I have two table
table#1
name_id | name
--------|-------
1 | abc
2 | def
table#2
subject_id| name_id |subject
----------|---------|--------
1 | 1 |malayalam
2 | 1 | english
3 | 1 | hindi
4 | 2 | malayalam
5 | 2 | hindi
i want to join these two tables and get output in the following format
id |name |subject1 |subject2 |subject3
---|-----|--------- |---------|--------
1 |abc |malayalam |english |hindi
2 |def |malayalam |hindi |
is it possible in mySql?
can anyone help?
I'll leave my other answer up because again, I think you should really consider redesigning your DB schema. You're using MySQL so have you tried using group_concat() ? It's not going to put it in table form like that but will give you the expected data, where the different subjects will be in CSV form.
SELECT one.name_id as id, name, group_concat(subject)
FROM one
INNER JOIN two
ON id = two.name_id
GROUP BY id;
Output
1 | abc | malayalam, english, hindi
2 | def | malayalam, hindi
So I'm not exactly sure about the scope of your schema here, because you have a subject_id that doesn't correlate to each subject. For example, subject_id 1 = malayalam but malayalm also equals subject_id 3, but subject_id 3 also equals hindi, while again hindi is also assigned to subject_id 4. Very confusing schema to work with.
You mention it's a one to many, but without knowing more context I don't see that being the case. A person can have more than one subject. One subject can belong to more than 1 person. This is a many to many relationship, which means you should be using 3 tables here.
For the sake of your question, you can use an inner join
SELECT one.name_id as id, name, subject
FROM one
INNER JOIN two
ON id = two.name_id
ORDER BY id;
The output of that query would be:
1 | abc | malayalam
1 | abc | english
1 | abc | hindi
2 | def | malayalam
2 | def | hindi
However, I would highly consider re-designing your database. I mean you want your data in that format, which looks nice with only 3 subjects. But what happens when there's 50 subjects for one person? Would you still want all subjects on one line like that? It's better to break it up, it will save you down the road when your database starts becoming more complex. In many cases, to create a many to many relationship you'll want a 3rd table that connects the two entities, in this case I'd create an Enrolled_Classes table to connect Names and Subjects
Names
name_id | name
--------------
1 | abc
2 | def
Subjects
subject_id | subject
--------------------
1 | malayalm
2 | english
3 | hindi
Enrolled_Classes
name_id | subject_id
--------------------
1 | 1
1 | 2
1 | 3
2 | 1
2 | 3
Then you'd need another join for your query:
SELECT name, subject
FROM Names
INNER JOIN Enrolled_Classes
ON Names.name_id = Enrolled_Classes.name_id
INNER JOIN Subjects
ON Enrolled_Classes.subject_id = Subjects.subject_id;
Same Output
abc | malayalam
abc | english
abc | hindi
def | malayalam
def | hindi
Related
I have a MySQL table with users who have completed a survey - in some cases, they have complete the survey multiple times. So it looks like this:
users|survey_attempt|question_num|response
---------------------------------------------
john | 1 | 1 | cat
john | 1 | 2 | dog
john | 1 | 3 | frog
john | 2 | 1 | dog
john | 2 | 2 | frog
john | 2 | 3 | dog
jim | 1 | 1 | frog
jim | 1 | 2 | bat
jim | 1 | 3 | bat
jim | 2 | 1 | cat
jim | 2 | 2 | frog
jim | 2 | 3 | bat
In this case, how would I find users who had common responses within the same attempt at the survey? So for instance, if I wanted to know who answered "frog" and "cat" within a unique attempt at the survey (regardless of which specific question the answer was for)?
In general, the database layout has flaws. I would suggest to use unique survey submission IDs. Because right now, you need to check user name AND survey attempt to determine if two or more rows belong to the same submission.
