I am having problem with storing the data in mysql. I want to save the data in an unordered way inside a database. for eg.
Number | Name | Section | Grades |
1 | x | A | 80% |
3 | z | B | 72% |
2 | y | C | 55% |
I want to save the data inside the database in such a way that the data is saved according to the number in order. It should be stored in order of number.
I saw the function GROUP BY which show the data in an arranged way. But I need to save the data in database file that way. Is there any way by which I can make the data in order inside database file.?
Thanks for your time.
this one is will going to display in grades format like as,3,2,1 :
SELECT number,name, section, grades FROM tablename ORDER BY grades DESC;
The order in which the data is save is not relevant to you as you can use the ORDER BY operator to order your select results.
An example query by sorting the data will be:
SELECT * FROM <table> ORDER BY name ASC;
You can see the full documentation on:
https://dev.mysql.com/doc/refman/5.0/en/sorting-rows.html
Reading the comments, a view may be usefull for you, so you can access the data in an ordered way from the other software:
CREATE VIEW <view_name> AS SELECT * FROM <table> ORDER BY name ASC;
And the you can access the view with
SELECT * from <view_name>
and your data will be in the order you defined in the view query.
You can find the full view reference on:
https://dev.mysql.com/doc/refman/5.0/en/create-view.html
Related
I have table region with fields
id | name | dates
Sample data
1 | local | "2018-01-01", "2018-01-02", "2018-01-03"
I want a query like this
SELECT * FROM region WHERE "2018-01-02" in (region.dates)
but this does not work. I do not use json data in this case. How can I change it?
(My)SQL doesn't work like that as it will see region.dates as a simple string. In which case, you can do SELECT * FROM region WHERE dates LIKE '%"2018-01-02"%';
However, a better solution would be to devolve that column into another table.
I have a column called "Permissions" in my table. The permissions are strings which can be:
"r","w","x","rw","wx","rwx","xwr"
etc. Please note the order of characters in the string is not fixed. I want to GROUP_CONCAT() on the "Permissions" column of my table. However this causes very large strings.
Example: "r","wr","wx" group concatenated is "r,wr,wx" but should be "r,w,x" or "rwx". Using distinct() clause doesn't seem to help much. I am thinking that if I could check if a permission value is a substring of the other column then I should not concatenate it, but I don't seem to find a way to accomplish that.
Any column based approach using solely string functions would also be appreicated.
EDIT:
Here is some sample data:
+---------+
| perm |
+---------+
| r,x,x,r |
| x |
| w,rw |
| rw |
| rw |
| x |
| w |
| x,x,r |
| r,x |
+---------+
The concatenated result should be:
+---------+
| perm |
+---------+
| r,w,x |
+---------+
I don't have control over the source of data and would like not to create new tables ( because of restricted privileges and memory constraints). I am looking for a post-processing step that converts each column value to the desired format.
A good idea would be to first normalize your data.
You could, for example try this way (I assume your source table is named Files):
Create simple table called PermissionCodes with only column named Code (type of string).
Put r, w, and x as values into PermissionCodes (three rows total).
In a subquery join Files to PermissionCodes on a condition that Code exists as a substring in Permissions.
Perform your GROUP_CONCAT aggregation on the result of the subquery.
If it is a case here, that for the same logical entires in Files there exists multiple permission sets that overlaps (i.e. for some file there is a row with rw and another row with w) then you would limit your subquery to distinct combinations of Files' keys and Code.
Here's a fiddle to demonstrate the idea:
http://sqlfiddle.com/#!9/6685d6/4
You can try something like:
SELECT user_id, GROUP_CONCAT(DISTINCT perm)
FROM Permissions AS p
INNER JOIN (SELECT 'r' AS perm UNION ALL
SELECT 'w' UNION ALL
SELECT 'x') AS x
ON p.permission LIKE CONCAT('%', x.perm, '%')
GROUP BY user_id
You can include any additional permission code in the UNION ALL of the derived table used to JOIN with Permissions table.
