I am trying to do multiple joins on the same MySQL table, but am not getting the results that I expect to get. Hopefully someone can point out my mistake(s).
Table 1 - cpe Table
|id | name
|----------
| 1 | cat
| 2 | dog
| 3 | mouse
| 4 | snake
-----------
Table 2 - AutoSelect
|id | name | cpe1_id | cpe2_id | cpe3_id |
|-----------------------------------------------
| 1 | user1 | 1 | 3 | 4 |
| 2 | user2 | 3 | 1 | 2 |
| 3 | user3 | 3 | 3 | 2 |
| 4 | user4 | 4 | 2 | 1 |
------------------------------------------------
I would like to see an output of
user1 | cat | mouse | snake |
user2 | mouse | snake | dog |
..etc
Here is what I have tried
SELECT * FROM AutoSelect
LEFT JOIN cpe ON
( cpe.id = AutoSelect.cpe1_id ) AND
( cpe.id = AutoSelect.cpe2_id ) AND
( cpe.id = AutoSelect.cpe3_id )
I get blank results. I thought i knew how to do these joins, but apparently when I'm trying to match cpe?_id with the name of the cpe table.
Thanks in advance for any assistance.
You need left join 3 times as well. Currently your query only joins 1 time with 3 critieria as to the join. This should do:
SELECT a.name, cpe1.name, cpe2.name, cpe3.name FROM AutoSelect as a
LEFT JOIN cpe as cpe1 ON ( cpe1.id = a.cpe1_id )
LEFT JOIN cpe as cpe2 ON ( cpe2.id = a.cpe2_id )
LEFT JOIN cpe as cpe3 ON ( cpe3.id = a.cpe3_id )
And you probably mean to INNER JOIN rather than LEFT JOIN unless NULL values are allowed in your AutoSelect table.
I think your design is wrong.
With tables like that, you get it the way it's meant to be in relational databases :
table 1 : animal
id name
1 cat
2 dog
3 mouse
4 snake
table 2 : user
|id | name |
|--------------
| 1 | user1 |
| 2 | user2 |
| 3 | user3 |
| 4 | user4 |
table 3 : association
|id_user | id_animal|
|--------------------
| 1 | 1 |
| 1 | 3 |
| 1 | 4 |
| 2 | 3 |
| 2 | 1 |
| 2 | 2 |
| 3 | 3 |
| 3 | 2
| 4 | 4 |
| 4 | 2 |
| 4 | 1 |
---------------------
Then :
select u.name, a.name from user u, animal a, association ass where ass.user_id = u.id and ass.animal_id = a.id;
In this case, your solution won't produce a good dynamic database. There are other ways to make combinations of multiple tables. I can show you by my own database what you should use and when you should use this solution. The scheme is in dutch, but you'll probably understand the keywords.
Like you, I had to combine my windmills with a kWh-meter, which has to measure the energyproduction of my windmills. What you should do, is this case, is making another table(in my case molenkWhlink). Make sure your tables are INNODB-types(for making Foreign keys). What I've done is combining my meters and mills by putting a pointer(a foreign key) of their ID(in Dutch Volgnummer) in the new table. An advantage you may not need, but I certainly did, is the fact I was able to extend the extra table with connection and disconnection info like Timestamps and metervalues when linking or unlinking. This makes your database way more dynamic.
In my case, I Also had a table for meassurements(metingoverzicht). As you can see in the scheme, I've got 2 lines going from Metingoverzicht to molenkwhlink. The reason for this is quite simple. All meassurements I take, will be saved in table Metingoverzicht. Daily meassurements(which are scheduled) will have a special boolean put on, but unscheduled meassurements, will also me saved here, with the bollean turned off. When switching meters, I need the endvalue from the leaving meter and the startvalue from the new meter, to calculate the value of todays eneryproduction. This is where your solution comes in and an extra table won't work. Usually, when you need just one value from another table a JOIN will be used. The problem in this case is, I've got 2 meassurementIDs in 1 link(1 for connecting and 1 for disconnecting). They both point to the same tablecolumn, because they both need to hold the same type of information. That is when you can use a double JOIN from one table towards the other. Because, both values will only be used once, and just needed to be saved in a different place to avoid having 1 action stored on different locations, which should always be avoided.
http://s1101.photobucket.com/user/Manuel_Barcelona/media/schemedatabase.jpg.html
Related
I created a table (t_subject) like this
| id | description | enabled |
|----|-------------|---------|
| 1 | a | 1 |
| 2 | b | 1 |
| 3 | c | 1 |
And another table (t_place) like this
| id | description | enabled |
|----|-------------|---------|
| 1 | d | 1 |
| 2 | e | 1 |
| 3 | f | 1 |
Right now data from t_subject is used for each of t_place records, to show HTML dropdowns, with all the results from t_subject.
