Mysql randomly queries several data entries

Mysql randomly queries several pieces of data. In mysql, You can query five non-Duplicate pieces of data BY using the following: SELECT * FROM 'table' order by rand () LIMIT 5. However, the test results show that the efficiency is very low. It takes more than 8 seconds to search Google for more than 5 pieces of data in a database with more than 0.15 million pieces of data. On the Internet, it is basically a query of max (id) * rand () to obtain random 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; 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. On the mysql Forum, someone uses SELECT * FROM 'table' WHERE id> = (select floor (MAX (id) * RAND ()) FROM 'table') order by id LIMIT 1; I tested it and it took 0.5 seconds. The speed was good, but there was a big gap with the above statements. I always feel that something is abnormal. So I changed the statement. SELECT * FROM 'table' WHERE id> = (SELECT floor (RAND () * (select max (id) FROM 'table') order by id LIMIT 1; next, the query efficiency is improved. The query time is only 0.01 seconds, and then the statement is improved with 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: 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 checked separately. For 10 queries, the former takes 0.147433 seconds, and the latter takes 0.015130 seconds. It seems that the JOIN syntax is much more efficient than using functions directly in the WHERE clause.