Demo here
I have 6 tables. These are simplified for this example.
user_items
ID | user_id | item_name | version
-------------------------------------
1 | 123 | test | 1
data
ID | name | version | info
----------------------------
1 | test | 1 | info
data_emails
ID | name | version | email_id
------------------------
1 | test | 1 | 1
2 | test | 1 | 2
emails
ID | email
-------------------
1 | email#address.com
2 | second#email.com
data_ips
ID | name | version | ip_id
----------------------------
1 | test | 1 | 1
2 | test | 1 | 2
ips
ID | ip
--------
1 | 1.2.3.4
2 | 2.3.4.5
What I am looking to achieve is the following.
The user (123) has the item with name 'test'. This is the basic information we need for a given entry.
There is data in our 'data' table and the current version is 1 as such the version in our user_items table is also 1. The two tables are linked together by the name and version. The setup is like this as a user could have an item for which we dont have data, likewise there could be an item for which we have data but no user owns..
For each item there are also 0 or more emails and ips associated. These can be the same for many items so rather than duplicate the actual email varchar over and over we have the data_emails and data_ips tables which link to the emails and ips table respectively based on the email_id/ip_id and the respective ID columns.
The emails and ips are associated with the data version again through the item name and version number.
My first query is is this a good/well optimized database setup?
My next query and my main question is joining this complex data structure.
What i had was:
PHP
- get all the user items
- loop through them and get the most recent data entry (if any)
- if there is one get the respective emails
- get the respective ips
Does that count as 3 queries or essentially infinite depending on the number of user items?
I was made to believe that the above was inefficient and as such I wanted to condense my setup into using one query to get the same data.
I have achieved that with the following code
SELECT user_items.name,GROUP_CONCAT( emails.email SEPARATOR ',' ) as emails, x.ip
FROM user_items
JOIN data AS data ON (data.name = user_items.name AND data.version = user_items.version)
LEFT JOIN data_emails AS data_emails ON (data_emails.name = user_items.name AND data_emails.version = user_items.version)
LEFT JOIN emails AS emails ON (data_emails.email_id = emails.ID)
LEFT JOIN
(SELECT name,version,GROUP_CONCAT( the_ips.ip SEPARATOR ',' ) as ip FROM data_ips
LEFT JOIN ips as the_ips ON data_ips.ip_id = the_ips.ID )
x ON (x.name = data.name AND x.version = user_items.version)
I have done loads of reading to get to this point and worked tirelessly to get here.
This works as I require - this question seeks to clarify what are the benefits of using this instead?
I have had to use a subquery (I believe?) to get the ips as previously it was multiplying results (I believe based on the complex joins). How this subquery works I suppose is my main confusion.
Summary of questions.
-Is my database setup well setup for my usage? Any improvements would be appreciated. And any useful resources to help me expand my knowledge would be great.
-How does the subquery in my sql actually work - what is the query doing?
-Am i correct to keep using left joins - I want to return the user item, and null values if applicable to the right.
-Am I essentially replacing a potentially infinite number of queries with 2? Does this make a REAL difference? Can the above be improved?
-Given that when i update a version of an item in my data table i know have to update the version in the user_items table, I now have a few more update queries to do. Is the tradeoff off of this setup in practice worthwhile?
Thanks to anyone who contributes to helping me get a better grasp of this !!
Given your data layout, and your objective, the query is correct. If you've only got a small amount of data it shouldn't be a performance problem - that will change quickly as the amount of data grows. However when you ave a large amount of data there are very few circumstances where you should ever see all your data in one go, implying that the results will be filtered in some way. Exactly how they are filtered has a huge impact on the structure of the query.
How does the subquery in my sql actually work
Currently it doesn't work properly - there is no GROUP BY
Is the tradeoff off of this setup in practice worthwhile?
No - it implies that your schema is too normalized.
I am thinking of returning a randomly ordered SQL response where the results are mixed up randomly, with a limit.
The thing is I need All the rows back, basically divided into groups (chunks of rows). I hope I am clear.
