I have a message id which i get when i search a text across all my chats. Once i click on it I go into that chat and now I want the messages that where around that message i.e 10 messages before and after. How do i get this limit done
The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement.
With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return.
SELECT * FROM tbl LIMIT 1,10; # Retrieve rows 1-10
You would have to know how many rows there are in total messages to implement correctly in your case
The alternative is using the limit with offset Define OFFSET for the query. For example
SELECT column FROM table LIMIT 10 OFFSET 10;
You could work out the total records with a server side language and then divide by the offset to work out how many iterations queries you need to perform
...
An alternative is to get all the records and page it with client side language like JS using something like pagination or a similar technique
Related
I want to achieve the below 2 scenarios in a single query.
(Note- This is just for reference, the actual query is different)
1. SELECT * FROM CUSTOMER.CUSTOMER LIMIT :startingRow, rowsCount; //WITH LIMIT
2. SELECT * FROM CUSTOMER.CUSTOMER; // NO LIMIT
Is it possible to write a single conditional query for this?
If I pass starting row and rows count params it should go for 1st condition and if no input params are passed, it should give me all records from a table.
The MySQL manual gives a tip for this:
https://dev.mysql.com/doc/refman/en/select.html
To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95,18446744073709551615;
The very large value used in this example is 264-1, or the greatest value of BIGINT UNSIGNED. There are certainly a smaller number of rows in your table.
In your case, you could use 0 as the default offset and a very large value like that as the default limit.
Speaking for myself, I would just run two different queries. One with a LIMIT clause, and the other with no LIMIT clause. Use some kind of if/then/else structure in your client code to determine which query to run, based on whether the function has specified the limit parameters or not.
My iPhone application connects to my PHP web service to retrieve data from a MySQL database, a request can return up to 500 results.
What is the best way to implement paging and retrieve 20 items at a time?
Let's say I receive the first 20 entries from my database, how can I now request the next 20 entries?
From the MySQL documentation:
The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must both be nonnegative integer constants (except when using prepared statements).
With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1):
SELECT * FROM tbl LIMIT 5,10; # Retrieve rows 6-15
To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95,18446744073709551615;
With one argument, the value specifies the number of rows to return from the beginning of the result set:
SELECT * FROM tbl LIMIT 5; # Retrieve first 5 rows
In other words, LIMIT row_count is equivalent to LIMIT 0, row_count.
For 500 records efficiency is probably not an issue, but if you have millions of records then it can be advantageous to use a WHERE clause to select the next page:
SELECT *
FROM yourtable
WHERE id > 234374
ORDER BY id
LIMIT 20
The "234374" here is the id of the last record from the prevous page you viewed.
This will enable an index on id to be used to find the first record. If you use LIMIT offset, 20 you could find that it gets slower and slower as you page towards the end. As I said, it probably won't matter if you have only 200 records, but it can make a difference with larger result sets.
Another advantage of this approach is that if the data changes between the calls you won't miss records or get a repeated record. This is because adding or removing a row means that the offset of all the rows after it changes. In your case it's probably not important - I guess your pool of adverts doesn't change too often and anyway no-one would notice if they get the same ad twice in a row - but if you're looking for the "best way" then this is another thing to keep in mind when choosing which approach to use.
If you do wish to use LIMIT with an offset (and this is necessary if a user navigates directly to page 10000 instead of paging through pages one by one) then you could read this article about late row lookups to improve performance of LIMIT with a large offset.
Define OFFSET for the query. For example
page 1 - (records 01-10): offset = 0, limit=10;
page 2 - (records 11-20) offset = 10, limit =10;
and use the following query :
SELECT column FROM table LIMIT {someLimit} OFFSET {someOffset};
example for page 2:
SELECT column FROM table
LIMIT 10 OFFSET 10;
There's literature about it:
Optimized Pagination using MySQL, making the difference between counting the total amount of rows, and pagination.
Efficient Pagination Using MySQL, by Yahoo Inc. in the Percona Performance Conference 2009. The Percona MySQL team provides it also as a Youtube video: Efficient Pagination Using MySQL (video),
The main problem happens with the usage of large OFFSETs. They avoid using OFFSET with a variety of techniques, ranging from id range selections in the WHERE clause, to some kind of caching or pre-computing pages.
There are suggested solutions at Use the INDEX, Luke:
"Paging Through Results".
"Pagination done the right way".
This tutorial shows a great way to do pagination.
Efficient Pagination Using MySQL
In short, avoid to use OFFSET or large LIMIT
you can also do
SELECT SQL_CALC_FOUND_ROWS * FROM tbl limit 0, 20
The row count of the select statement (without the limit) is captured in the same select statement so that you don't need to query the table size again.
You get the row count using SELECT FOUND_ROWS();
Query 1: SELECT * FROM yourtable WHERE id > 0 ORDER BY id LIMIT 500
Query 2: SELECT * FROM tbl LIMIT 0,500;
Query 1 run faster with small or medium records, if number of records equal 5,000 or higher, the result are similar.
