full_name AS sender_name
-- , receiver.Full_name AS receiver_name
FROM relationships AS rel
JOIN users AS sender ON sender.user_ID = rel.user_sender
JOIN users As receiver ON receiver.user_ID = rel.user_receiver
WHERE receiver.Full_name = 'Bill Gates'
If you already know the user_receiver number, and you only want the ID's
SELECT *
FROM relationships
WHERE user_receiver = 2
I have a table A that contains tree columns, id, users ids and vehicle id. And a table B that contains vehicleid, and vehicle name.
Table A
---------------------------
| Id | User_id |Vehicle_id|
---------------------------
| 1 | 1 | 2 |
| 2 | 1 | 3 |
| 3 | 1 | 4 |
| 4 | 2 | 2 |
| 5 | 2 | 3 |
| 6 | 4 | 5 |
---------------------------
Table B
-------------------
| Id |Vehicle_name|
-------------------
| 1 | Car |
| 2 | Bike |
| 3 | Plane |
| 4 | Boat |
| 5 | Rocket |
-------------------
Given a user id, I need to get all vehicle names, that doesn't match with table A. I've tried Outer joins, but I can't manage to do get the info that i need.
For example: Given user id 1, the query should return Car and Rocket.
thanks in advance
This is simple enough using not in or not exists:
select b.*
from b
where not exists (select 1
from a
where a.vehicle_id = b.id and a.user_id = #a_user_id
);
I also thought of using a cross join and was able to get the output in case you are more comfortable with join logic.
SELECT CJOIN.USER_ID, CJOIN.VEHICLE_ID, CJOIN.VEHICLE_NAME
FROM
(SELECT DISTINCT A.USER_ID, B.ID AS VEHICLE_ID, B.VEHICLE_NAME FROM TABLE_A A CROSS JOIN TABLE_B B) CJOIN
LEFT JOIN
TABLE_A D
ON CJOIN.USER_ID = D.USER_ID AND CJOIN.VEHICLE_ID = D.VEHICLE_ID
WHERE D.USER_ID IS NULL AND D.VEHICLE_ID IS NULL;
First, I got all possible combinations of USER_ID x VEHICLE_ID by a cross join and used this table in a left join to pull records for which there is no match.
I have three tables
Table a
+-----+-------+
| aid | value |
+-----+-------+
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
+-----+-------+
Table b
+-----+------+
| bid | name |
+-----+------+
| 1 | A |
| 2 | B |
| 3 | C |
+-----+------+
Table ba (mapping of table a and table b)
+-----+-----+
| bid | aid |
+-----+-----+
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 3 | 2 |
| 1 | 3 |
| 2 | 3 |
| 2 | 4 |
+-----+-----+
From these tables I want a query like
SELECT aid, mapped('true'-if(aid exist in ba) 'false'-otherwise)
FROM a
JOIN b
JOIN ba
WHERE bid=1
to get a result from where I can generate a list
(when bid=1)
A-mapped
B-not mapped
C-mapped
D-not mapped
(when bid=2)
A-mapped
B-not mapped
C-mapped
D-mapped
(when bid=3)
A-mapped
B-mapped
C-not mapped
D-not mapped
Right now I am generating the list in a while loop for all the rows of table 'a' and inside the loop a query is executed for each iteration to check the existence in table 'ba'.
I think this is supposed to be table b independent:
SELECT CONCAT_WS('-', a.value, IF(ba.aid IS NULL, "-not mapped", "-mapped"))
FROM a LEFT JOIN ba ON a.aid = ba.aid AND ba.bid = 1
ORDER BY a.aid
Note: I took "a" table as the base table since your samples included all values from "a" table.
