使用存储过程在MySQL中随机播放表

Question

I want to create a procedure in MySQL that can shuffle all my records inside a table. So far this is what i have now.

/*
    SHUFFLE A TABLE PROCEDURE
*/

DELIMITER $$

USE `nologinsdb`$$

DROP PROCEDURE IF EXISTS `shuffle_record_table`$$

CREATE DEFINER=`root`@`localhost` PROCEDURE `shuffle_record_table`()
BEGIN
    DROP TABLE IF EXISTS record_table2;

    CREATE TABLE record_table2 LIKE record_table;

    INSERT INTO record_table2 SELECT * FROM record_table ORDER BY RAND();

    DROP TABLE record_table;

    RENAME TABLE record_table2 TO record_table;
END$$

DELIMITER ;


CALL shuffle_record_table();

and i don't understand something

SELECT * FROM record_table ORDER BY RAND()

gives me random order, but when i inserting in the record_table2 table it has the same order as the record_table table.

i don't understand.

Thanks.

Answer 1

There is no point to shuffle rows in a table. You can shuffle them while retrieving the data using order by.

The physical order of the rows is of no interest to the user. The storage engine is placing the in the order of the index to optimize the access.

Answer 2

If you use InnoDB tables, they are always stored in primary key order. So if you SELECT from the table without specifying any other ORDER BY , then you'll the rows in that order.

---

Error Code: 1075 Incorrect table definition; there can be only one auto column and it must be defined as a key ==> seems lie i cannot drop primary key, why? – Damian

InnoDB requires that if you have an AUTO_INCREMENT column, it must be the first column in an index. Typically people define it as the primary key index, but if you drop the primary key, you must at least define a different index for it.

Example: create a test table that has an auto-increment primary key.

mysql> create table record_table (id int auto_increment primary key);

mysql> insert into record_table values (1), (2), (3);

It's not allowed to have an auto-increment column that is not part of any key.

mysql> alter table record_table drop primary key;
ERROR 1075 (42000): Incorrect table definition; 
there can be only one auto column and it must be defined as a key

If we drop the primary key but add a secondary key on the same column, this satisfies InnoDB.

mysql> alter table record_table drop primary key, add key (id);
Query OK, 3 rows affected (0.13 sec)

But if we try to make that column have no key, it's also an error.

mysql> alter table record_table drop key id;
ERROR 1075 (42000): Incorrect table definition; 
there can be only one auto column and it must be defined as a key

---

One workaround is to create an extra column in the table and assign random values to it, then you can ORDER BY that column.

mysql> alter table record_table add column sort float;

mysql> update record_table set sort = rand();

mysql> select * from record_table ;
+----+----------+
| id | sort     |
+----+----------+
|  1 | 0.439593 |
|  2 | 0.416936 |
|  3 |   0.7659 |
+----+----------+

mysql> select * from record_table order by sort;
+----+----------+
| id | sort     |
+----+----------+
|  2 | 0.416936 |
|  1 | 0.439593 |
|  3 |   0.7659 |
+----+----------+

You can "reshuffle" the table without having to copy data back and forth, just UPDATE the random values.

mysql> update record_table set sort = rand();

mysql> select * from record_table order by sort;
+----+----------+
| id | sort     |
+----+----------+
|  1 | 0.137319 |
|  3 | 0.329505 |
|  2 | 0.348292 |
+----+----------+

Answer 3

Here http://dev.mysql.com/doc/refman/5.1/en/create-table.html
they say that:

Use LIKE to create an empty table based on the definition of another table, including any column attributes and indexes defined in the original table :

It means, that a primary key is also copied to a new table, and preservers the rows order.
Just drop the primary key in the new table :

CREATE TABLE record_table2 LIKE record_table;
ALTER TABLE record_table2 DROP PRIMARY KEY;
INSERT INTO record_table2 SELECT * FROM record_table ORDER BY RAND();

----- EDIT -----

If the above solution doesn't work since there is an auto_icrement on the primary key column,
then try CREATE TABLE x AS SELECT * FROM y , this doesn't copy indexes and keys:

create table x( 
  x int primary key auto_increment,
  y int );

insert into x( y ) values( 1 ), (3 ), (2),(5),(4);

select * from x;
+ ------ + ------ +
| x      | y      |
+ ------ + ------ +
| 1      | 1      |
| 2      | 3      |
| 3      | 2      |
| 4      | 5      |
| 5      | 4      |
+ ------ + ------ +

5 rows

create table y as select * from x order by rand();

select * from y;

+ ------ + ------ +
| x      | y      |
+ ------ + ------ +
| 2      | 3      |
| 4      | 5      |
| 3      | 2      |
| 1      | 1      |
| 5      | 4      |
+ ------ + ------ +

5 rows

Answer 4

Look i had same question here and the solution has:

CREATE TABLE origintable SELECT * FROM origintable2;
ALTER TABLE origintable2 DROP idfield;
CREATE TABLE origintable3 LIKE origintable2;
INSERT INTO origintable3 SELECT * FROM origintable2 ORDER BY RAND();
DROP TABLE origintable2;
ALTER TABLE origintable3 ADD COLUMN idfield INT NOT NULL AUTO_INCREMENT FIRST
, ADD PRIMARY KEY (idfield);
RENAME TABLE origintable TO origintable_bkp;
RENAME TABLE origintable3 TO origintable;
DROP TABLE origintable3;

Works like a charm! new table, suffle data, same sctructure!

Hope helps somebody