So I simply do
SELECT * FROM t_subject WHERE enabled = 1
Now just for one of t_place records, one record from t_subject should be hidden.
I don't want to simply delete it with javascript, since I want to be able to customize all of the dropdowns if anything changes.
So the first thing I though was to add a place_id column to t_subject.
But this means I have to duplicate all of t_subject records, I would have 3 of each, except one that would have 2.
Is there any way to avoid this??
I thought adding an id_exclusion column to t_subject so I could duplicate records only whenever a record is excluded from another id from t_place.
How bad would that be?? This way I would have no duplicates, so far.
Hope all of this makes sense.
While you only need to exclude one course, I would still recommend setting up a full 'place-course' association. You essentially have a many-to-many relationship, despite not explicitly linking your tables.
I would recommend an additional 'bridging' or 'associative entity' table to represent which courses are offered at which places. This new table would have two columns - one foreign key for the ID of t_subject, and one for the ID of t_place.
For example (t_place_course):
| place_id | course_id |
|----------|-----------|
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
| 2 | 2 |
| 2 | 3 |
| 3 | 1 |
| 3 | 3 |
As you can see in my example above, place 3 doesn't offer course 2.
From here, you can simply query all of the courses available for a place by querying the place_id:
SELECT * from t_place_course WHERE place_id = 3
The above will return both courses 1 and 3.
You can optionally use a JOIN to get the other information about the course or place, such as the description:
SELECT `t_course`.`description`
FROM `t_course`
INNER JOIN `t_place_course`
ON `t_course`.`id` = `t_place_course`.`course_id`
INNER JOIN `t_place`
ON `t_place`.`id` = `place_id`
Two tables...
people (personid, name, mainordering)
entries (userid, personid, altordering)
"personid" is the common field. My app displays a draggable list users can move around. When done, they click to "lock" in their order.
Table : people
+----------+---------+--------------+
| personid | name | mainordering |
+----------+---------+--------------+
| 1 | Bob | 2 |
| 2 | Charlie | 4 |
| 3 | Jim | 1 |
| 4 | Doug | 3 |
+----------+---------+--------------+
So using mainordering, it would display:
Jim
Bob
Doug
Charlie
entries table might have (for user 16):
+--------+----------+-------------+
| userid | personid | altordering |
+--------+----------+-------------+
| 16 | 1 | 3 |
| 16 | 2 | 1 |
| 16 | 3 | 2 |
| 16 | 4 | 4 |
+--------+----------+-------------+
So if user 16 has already submitted his entry BUT NOT LOCKED IT IN, I want to display his list using altordering. i.e.
Charlie
Jim
Bob
Doug
I'm struggling with the proper join to use. Here is what I tried and isn't working (it's simply ordering by mainordering still)...
$sql = "SELECT * from entries
WHERE userid=".$_SESSION['userid']."
LEFT JOIN people ON entries.personid = people.personid
ORDER BY altordering";
Any thoughts would be much appreciated. Thank you...
Are you sure you don't get an error when using WHERE before JOIN?
It should work like this:
SELECT people.*
FROM people
JOIN entries ON entries.personid = people.personid
WHERE entries.userid={$_SESSION['userid']}
ORDER BY entries.altordering
I assume entries.personid will always have a matching person in people, so you should use an INNER JOIN. You would use FROM entries LEFT JOIN people if you wanted to retrieve altordering even for non-existing people.
I have a question about my DB table structure. I want to know if i'm on the right track or if I have missed a good alternative. Here is the case:
To make it easy to read, I haven't pasted the full contents as my question is only about the structure.
2 tables:
1: id (AI), task
2: id, name, task
Table 1 presents dynamic check-boxes which can be altered by an admin panel so the contents would be like this
1 task1
2 task2
5 task5
(3 & 4 are missing cause the administrator deleted those records).
In table number two are the people who should do the tasks from table 1. And the goal is that the tasks wich are not checked will be displayed.
So the contents of table 2 would be:
1 Name1 1,5
2 Name2 1,2
3 Name3 1,2,5
The numbers in table 2 represent the checked boxes from table 1. So with a query i can compare the numbers from table 2 with the id's from table 1 and display the missing ids as "todo".
In my opinion this looks very overdone, and there must be an easier way to create dynamic options which can be compared and stored as a todo.
Suggestions are welcome!
