How to Weight Random Selections in MySQL?

Weighting Random Selections in MySQL

When working with large datasets, it may be necessary to select an entry at random while considering the weight associated with each entry. In MySQL, this can be achieved by using a combination of functions and weighting multipliers.

To select a single entry from a table, use the SELECT statement. To randomize the selection, use the RAND() function. However, to introduce weighting, an additional step is required.

Consider a table with entries and a "Multiplier" column that represents their weights. To weight the selection towards entries with higher multipliers, modify the statement with the following syntax:

SELECT *
FROM table_name
ORDER BY -LOG(1.0 - RAND()) / Multiplier
LIMIT 1;

In this expression, the ORDER BY clause sorts the results in ascending order of the calculated value. The value is derived by dividing the negative logarithm of a random number between 0 and 1 by the multiplier. Entries with higher multipliers will have smaller calculated values and appear earlier in the sorted list. The LIMIT 1 clause then selects the top entry as the weighted random selection.

Note that this method is not affected by multiplier values of 0, as a zero multiplier will still result in the entry appearing at the end of the sorted list. If desired, entries with a multiplier of 0 can be excluded from the selection using a WHERE clause:

SELECT *
FROM table_name
WHERE Multiplier > 0
ORDER BY -LOG(1.0 - RAND()) / Multiplier
LIMIT 1;

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