Result for 500 records:
Query1 take 9.9999904632568 milliseconds
Query2 take 19.999980926514 milliseconds
Result for 8,000 records:
Query1 take 129.99987602234 milliseconds
Query2 take 160.00008583069 milliseconds
Here's how I'm solving this problem using node.js and a MySQL database.
First, lets declare our variables!
const
Key = payload.Key,
NumberToShowPerPage = payload.NumberToShowPerPage,
Offset = payload.PageNumber * NumberToShowPerPage;
NumberToShowPerPage is obvious, but the offset is the page number.
Now the SQL query...
pool.query("SELECT * FROM TableName WHERE Key = ? ORDER BY CreatedDate DESC LIMIT ? OFFSET ?", [Key, NumberToShowPerPage, Offset], (err, rows, fields) => {}));
I'll break this down a bit.
Pool, is a pool of MySQL connections. It comes from mysql node package module. You can create a connection pool using mysql.createPool.
The ?s are replaced by the variables in the array [PageKey, NumberToShow, Offset] in sequential order. This is done to prevent SQL injection.
See at the end were the () => {} is? That's an arrow function. Whatever you want to do with the data, put that logic between the braces.
Key = ? is something I'm using to select a certain foreign key. You would likely remove that if you don't use foreign key constraints.
Hope this helps.
If you are wanting to do this in a stored procedure you can try this
SELECT * FROM tbl limit 0, 20.
Unfortunately using formulas doesn't work so you can you execute a prepared statement or just give the begin and end values to the procedure.
I'm using MySQL to store financial stuff, and using the data to build, among other things, registers of all the transactions for each account. For performance reasons - and to keep the user from being overwhelmed by a gargantuan table - I paginate the results.
Now, as part of the register, I display a running balance for the account. So if I'm displaying 20 transactions per page, and I'm displaying the second page, I use the data as follows:
Transactions 0 - 19: Ignore them - they're more recent than the page being looked at.
Transactions 20 - 39: Select everything from these - they'll be displayed.
Transactions 40 - ??: Sum the amounts from these so the running balance is accurate.
It's that last one that's annoying me. It's easy to select the first 40 transactions using a LIMIT clause, but is there something comparable for everything but the first 40? Something like "LIMIT -40"?
I know I can do this with a COUNT and a little math, but the actual query is a bit ugly (multiple JOINs and GROUP BYs), so I'd rather issue it as few times as possible. And this seems useful enough to be included in SQL - and I just don't know about it. Does anybody else?
The documentation says:
The LIMIT clause can be used to constrain the number of rows returned
by the SELECT statement. LIMIT takes one or two numeric arguments,
which must both be nonnegative integer constants, with these
exceptions:
Within prepared statements, LIMIT parameters can be specified
using ? placeholder markers.
Within stored programs, LIMIT parameters can be specified using
integer-valued routine parameters or local variables as of MySQL
5.5.6.
With two arguments, the first argument specifies the offset of the
first row to return, and the second specifies the maximum number of
rows to return. The offset of the initial row is 0 (not 1):
SELECT * FROM tbl LIMIT 5,10; # Retrieve rows 6-15
To retrieve all rows from a certain offset up to the end of the result
set, you can use some large number for the second parameter. This
statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95,18446744073709551615;
Next time, please use the documentation as your first port of call.
You can hack it this way:
select sel.*
from
(
SELECT #rownum:=#rownum+1 rownum, t.*
FROM (SELECT #rownum:=0) r, YourTableOrYourSubSelect t
) sel
where rownum > 40
It's kinda like having Oracle's rownum in MySQL.
Good Afternoon
Please can someone help me, I’m nearly a total noob. I have a very simple DB which has thousands of rows and very few columns. I have an ID, Name, Image, Information, and Date Added. Really basic!
Now I’m trying to display only a single row of data at a time so there is no need for loops and things in this request. Sounds very simple in theory?.
I can display a row in date order, and by the most recent or oldest, ascending or descending. But I want to be able to display for example: =
The 6th newest entry. And perhaps somewhere else on my sites the 16 most recent entry and so on. This could even be the 1232 most recent entry.
Sounds to me like it would be a common task but I can’t find the answer anywhere. Can someone provide me with the very short command for doing this? I probably missing something really daft and fundamental.
Thanks
Leah
The LIMIT clause can be used to constrain the number of rows returned by
the SELECT statement. LIMIT takes one or two numeric arguments, which
must both be nonnegative integer constants (except when using prepared
statements).
With two arguments, the first argument specifies the offset of the first
row to return, and the second specifies the maximum number of rows to
return. The offset of the initial row is 0 (not 1):
SELECT * FROM tbl LIMIT 5,10; # Retrieve rows 6-15
http://dev.mysql.com/doc/refman/5.1/en/select.html
So if you want the 1232nd row from your table you can something like this:
SELECT * FROM tbl ORDER BY date_added LIMIT 1231,1;
In your query use LIMIT e.g.
LIMIT 6,1 // Starts at row 6 and retrieves one result.