I suggest you to use basic structure for many-to-many relationship:
tasks users user_tasks
+----+-----------+ +----+-------+ +---------+---------+
| id | name | | id | name | | user_id | task_id |
+----+-----------+ +----+-------+ +---------+---------+
| 1 | Buy milk | | 1 | John | | 1 | 2 |
| 2 | Get drunk | | 2 | Tim | | 3 | 2 |
| 3 | Have fun | | 3 | Steve | | 2 | 4 |
| 4 | Go home | +----+-------+ | 3 | 4 |
+----+-----------+ +---------+---------+
And you can fetch unassigned tasks using following query:
SELECT
tasks.*
FROM
tasks
LEFT JOIN
user_tasks
ON (tasks.id = user_tasks.task_id)
WHERE
user_tasks.user_id IS NULL
You also can fetch users who have no assigned tasks:
SELECT
users.*
FROM
users
LEFT JOIN
user_tasks
ON (users.id = user_tasks.user_id)
WHERE
user_tasks.user_id IS NULL
Hope this will help you.
I have a table of users where 1 column stores user's "roles".
We can assign multiple roles to particular user.
Then I want to store role IDs in the "roles" column.
But how can I store multiple values into a single column to save memory in a way that is easy to use? For example, storing using a comma-delimited field is not easy and uses memory.
Any ideas?
If a user can have multiple roles, it is probably better to have a user_role table that stores this information. It is normalised, and will be much easier to query.
A table like:
user_id | role
--------+-----------------
1 | Admin
2 | User
2 | Admin
3 | User
3 | Author
Will allow you to query for all users with a particular role, such as SELECT user_id, user.name FROM user_role JOIN user WHERE role='Admin' rather than having to use string parsing to get details out of a column.
Amongst other things this will be faster, as you can index the columns properly and will take marginally more space than any solution that puts multiple values into a single column - which is antithetical to what relational databases are designed for.
The reason this shouldn't be stored is that it is inefficient, for the reason DCoder states on the comment to this answer. To check if a user has a role, every row of the user table will need to be scanned, and then the "roles" column will have to be scanned using string matching - regardless of how this action is exposed, the RMDBS will need to perform string operations to parse the content. These are very expensive operations, and not at all good database design.
If you need to have a single column, I would strongly suggest that you no longer have a technical problem, but a people management one. Adding additional tables to an existing database that is under development, should not be difficult. If this isn't something you are authorised to do, explain to why the extra table is needed to the right person - because munging multiple values into a single column is a bad, bad idea.
You can also use bitwise logic with MySQL. role_id must be in BASE 2 (0, 1, 2, 4, 8, 16, 32...)
role_id | label
--------+-----------------
1 | Admin
2 | User
4 | Author
user_id | name | role
--------+-----------------
1 | John | 1
2 | Steve | 3
3 | Jack | 6
Bitwise logic allows you to select all user roles
SELECT * FROM users WHERE role & 1
-- returns all Admin users
SELECT * FROM users WHERE role & 5
-- returns all users who are admin or Author because 5 = 1 + 4
SELECT * FROM users WHERE role & 6
-- returns all users who are User or Author because 6 = 2 + 4
From your question what I got,
Suppose, you have to table. one is "meal" table and another one is "combo_meal" table. Now I think you want to store multiple meal_id inside one combo_meal_id without separating coma[,]. And you said that it'll make your DB to more standard.
If I not getting wrong from your question then please read carefully my suggestion bellow. It may be help you.
First think is your concept is right. Definitely it'll give you more standard DB.
For this you have to create one more table [ example table: combo_meal_relation ] for referencing those two table data. May be one visible example will clear it.
meal table
+------+--------+-----------+---------+
| id | name | serving | price |
+------+--------+-----------+---------+
| 1 | soup1 | 2 person | 12.50 |
+------+--------+-----------+---------+
| 2 | soup2 | 2 person | 15.50 |
+------+--------+-----------+---------+
| 3 | soup3 | 2 person | 23.00 |
+------+--------+-----------+---------+
| 4 | drink1 | 2 person | 4.50 |
+------+--------+-----------+---------+
| 5 | drink2 | 2 person | 3.50 |
+------+--------+-----------+---------+
| 6 | drink3 | 2 person | 5.50 |
+------+--------+-----------+---------+
| 7 | frui1 | 2 person | 3.00 |
+------+--------+-----------+---------+
| 8 | fruit2 | 2 person | 3.50 |
+------+--------+-----------+---------+
| 9 | fruit3 | 2 person | 4.50 |
+------+--------+-----------+---------+
combo_meal table
+------+--------------+-----------+
| id | combo_name | serving |
+------+--------------+-----------+
| 1 | combo1 | 2 person |
+------+--------------+-----------+
| 2 | combo2 | 2 person |
+------+--------------+-----------+
| 4 | combo3 | 2 person |
+------+--------------+-----------+
combo_meal_relation
+------+--------------+-----------+
| id | combo_meal_id| meal_id |
+------+--------------+-----------+
| 1 | 1 | 1 |
+------+--------------+-----------+
| 2 | 1 | 2 |
+------+--------------+-----------+
| 3 | 1 | 3 |
+------+--------------+-----------+
| 4 | 2 | 4 |
+------+--------------+-----------+
| 5 | 2 | 2 |
+------+--------------+-----------+
| 6 | 2 | 7 |
+------+--------------+-----------+
When you search inside table then it'll generate faster result.
