Why does RAND sometimes produce multiple results in a MySQL call? - mysql

Question

I have table agent which includes 13 agent IDs and agent names.
agent_id|agent_name|
--------|----------|
1|Jack |
2|Jill |
3|Jo |
...
When I run this query...
select agent_name from agent where agent_id = (FLOOR( 1 + RAND( ) * 13 ))
...MySQL sometimes returns 0 names and sometimes returns many names. Since (FLOOR( 1 + RAND( ) * 13 )) on its own always seems to return a single, non-zero integer, I would expect to get a single name back, but this is not the case. On testing the above query, the following number of names are returned on each execution;
Execution | Total Names Returned
1 | 3
2 | 2
3 | 1
4 | 0
5 | 1
6 | 1
7 | 0
8 | 0
9 | 1
10 | 4
Clearly, when one runs...
select agent_name from agent where agent_id = 3
...the same, single, name is returned each time.
I see that the docs explain that
RAND() in a WHERE clause is evaluated for every row (when selecting from one table) or combination of rows (when selecting from a multiple-table join). Thus, for optimizer purposes, RAND() is not a constant value and cannot be used for index optimizations
I am not sure why this would mean that a single call would return many rows.
If I add LIMIT 1
select * from agent a where agent_id = (FLOOR( 1 + RAND() * 13 )) limit 1
...then the query sometimes returns NULL.
Questions
Why does RAND return different numbers of records?
What is the proper way to return a single random agent?

Answer 1

As the documentation says, it is evaluated for each row. It means, the following:
MySQL gets all rows
For each row, executes RAND() and if RAND() == id, return the row
For example
Get all rows
ID = 1, RAND() = 3, no return
ID = 2, RAND() = 2, return
ID = 3, RAND() = 4, no return
ID = 4, RAND() = 4, return
In this case, you have two results.
If you want to get random agent, maybe better approach would be
SELECT * FROM `agent` ORDER BY RAND() LIMIT 1

Answer 2

I assume that your first query is sometimes returning multiple records because RAND() is being evaluted for each record in the query. If you just want a single random agent, then use:
SELECT agent_name
FROM agent
ORDER BY RAND()
LIMIT 1;
To further explain your current observations, the following expression is being evaluated once per each agent record:
(FLOOR( 1 + RAND( ) * 13 ))
Imagine that for each record, the above happens, by chance, to equal the agent_id. If so, then your first query would return all 13 records. By the way, the documentation link you cited basically says this.