This is a tricky question, but the difficult part is in figuring out how to formulate the query. Once that is out of the way, it is downhill from there. One approach is to use a cross join between the A and B tables to obtain all possible mappings. Then LEFT JOIN to the mapping table to determine which pairs are being mapped and which are not. Try the following query:
SELECT tb.bid, ta.value,
CASE WHEN ba.bid IS NOT NULL THEN 'mapped' ELSE 'not mapped' END AS label
FROM tb INNER JOIN ta -- cross join to obtain all bid/aid pairs
LEFT JOIN ba -- to determine which pairs are mapped/not mapped
ON ta.aid = ba.aid AND tb.bid = ba.bid
ORDER BY tb.bid, ta.value
Demo here:
SQLFiddle
I want to drop all rows in a table of mySQL that have a duplicate using GROUP BY. My table has fields name date position email and looks like
+----------+---------------+----------+--------------------+
| M | 1976-10-03 | 1 | m#gmail |
| R | 1982-03-26 | 2 | r#gmail.com |
| C | 1987-09-03 | 3 | c#gmail.com |
| M | 1976-10-03 | 1 | m#gmail |
+----------+---------------+----------+--------------------+
I want to get
+----------+---------------+----------+--------------------+ |
| R | 1982-03-26 | 2 | r#gmail.com |
| C | 1987-09-03 | 3 | c#gmail.com |
+----------+---------------+----------+--------------------+
My attempt (from the answers to similar questions)
DELETE FROM ts1 WHERE * IN (SELECT * FROM ts1 GROUP BY * HAVING COUNT(*)>1);
Where are the errors? I understand I'm using too many * but I want to avoid naming all columns because they are too many in my actual table. Notice that I want to check for duplicates over the entire row.
You can't use GROUP BY * - you want to use GROUP BY name:
DELETE FROM ts1 WHERE name IN (SELECT name FROM ts1 GROUP BY name HAVING COUNT(*)>1);
Note that this would assume that users have unique names.
So you may actually want to check their emails instead:
DELETE FROM ts1 WHERE email IN (SELECT email FROM ts1 GROUP BY email HAVING COUNT(*)>1);
Say I have a database that has people, grocery stores, and items you can buy in the store, like so:
Stores People Foods
----------------- ------------------ ------------------
| id | name | | id | name | | id | name |
----------------- ------------------ ------------------
| 1 | Giant | | 1 | Jon Skeet | | 1 | Tomatoes |
| 2 | Vons | | 2 | KLee1 | | 2 | Apples |
| 3 | Safeway | ------------------ | 3 | Potatoes |
----------------- ------------------
I have an additional table which keep track of which stores sell what:
Inventory
--------------------
| store_id| food_id|
--------------------
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
| 3 | 1 |
| 3 | 2 |
| 3 | 3 |
--------------------
And I have another table that has shopping lists on it
Lists
---------------------
| person_id| food_id|
---------------------
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
| 2 | 3 |
---------------------
My question is, given a person, or their id, what is the best way to figure out what stores they can go to so they will get everything on their list. Is there a pattern for these types of computations in MySQL?
My attempt (very ugly and messy) is something like:
-- Given that _pid is the person_id we want to get the list of stores for.
SELECT stores.name, store_id, num, COUNT(*) AS counter
FROM lists
INNER JOIN inventory
ON (lists.food_id=inventory.food_id)
INNER JOIN (SELECT COUNT(*) AS num
FROM lists WHERE person_id=_pid
GROUP BY person_id) AS T
INNER JOIN stores ON (stores.id=store_id)
WHERE person_id=_pid
GROUP BY store_id
HAVING counter >= num;
Thanks for your time!
Edit SQL Fiddle with Data
If I were to solved the problem, I'll join the four tables with their linking column (specifically the foreign keys) then a subquery on the HAVING clause to count the number of items on the list for each person. Give this a try,
SET #personID := 1;
SELECT c.name
FROM Inventory a
INNER JOIN Foods b
ON a.food_id = b.id
INNER JOIN Stores c
ON a.store_id = c.id
INNER JOIN Lists d
ON d.food_id = b.id
WHERE d.person_id = #personID
GROUP BY c.name
HAVING COUNT(DISTINCT d.food_id) =
(
SELECT COUNT(*)
FROM Lists
WHERE person_ID = #personID
)
SQLFiddle Demo
#JohnWoo: why DISTINCT?
Another one...
SET #pid=2;
SELECT store_id, name
FROM inventory
JOIN lists ON inventory.food_id=lists.food_id
JOIN stores ON store_id=stores.id
WHERE person_id=#pid
GROUP BY store_id
HAVING COUNT(*)=(
SELECT COUNT(*)
FROM lists
WHERE person_id=#pid
);