search query:
SELECT m.*
FROM combo_meal cm
JOIN meal m
ON m.id = cm.meal_id
WHERE cm.combo_id = 1
Hopefully you understand :)
You could do something like this
INSERT INTO table (id, roles) VALUES ('', '2,3,4');
Then to find it use FIND_IN_SET
As you might already know, storing multiple values in a cell goes against 1NF form. If youre fine with that, using a json column type is a great way and has good methods to query properly.
SELECT * FROM table_name
WHERE JSON_CONTAINS(column_name, '"value 2"', '$')
Will return any entry with json data like
[
"value",
"value 2",
"value 3"
]
Youre using json, so remember, youre query performance will go down the drain.
Some background: an 'image' is part of one 'photoshoot', and may be a part of zero or many 'galleries'. My tables:
'shoots' table:
+----+--------------+
| id | name |
+----+--------------+
| 1 | Test shoot |
| 2 | Another test |
| 3 | Final test |
+----+--------------+
'images' table:
+----+-------------------+------------------+
| id | original_filename | storage_location |
+----+-------------------+------------------+
| 1 | test.jpg | store/test.jpg |
| 2 | test.jpg | store/test.jpg |
| 3 | test.jpg | store/test.jpg |
+----+-------------------+------------------+
'shoot_images' table:
+----------+----------+
| shoot_id | image_id |
+----------+----------+
| 1 | 1 |
| 1 | 2 |
| 3 | 3 |
+----------+----------+
'gallery_images' table:
+------------+----------+
| gallery_id | image_id |
+------------+----------+
| 1 | 1 |
| 1 | 2 |
| 2 | 3 |
| 3 | 1 |
| 4 | 1 |
+------------+----------+
What I'd like to get back, so I can say 'For this photoshoot, there are X images in total, and these images are featured in Y galleries:
+----+--------------+-------------+---------------+
| id | name | image_count | gallery_count |
+----+--------------+-------------+---------------+
| 3 | Final test | 1 | 1 |
| 2 | Another test | 0 | 0 |
| 1 | Test shoot | 2 | 4 |
+----+--------------+-------------+---------------+
I'm currently trying the SQL below, which appears to work correctly but only ever returns one row. I can't work out why this is happening. Curiously, the below also returns a row even when 'shoots' is empty.
SELECT shoots.id,
shoots.name,
COUNT(DISTINCT shoot_images.image_id) AS image_count,
COUNT(DISTINCT gallery_images.gallery_id) AS gallery_count
FROM shoots
LEFT JOIN shoot_images ON shoots.id=shoot_images.shoot_id
LEFT JOIN gallery_images ON shoot_images.image_id=gallery_images.image_id
ORDER BY shoots.id DESC
Thanks for taking the time to look at this :)
You are missing the GROUP BY clause:
SELECT
shoots.id,
shoots.name,
COUNT(DISTINCT shoot_images.image_id) AS image_count,
COUNT(DISTINCT gallery_images.gallery_id) AS gallery_count
FROM shoots
LEFT JOIN shoot_images ON shoots.id=shoot_images.shoot_id
LEFT JOIN gallery_images ON shoot_images.image_id=gallery_images.image_id
GROUP BY 1, 2 -- Added this line
ORDER BY shoots.id DESC
Note: The SQL standard allows GROUP BY to be given either column names or column numbers, so GROUP BY 1, 2 is equivalent to GROUP BY shoots.id, shoots.name in this case. There are many who consider this "bad coding practice" and advocate always using the column names, but I find it makes the code a lot more readable and maintainable and I've been writing SQL since before many users on this site were born, and it's never cause me a problem using this syntax.
FYI, the reason you were getting one row before, and not getting and error, is that in mysql, unlike any other database I know, you are allowed to omit the group by clause when using aggregating functions. In such cases, instead of throwing a syntax exception, mysql returns the first row for each unique combination of non-aggregate columns.
Although at first this may seem abhorrent to SQL purists, it can be incredibly handy!
You should look into the MySQL function group by.