To query five non-duplicate data records in mysql, use the following:
Copy codeThe Code is as follows:
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
Search for Google. Basically, data is randomly obtained by querying max (id) * rand () on the Internet.
Copy codeThe Code is as follows:
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;
However, five consecutive records are generated. The solution is to query only one item at a time and query five times. Even so, it is worthwhile because it takes less than 0.15 million seconds to query 0.01 tables.
The preceding statement uses JOIN, Which is used on the mysql forum.
Copy codeThe Code is as follows:
SELECT *
FROM 'table'
WHERE id> = (select floor (MAX (id) * RAND () FROM 'table ')
Order by id LIMIT 1;
I tested it. It took 0.5 seconds and the speed was good, but there was still a big gap with the above statements. I always feel that something is abnormal.
So I changed the statement.
Copy codeThe Code is as follows:
SELECT * FROM 'table'
WHERE id> = (SELECT floor (RAND () * (select max (id) FROM 'table ')))
Order by id LIMIT 1;
The query efficiency is improved, and the query time is only 0.01 seconds.
Finally, complete the statement and add the MIN (id) judgment. At the beginning of the test, because I did not add the MIN (id) Judgment, half of the time is always the first few rows in the table.
The complete query statement is:
Copy codeThe Code is as follows:
SELECT * FROM 'table'
WHERE id> = (SELECT floor (RAND () * (select max (id) FROM 'table')-(select min (id) FROM 'table ')) + (select min (id) FROM 'table ')))
Order by id LIMIT 1;
SELECT *
FROM 'table' AS t1 JOIN (select round (RAND () * (select max (id) FROM 'table')-(select min (id) FROM 'table ')) + (select min (id) FROM 'table') AS id) AS t2
WHERE t1.id> = t2.id
Order by t1.id LIMIT 1;
Finally, the two statements are queried 10 times respectively,
The former takes 0.147433 seconds.
The latter takes 0.015130 seconds.
It seems that using the JOIN syntax is much more efficient than using functions directly in the WHERE clause.