23-07-2006, 02:28
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חבר מתאריך: 03.07.05
הודעות: 284
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ציטוט:
RAND(), RAND(N) Returns a random floating-point value v between 0 and 1 inclusive (that is, in the range 0 <= v <= 1.0). If an integer argument N is specified, it is used as the seed value, which produces a repeatable sequence.
mysql> SELECT RAND();
-> 0.9233482386203
mysql> SELECT RAND(20);
-> 0.15888261251047
mysql> SELECT RAND(20);
-> 0.15888261251047
mysql> SELECT RAND();
-> 0.63553050033332
mysql> SELECT RAND();
-> 0.70100469486881
mysql> SELECT RAND(20);
-> 0.15888261251047
To obtain a random integer R in the range i <= R <= j, use the expression FLOOR(i + RAND() * (j – i). For example, to obtain a random integer in the range of 7 to 12 inclusive, you could use the following statement:
SELECT FLOOR(7 + (RAND() * 5));
You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. However, you can retrieve rows in random order like this:
mysql> SELECT * FROM tbl_name ORDER BY RAND(); ORDER BY RAND() combined with LIMIT is useful for selecting a random sample from a set of rows:
mysql> SELECT * FROM table1, table2 WHERE a=b AND c<d -> ORDER BY RAND() LIMIT 1000; Note that RAND() in a WHERE clause is re-evaluated every time the WHERE is executed.
RAND() is not meant to be a perfect random generator, but instead is a fast way to generate ad hoc random numbers which is portable between platforms for the same MySQL version.
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