Anyways, you would need to self join the table and check for the answer you want but disregard the question:
SELECT A.users, A.survey_attempt
FROM table A
INNER JOIN table B ON A.users = B.users AND A.survey_attempt = B.survey_attempt
WHERE A.response = 'frog'
AND B.response = 'cat';
The table is matched with itself, in each result table you'll have all columns two times. Then the query will only select these rows where both user names and survey attempt numbers are equal. Finally, the WHERE statement checks for the answers you wanted. Nowhere, the question number is checked as you wanted to get the result regardless of specific questions.
How to use IN() Operator not working it's.
Those table are example and look the same as the real database I have.I don't have the permitting to add tables or change
Those are the tables:
students
+------+------+
| id | name |
+------+------+
| 1 | ali |
| 2 | man |
| 3 | sos |
+------+------+
Classes
+------+---------+
| c_id | students|
+------+---------+
| 1 | 1,2,3,4 |
| 2 | 88,33,55|
| 3 | 45,23,72|
+------+---------+
When I use this query it return me only the student with id =1
because "id IN (students)" return 1 when the first value are equal.
select name,c_id from students,classes where id IN (students);
when I get the list out on PHP than add it. it work fine.But, this solution need a loop and cost many queries.
select name,c_id from students,classes where id IN (1,2,3,4);
FIND_IN_SET()
the same happened, it's only return 1 but if the value on other position it return 0.
The IN operator works just fine, where it's applicable for what it does.
First, consider restructuring your data to be normalized, and avoid storing values as comma separated lists.
Second, if you absolutely have to deal with columns containing comma separated lists of values, MySQL provides the FIND_IN_SET() function.
FOLLOWUP
Ditch the old-school comma syntax for the join operation, and use the JOIN keyword instead. And relocate the join predicates from the WHERE clause to the ON clause. Fully qualify column references, eg.
SELECT s.name
, c.c_id
FROM students s
JOIN classes c
ON FIND_IN_SET(s.student_id,c.students)
ORDER BY s.name, c.c_id
To reiterate, storing a "comma separated list" in a column is an anti-pattern; it flies against relational theory and normalization, and disregards the best practices around relational databases. O
One might argue for improved performance, but this pattern doesn't improve performance; rather it adds unnecessary complexity in query and DML operations.
You need three tables.
One table students, one table classes, and then one table, say, students_to_classes containing something like
c_id | student_id
1 | 1
1 | 2
1 | 3
1 | 4
2 | 88
and so on.
Then you can query
select c_id from students_to_classes where student_id in (1,2,3,4)
Google "n:m relationship" for background on this.
EDIT
I know you're not specifically asking for another table structure, but this is a way of having a data type (a single number) that works with IN. Please believe me that this is the right way to do it, the reason you run into trouble with something as simple as IN is that you're using a non-standard approach, which, for such a standard problem, is typically not a good idea.
That's not how the function IN is supposed to work. You use IN when you have a list of possible matches like:
instead of:
WHERE id=1 or id=2 or id=3 or id=4
you use:
WHERE id IN (1,2,3,4)
Anyhow, your logic is not correct. The relation of Class and Student is Many-to-Many, thus a third table is needed. Let's call it studend_class, where you can store the students of each class.
student
+------+------+
| id | name |
+------+------+
| 1 | ali |
| 2 | man |
| 3 | sos |
+------+------+
class
+------+---------+
| id | name |
+------+---------+
| 1 | math |
| 2 | english |
| 3 | science |
+------+---------+
student_class
+------------+-------------+
| class_id | student_id |
+------------+-------------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 3 | 3 |
+--------------+-----------+
In the example above all students are in math class and ali is also in science class.
Finally, if you whant to know which students are in what class, let's say Math, you can use:
SELECT s.id, s.name, c.name
FROM student s
INNER JOIN student_class sc ON sc.student_id=s.id
INNER JOIN class c ON sc.class_id = c.id
WHERE c.name="math";
MORE DETAILS:
Both of you recomend using JOIN. But the main problem is how to assign multiple SUBJECTS PER EACH CLASS without using multiple duplicate values. I will have ~200 de classes, with ~30 subjects per class. That means if 2 classes share the same 20 subjects, i will have 40 rows, all with "class_id = 1" but with "subjects_Id =1, subjects_id=2, etc" Its not very ergonomic. Any other ideas? Thanks for your time!
