Label:MySQL random query: MySQL Order by Rand () efficiency problem has always been a developer's frequently asked questions, we are not DBA, not so cow B, can only slowly study, recently due to project issues, need to probably study the MySQL random extraction implementation method To randomly extract a record from the TableName table, the general notation is: SELECT * from TableName ORDER by RAND () LIMIT
In SQL Server, random functions have rand (), NewID (), where Rand is randomly fetched in 0 to 1, and NewID is a random uniqueidentifier unique identifier.
SELECT * from Northwind.. Orders by NEWID ()--Random sort
SELECT Top * from Northwind.. Orders by NEWID ()--Randomly remove 10 records from the Orders table
Example
A. Using NEWID functions on variables
The following example uses NEWID () to assign
Recently, because of the need to study the MySQL random extraction implementation method.For example, to randomly extract a record from the TableName table, the general way to write isSELECT * FROM TableName ORDER by RAND () LIMIT 1But then I looked at the official MySQL manual, which was given to Rand () with the following hints:You can ' t use a column with RAND
Random numbers of computers are generated by pseudo random numbers, which are produced by the small m polynomial sequence, in which each small sequence has an initial value, i.e. random seed. (Note: The cycle of a small m polynomial sequence is 65535, that is, the cycle of random numbers generated by a random seed each time is 65535, and they recur when you get 65,535 random numbers.) )
We know that the rand () function can be used to generate ran
Recently, we have studied the implementation of MYSQL random extraction. For example, to randomly extract a record FROM the tablename table, the general syntax is: SELECT * FROM tablename order by rand () LIMIT 1.However, I checked the MYSQL official manual. The prompt for RAND () indicates that the RAND () function cannot be used in the ORDER BY clause, this wil
Srand ((unsigned) time (NULL)) uses the value of the system Timer/counter as a random seed. Each seed corresponds to a set of pre-generated random numbers based on the algorithm, so, in the same platform environment, the random number generated at different times will be different, correspondingly, if Srand (unsigned) time (NULL) is changed to Srand (TP) (TP is any constant), then whenever run, The number of "random numbers" that are run is a fixed set of sequences, so the random number generate
-----------------------------------------Principle
In C language, the pseudo-random number generation algorithm actually uses the "linear same remainder method ". The specific calculation is as follows:
I = (Xi-1 * A + C) mod m
Where a, c, and m are constants (usually the prime number ). When C = 0, it is called the multiplication same method. The concept of seed is introduced. It will be substituted into the above formula as x0, and each time the rand
Inin the SQL language, the random function newid and rand () are not used in the same way, the following will give you a sample analysis of the differences between these two random functions, for your reference, you want to understand the SQL function can play a role. Example: SELECT * from TABLENAMEORDER by NEWID () --Random sort select TOP * from TABLENAME ORDER by NEWID ()--take 10 records randomly from the TABLENAME table example: use NEWID funct
Use Rand () in MySQL to generate random numbers with random numbers and random name strings.
Use Rand () in MySQL to generate random numbers with random numbers and random name strings.
Preface:The RAND function returns a random floating point value ranging from 0 to 1.0. If an integer parameter N is specified, it is used as a seed value.
1. A friend's quest
I always thought that MySQL would randomly query several pieces of data.
Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->Select
*
From
'
Table
'
Order
By
Rand
() Limit
5
You can.However, the test results show that the efficiency is very low. It takes more than 8 seconds to query 5 data records in a database with more than 0.15 million entries
According to the official manual,
Obtain two random records from table A and use select top 10 * From ywle order by newid ()Order by is generally sorted by a certain field. The return value of newid () is uniqueidentifier. How does order by newid () randomly select records?Newid () generates a value when scanning each record, and the generated value is random, with no case sequence. so the final result is sorted by this sort, and the sorting result is of course unordered.OrSelect top 10 *, newid () as random from ywle where ywle
1. In C ++, the random () function cannot be used as a non-ansi c standard and cannot be compiled by GCC, Vc, or other compilers. You can use the rand function in C ++. 1. The C ++ standard function library provides a random number generator Rand, which returns a pseudo random integer evenly distributed between 0 and rand_max. Rand_max must be 32767 at least. The rand
Using some computer intelligent algorithms (GA, PSO, Ann etc .) during simulation, you often need to randomly generate the initial population (initial sample) to see 1. Generate a real number and an integer between [a, B]
[CPP]View plaincopy
/*
Define the int rand (void) function );
Function Description rand () returns a random value ranging from 0 to rand_max.
Before calling this function to generate
Recently because of the need to study the MySQL random extraction implementation method. For example, to randomly extract a record from the TableName table, the general wording would be: SELECT * from TableName ORDER by RAND () LIMIT 1. However, I later looked at the official MySQL manual, where the hints for rand () probably meant that the rand () function cou
Srand (unsigned) time (NULL) and use and understanding of rand () functions, srandrand
1. First, let's talk about the rand () function.
The rand () function is equivalent to a random number generator. The generated number ranges from 0 to 32767 (Srand (seed) was not called previously)).
So why is the range 0?
0: srand (seed) is not called before the
The rand () function in C + +
Category: Programming languages /C #/articles The generation of random number seeds in C + + is always confusing for beginners. As you know, there's a special Srand (N) function in C that can be easily implemented, but in C + + It's a bit more complicated. The following is the author learn a little bit of experience, I hope we can help. (We still need to use the rand () funct
The code is as follows
Copy Code
SELECT * FROM table_name ORDER BY RAND () LIMIT 5;
That's what Rand said in the handbook:RAND ()RAND (N)Returns a random floating-point value in the range 0 through 1.0. If an integer parameter n is specified, it is used as a seed value.
code is as follows
copy code
mysql>
Label:PHP MySQL uses the RAND function to fetch records randomly How to use random numbers in MySQL, how to write a statement can update hundreds of MySQL data! Need to test MySQL database, there is a database of tens of thousands of data, how to write a PHP file each time update hundreds of information, I am writing a cycle update a message, so I know with while write can be, if one update is like 100 data change how to write it! The correct answer i
C + + generates a random number using the rand (), Srand () function, which summarizes two function features and uses.1. Rand ()#include 2. Srand ()#include 3. Formula for generating a certain range of random numbersGets the random integer [A, b], using (rand ()% (b-a)) + A;Gets a random integer [A, b], using (rand ()%
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