I am using Mysql and I have two tables-
BusDetails
+-------+-----------+
| busId | BusName |
+-------+-----------+
| 1 | A TRAVELS |
| 2 | B TRAVELS |
| 3 | C TRAVELS |
+-------+-----------+
AreaDetails
+--------+----------+
| cityId | cityName |
+--------+----------+
| 1 | ABC |
| 2 | DEF |
| 3 | GHI |
| 4 | JKL |
+--------+----------+
Now I have to create third table which will map bus table to city table. Suppose busId 1 stops at cityId 2 and 3 and bustId 2 stops at cityId 1 and 4. To create this scenario I have 2 options-
first option-
+-------+--------+
| busId | areaId |
+-------+--------+
| 1 | 3,2 |
| 2 | 4,1 |
+-------+--------+
second option-
+-------+--------+
| busId | areaId |
+-------+--------+
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
| 2 | 4 |
+-------+--------+
In future when there will be large number of records then which table will give better performance and why ?
The first option is poor because comma-separated lists do not get indexed. If you want to find all the busses in area 2, you would have to use
SELECT busID
FROM bus_areas
WHERE FIND_IN_SET('2', areaID)
This will have to perform a full table scan, parse the areaID column on each row, and test whether 2 is a member of the resulting array.
With the second version you can do:
SELECT busID
FROM bus_areas
WHERE areaID = 2
If you have an index on areaID, this will be extremely efficient.
If you wanted to know how many busses are in each area, it's easy with the second option:
SELECT areaID, COUNT(*)
FROM bus_areas
GROUP BY areaID
With the first option it would be more cumbersome:
SELECT cityID, COUNT(*)
FROM areaDetails a
JOIN bus_areas ba ON FIND_IN_SET(a.cityID, ba.areaID)
GROUP BY cityID
This will be very inefficient because it has to perform M*N FIND_IN_SET operations, and as I explained above this cannot be indexed. Notice that I had to join with the areaDetails table because there's no way to enumerate all the areas in the comma-separated lists in SQL.
The answer depends upon your use.
Although first option is not recommended but if you have very large data and you are not planning to perform wide range of Database operations (probably for self or small project) you can use it.
Second options has it's own advantage and recommended by relational model. It will give you more flexibility and scalability as this minimize redundancy.
Dear Second Table Is Better For All Reason Baecause At long time you have big data second type in save so many rows but better for getting report easy in easy for SQL query easy. you can all type join easy.
Related
In my table I have two columns "sku" and "fitment". The sku represents a part and the fitment represents all the vehicles this part will fit on. The problem is, in the fitment cells, there could be up to 20 vehicles in there, separated by ^^. For example
**sku -- fitment**
part1 -- Vehichle 1 information ^^ vehichle 2 information ^^ vehichle 3 etc
I am looking to split the cells in the fitment column, so it would look like this:
**sku -- fitment**
part1 -- Vehicle 1 information
part1 -- Vehicle 2 information
part1 -- Vehicle 3 information
Is this possible to do? And if so, would a mySQL db be able to handle hundreds of thousands of items "splitting" like this? I imagine it would turn my db of around 250k lines to about 20million lines. Any help is appreciated!
Also a little more background, this is going to be used for a drill down search function so I would be able to match up parts to vehicles (year, make, model, etc) so if you have a better solution, I am all ears.
Thanks
Possible duplicate of this: Split value from one field to two
Unfortunately, MySQL does not feature a split string function. As in the link above indicates there are User-defined Split function's.
A more verbose version to fetch the data can be the following:
SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(fitment, '^^', 1), '^^', -1) as fitmentvehicle1,
SUBSTRING_INDEX(SUBSTRING_INDEX(fitment, '^^', 2), '^^', -1) as fitmentvehicle2
....
