Question

Let’s say I had a table full of records that I wanted to pull random records from. However, I want certain rows in that table to appear more often than others (and which ones vary by user). What’s the best way to go about this, using SQL?

The only way I can think of is to create a temporary table, fill it with the rows I want to be more common, and then pad it with other randomly selected rows from the table. Is there a better way?

Answer 1

One way I can think of is to create another column in the table which is a rolling sum of your weights, then pull your records by generating a random number between 0 and the total of all your weights, and pull the row with the highest rolling sum value less than the random number.

For example, if you had four rows with the following weights:

+---+--------+------------+
|row| weight | rollingsum |
+---+--------+------------+
| a |      3 |          3 |
| b |      3 |          6 |
| c |      4 |         10 |
| d |      1 |         11 |  
+---+--------+------------+

Then, choose a random number n between 0 and 11, inclusive, and return row a if 0<=n<3, b if 3<=n<6, and so on.

Here are some links on generating rolling sums:

http://dev.mysql.com/tech-resources/articles/rolling_sums_in_mysql.html

http://dev.mysql.com/tech-resources/articles/rolling_sums_in_mysql_followup.html