For example, from table A:
ID | NAME | PROFESSION
++++++++++++++++++++++++++++++++
1 | Jack | Carpenter
2 | Rob | Manager
3 | Phil | Driver
4 | Mary | Cook
5 | Tim | Postman
6 | Bob | Programmer
The query would return something like this:
With a limit of 0,2:
6 | Bob | Programmer
4 | Mary | Cook
With a limit of 2,2:
1 | Jack | Carpenter
5 | Tim | Postman
With a limit of 4,2:
3 | Phil | Driver
2 | Rob | Manager
Note: all the table rows were returned. In my page I need to have a << >> buttons that will show the user the needed "group"s of data.
How do I go about writing such a query ?
A better name for your explained problem would be randomly shuffled records. That is true that the order is random but since the order needs to be remembered, you have no choice but to save it in a column. You can do this by saving a randomly populated field and ordering your records based on that. This way you have ordered your records in no specific order while the order is remembered for future select queries. And whenever you got tired of the order, you can update the mentioned field with new randomly generated values to shuffle them again. This is the technique used by players to shuffle a playlist without replaying a song twice.
[EDIT]
While the first given solution stands as the general answer, there's a hack you can use in MySQL to randomly order records. In this way, all you need to store for remembering an order is its seed.
SELECT * FROM tbl ORDER BY RAND(s);
For instance, if you want each user see the records in some different randomly ordered, you can use their user_id as the seed. This way the order each user will ever see the records in, will remain the same while it is random and different from other users.
I can think of two things here:
If the data in the table is huge, add a column that tells the group to which a row belongs. When the user clicks on >> or << buttons, get the rows for that particular group.
If you are dealing with small amount of data, you could do this in the code itself.
If you use ORDER BY RAND() then you will have to flag selected records somewhere which is no advisable.
You can use some intelligent algorithm with combination of total_pages and ID e.g.
SELECT *
FROM my_table
ORDER BY MOD(ID, total_pages);
Add a column to the table called something like random_col
Then each time you need to randomise the table you run
UPDATE table SET random_col = RAND()
And now each time you want to retrieve results you run a normal select
SELECT * FROM table ORDER BY random_col ASC LIMIT x,y
And the results will appear in the same order until you randomise them again by running the 'UPDATE'
I have two simple Mysql tables:
SYMBOL
| id | symbol |
(INT(primary) - varchar)
PRICE
| id | id_symbol | date | price |
(INT(primary), INT(index), date, double)
I have to pass two symbols to get something like:
DATE A B
2001-01-01 | 100.25 | 25.26
2001-01-02 | 100.23 | 25.25
2001-01-03 | 100.24 | 25.24
2001-01-04 | 100.25 | 25.26
2001-01-05 | 100.26 | 25.28
2001-01-06 | 100.27 | 30.29
Where A and B are the symbols i need to search and the date is the date of the prices. (because i need the same date to compare symbol)
If one symbol doesn't have a date that has the other I have to jump it. I only need to retrive the last N prices of those symbols.
ORDER: from the earliest date to latest (example the last 100 prices of both)
How could I implement this query?
Thank you
Implementing these steps should bring you the desired result:
Get dates and prices for symbol A. (Inner join PRICE with SYMBOL to obtain the necessary rows.)
Similarly get dates and prices for symbol B.
Inner join the two result sets on the date column and pull the price from the first result set as the A column and the other one as B.
This should be simple if you know how to join tables.
I think you should update your question to resolve any of the mistakes you made in representing your data. I'm having a hard time following the details. However, I think based on what I am seeing there are four MySQL concepts you need to solve your problem.
The first is JOINS you would use a join to put two tables together so you may select related data using the key that you describe as "id_symbol"
The second would be to use LIMIT which will allow you to specify the number of records to return such as that if you wanted one record you would use the keywould LIMIT 1 or if you wanted a hundred records LIMIT 100
The third would be to use a WHERE clause to allow you to search for a specific value in one of your fields from the table you are querying.
The last is the ORDER BY which will allow you to specify a field to sort your returned records and the direction you want them sorted ASC or DESC
An Example:
SELECT *
FROM table1
JOIN table2 ON table1.id = table2.table1_id
WHERE table1.searchfield = 'search string'
LIMIT 100
ORDER BY table1.orderfield DESC
(This is pseudo code so this query may not actually work but is close and should provide you with the correct idea.)
I suggest referencing the MySQL documentation found here it should provide everything you need to keep going.