SUBSTRING_INDEX(SUBSTRING_INDEX(fitment, '^^', n), '^^', -1) as fitmentvehiclen
FROM table_name;
Since your requirement asks for a normalized format (i.e. not separated by ^^) to be retrieved, it is always better to store it in that way in the first place. And w.r.t the DB size bloat up, you might want to look into possibilities of archiving older data and deleting the same from the table.
Also, you should partition your table using an efficient partitioning strategy based on your requirement. It would be more easier to archive and truncate a partition of the table itself, instead of row by row.
E.g.
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table (user_id INT NOT NULL PRIMARY KEY,stuff VARCHAR(50) NOT NULL);
INSERT INTO my_table VALUES (101,'1,2,3'),(102,'3,4'),(103,'4,5,6');
SELECT *
FROM my_table;
+---------+-------+
| user_id | stuff |
+---------+-------+
| 101 | 1,2,3 |
| 102 | 3,4 |
| 103 | 4,5,6 |
+---------+-------+
SELECT * FROM ints;
+---+
| i |
+---+
| 0 |
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
| 9 |
+---+
SELECT DISTINCT user_id
, SUBSTRING_INDEX(SUBSTRING_INDEX(stuff,',',i2.i*10+i1.i+1),',',-1) x
FROM my_table
, ints i1
, ints i2
ORDER
BY user_id,x;
+---------+---+
| user_id | x |
+---------+---+
| 101 | 1 |
| 101 | 2 |
| 101 | 3 |
| 102 | 3 |
| 102 | 4 |
| 103 | 4 |
| 103 | 5 |
| 103 | 6 |
+---------+---+
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";
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:
+----+--------+----------+
| id | doc_id | next_req |
+----+--------+----------+
| 1 | 1 | 4 |
| 2 | 1 | 3 |
| 3 | 1 | 0 |
| 4 | 1 | 2 |
+----+--------+----------+
id - auto incerement primary key.
nex_req - represent an order of records. (next_req = id of record)
How can I build a SQL query get records in this order:
+----+--------+----------+
| id | doc_id | next_req |
+----+--------+----------+
| 1 | 1 | 4 |
| 4 | 1 | 2 |
| 2 | 1 | 3 |
| 3 | 1 | 0 |
+----+--------+----------+
Explains:
record1 with id=1 and next_req=4 means: next must be record4 with id=4 and next_req=2
record4 with id=5 and next_req=2 means: next must be record2 with id=2 and next_req=3
record2 with id=2 and next_req=3 means: next must be record3 with id=1 and next_req=0
record3 with id=3 and next_req=0: means that this is a last record
I need to store an order of records in table. It's important fo me.
If you can, change your table format. Rather than naming the next record, mark the records in order so you can use a natural SQL sort:
+----+--------+------+
| id | doc_id | sort |
+----+--------+------+
| 1 | 1 | 1 |
| 4 | 1 | 2 |
| 2 | 1 | 3 |
| 3 | 1 | 4 |
+----+--------+------+
Then you can even cluster-index on doc_id,sort for if you need to for performance issues. And honestly, if you need to re-order rows, it is not any more work than a linked-list like you were working with.
Am able to give you a solution in Oracle,
select id,doc_id,next_req from table2
start with id =
(select id from table2 where rowid=(select min(rowid) from table2))
connect by prior next_req=id
fiddle_demo
I'd suggest to modify your table and add another column OrderNumber, so eventually it would be easy to order by this column.
Though there may be problems with this approach:
1) You have existing table and need to set OrderNumber column values. I guess this part is easy. You can simply set initial zero values and add a CURSOR for example moving through your records and incrementing your order number value.
2) When new row appears in your table, you have to modify your OrderNumber, but here it depends on your particular situation. If you only need to add items to the end of the list then you can set your new value as MAX + 1. In another situation you may try writing TRIGGER on inserting new items and calling similar steps to point 1). This may cause very bad hit on performance, so you have to carefully investigate your architecture and maybe modify this unusual construction.
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