The implementation of MySQL random number

SELECT FLOOR (7 + (RAND () * 6)); Randomly generating a number in a range
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A friend asked how to use random numbers in MySQL

How to write a statement to update hundreds of MySQL data!
Need to test the MySQL database, which has a database of tens of thousands of data, how to write a PHP file every time update hundreds of information, I am writing a cycle to update a message, so I know with a while to write on it, if an update like 100 data changes how to write it!

The correct answer is: UPDATE cdb_posts SET views = rand ();

By the way, I got you some examples of the MySQL rand function, as follows:

In the Insert command, the value () is in Rand (), and note that the width of the field is always thought to be a random query of MySQL data, using
SELECT * from ' table ' ORDER by RAND () LIMIT 5
It's OK.
But the real test was found to be very inefficient. A library of more than 150,000, query 5 data, incredibly more than 8 seconds
View the official manual, also said that Rand () is executed multiple times in the ORDER BY clause, which is naturally inefficient and very low.

You are cannot use a column with RAND () of the clause, because order by would evaluate the column multiple time S.

Search Google, the Internet is basically query Max (ID) * RAND () to randomly obtain data.

SELECT *
From ' table ' as T1 JOIN (select ROUND (RAND () * (SELECT MAX (ID) from ' table ') as ID) as T2
WHERE t1.id >= t2.id
ORDER BY T1.id ASC LIMIT 5;
But this will produce a continuous 5 records. The solution can only be one query at a time, query 5 times. Even so, because of the 150,000 table, the query only needs 0.01 seconds.

The above statement uses a join,mysql forum where someone uses

SELECT *
From ' table '
WHERE ID >= (SELECT FLOOR (MAX (ID) * RAND ()) from ' table '
ORDER by ID LIMIT 1;

I tested it, it takes 0.5 seconds, and the speed is good, but there is still a big gap with the above statement. There is something wrong with the total sleep.

So I rewrote the statement.

SELECT * from ' table '
WHERE ID >= (SELECT Floor (RAND () * (select MAX (ID) from ' table '))
ORDER by ID LIMIT 1;

This, the efficiency is increased, the query time is only 0.01 seconds

Finally, the statement to improve, plus the min (id) judgment. I was at the beginning of the test, because I did not add min (id) judgment, the result is half of the time is always query to the first few lines in the table.
The full query statement is:


SELECT * from ' table '
WHERE ID >= (select floor (ID) from ' table ') + (select min (ID) () () + (()) + (() () + (() () () () + ("()") M ' table '))
ORDER by ID LIMIT 1;


SELECT *
From ' table ' as T1 JOIN (select ROUND (select MAX (ID) from ' table ') (select min (IDs) from ' tables ')) + (select min () d) from ' table ') as ID) as T2
WHERE t1.id >= t2.id
ORDER by t1.id LIMIT 1;

Finally in PHP, the two statements are queried separately 10 times,
The former takes 0.147433 seconds.
The latter takes time 0.015130 seconds
It seems that the syntax for join is much higher than the efficiency of using functions directly in a where.

---------Sql random functions newid () and rand ()-------------
SQL random functions newid () and Rand ()
SQL Server's random functions newid () and Rand ()
SELECT * from Northwind.. Orders by NEWID ()