My iPhone application connects to my PHP web service to retrieve data from a MySQL database, a request can return up to 500 results.
What is the best way to implement paging and retrieve 20 items at a time?
Let's say I receive the first 20 entries from my database, how can I now request the next 20 entries?
From the MySQL documentation:
The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must both be nonnegative integer constants (except when using prepared statements).
With two arguments, the first argument specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1):
SELECT * FROM tbl LIMIT 5,10; # Retrieve rows 6-15
To retrieve all rows from a certain offset up to the end of the result set, you can use some large number for the second parameter. This statement retrieves all rows from the 96th row to the last:
SELECT * FROM tbl LIMIT 95,18446744073709551615;
With one argument, the value specifies the number of rows to return from the beginning of the result set:
SELECT * FROM tbl LIMIT 5; # Retrieve first 5 rows
In other words, LIMIT row_count is equivalent to LIMIT 0, row_count.
For 500 records efficiency is probably not an issue, but if you have millions of records then it can be advantageous to use a WHERE clause to select the next page:
SELECT *
FROM yourtable
WHERE id > 234374
ORDER BY id
LIMIT 20
The "234374" here is the id of the last record from the prevous page you viewed.
This will enable an index on id to be used to find the first record. If you use LIMIT offset, 20 you could find that it gets slower and slower as you page towards the end. As I said, it probably won't matter if you have only 200 records, but it can make a difference with larger result sets.
Another advantage of this approach is that if the data changes between the calls you won't miss records or get a repeated record. This is because adding or removing a row means that the offset of all the rows after it changes. In your case it's probably not important - I guess your pool of adverts doesn't change too often and anyway no-one would notice if they get the same ad twice in a row - but if you're looking for the "best way" then this is another thing to keep in mind when choosing which approach to use.
If you do wish to use LIMIT with an offset (and this is necessary if a user navigates directly to page 10000 instead of paging through pages one by one) then you could read this article about late row lookups to improve performance of LIMIT with a large offset.
Define OFFSET for the query. For example
page 1 - (records 01-10): offset = 0, limit=10;
page 2 - (records 11-20) offset = 10, limit =10;
and use the following query :
SELECT column FROM table LIMIT {someLimit} OFFSET {someOffset};
example for page 2:
SELECT column FROM table
LIMIT 10 OFFSET 10;
There's literature about it:
Optimized Pagination using MySQL, making the difference between counting the total amount of rows, and pagination.
Efficient Pagination Using MySQL, by Yahoo Inc. in the Percona Performance Conference 2009. The Percona MySQL team provides it also as a Youtube video: Efficient Pagination Using MySQL (video),
The main problem happens with the usage of large OFFSETs. They avoid using OFFSET with a variety of techniques, ranging from id range selections in the WHERE clause, to some kind of caching or pre-computing pages.
There are suggested solutions at Use the INDEX, Luke:
"Paging Through Results".
"Pagination done the right way".
This tutorial shows a great way to do pagination.
Efficient Pagination Using MySQL
In short, avoid to use OFFSET or large LIMIT
you can also do
SELECT SQL_CALC_FOUND_ROWS * FROM tbl limit 0, 20
The row count of the select statement (without the limit) is captured in the same select statement so that you don't need to query the table size again.
You get the row count using SELECT FOUND_ROWS();
Query 1: SELECT * FROM yourtable WHERE id > 0 ORDER BY id LIMIT 500
Query 2: SELECT * FROM tbl LIMIT 0,500;
Query 1 run faster with small or medium records, if number of records equal 5,000 or higher, the result are similar.
Result for 500 records:
Query1 take 9.9999904632568 milliseconds
Query2 take 19.999980926514 milliseconds
Result for 8,000 records:
Query1 take 129.99987602234 milliseconds
Query2 take 160.00008583069 milliseconds
Here's how I'm solving this problem using node.js and a MySQL database.
First, lets declare our variables!
const
Key = payload.Key,
NumberToShowPerPage = payload.NumberToShowPerPage,
Offset = payload.PageNumber * NumberToShowPerPage;
NumberToShowPerPage is obvious, but the offset is the page number.
Now the SQL query...
pool.query("SELECT * FROM TableName WHERE Key = ? ORDER BY CreatedDate DESC LIMIT ? OFFSET ?", [Key, NumberToShowPerPage, Offset], (err, rows, fields) => {}));
I'll break this down a bit.
Pool, is a pool of MySQL connections. It comes from mysql node package module. You can create a connection pool using mysql.createPool.
The ?s are replaced by the variables in the array [PageKey, NumberToShow, Offset] in sequential order. This is done to prevent SQL injection.
See at the end were the () => {} is? That's an arrow function. Whatever you want to do with the data, put that logic between the braces.
Key = ? is something I'm using to select a certain foreign key. You would likely remove that if you don't use foreign key constraints.
Hope this helps.
If you are wanting to do this in a stored procedure you can try this
SELECT * FROM tbl limit 0, 20.
Unfortunately using formulas doesn't work so you can you execute a prepared statement or just give the begin and end values to the procedure.