So, I am here again asking for your time and help friends.
I have a database that its almost ok. But I am stuck at trying how to link multiple values from a table to on collumn on another.
Let me be more explicit.
I have this table:
CLASSES
id | class_name | Matters |
-----------------------------
1 | Class1 | 13.4.2013 |
2 | Class2 | 14.4.2013 |
And this table:
Subjects
mat_id | show title |
-----------------
1 | English |
2 | French |
Now the problem is this. Each CLASS (e.g. CLASS1) should be able to study more Subjects at once. For example, CLASS 1 should be linked with subject (mat_id) 1, 3, 5, 6.
How to do this without repeating myself, and optimize the database? I tought that I should do it like so, but its not convenient :
CREATE A NEW TABLE named
SUBJECTS_PER_CLASS
id | class_id | mat_id |
----------------------------
1 | 1 | 1 |
2 | 1 | 3 |
BUT then I dont know how to query it. Any ideas? Any help will be greatly appreciated!
THANKS!
SELECT
*
FROM
CLASSES
JOIN
SUBJECTS_PER_CLASS
ON
CLASSES.ID = SUBJECTS_PER_CLASS.class_id
JOIN
Subjects
ON
Subjects.id = SUBJECTS_PER_CLASS.mat_id
You can use join command.
Reference 1
Reference 2
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.
I have table like:
user :
uid | course_id | subjects
---------------------------
1 | 1 | html,php
2 | 1 | java,html,sql
3 | 1 | java
4 | 1 | fashion,html,php,sql,java
I want to run a query which can return most liked subjects in query and then second most and so on...
For Example :
select * from user where subjects like '%java%' or '%php%' or '%html%';
this query will return data like this:
uid | course_id | subjects
---------------------------
2 | 1 | java,html,sql
3 | 1 | java
4 | 1 | fashion,html,php,sql,java
but i want output like this :
uid | course_id | subjects
---------------------------
4 | 1 | fashion,html,php,sql,java
2 | 1 | java,html,sql
1 | 1 | html,php
3 | 1 | java
so the most matched subjects 1st then 2nd most matched subjects and so on....
Is there any modification in my query so that i can get this type of sorted output.
Never, never, never store multiple values in one column!
Like you see now this will only give you headaches. Normalize your user table. Then you can select normally.
It should look like this
uid | course_id | subjects
---------------------------
1 | 1 | html
1 | 1 | php
2 | 1 | java
2 | 1 | html
2 | 1 | sql
3 | 1 | java
...
or better introduce an new table subjects and then make a mapping table called course_subjects
subject
id | name
------------
1 | html
2 | sql
3 | java
...
course_subjects
uid | course_id | subject_id
---------------------------
1 | 1 | 1
1 | 1 | 2
...
Based on the way you want your results, it looks like you want to order by the number of subjects (or tags) within subject. This can be accomplished by counting the number of , (commas).
The way to count the number of occurances of a character is to subtract the original length by the length when the character is removed.
Example:
SELECT *
FROM USER
WHERE subjects LIKE '%java%'
OR '%php%'
OR '%html%'
ORDER BY ( Length(subjects) - Length(Replace(subjects, ',', '')) ) DESC;
SQLFiddle: http://sqlfiddle.com/#!2/cc793/4
Result:
UID COURSE_ID SUBJECTS
4 1 fashion,html,php,sql,java
2 1 java,html,sql
3 1 java
Note:
As juergen says it is a bad idea to store multiple values in one column.
With MyISAM storage engine you can do match against.
The simplest example:
SELECT *,
MATCH (subjects) AGAINST ('java php html') AS relevance
FROM `user`
WHERE MATCH (subjects) AGAINST ('java php html')
ORDER BY relevance DESC
In MySQL 5.6 full-text search is available with InnoDB too but needs a bit extra to make it work. For more info checkout the following post: http://www.mysqlperformanceblog.com/2013/03/04/innodb-full-text-search-in-mysql-5-6-part-2-the-queries/