Question

Why does SELECT * FROM TABLE ORDER BY RAND() Work? I thought ORDER BY only works for columns.

So what exactly does it mean to ORDER BY RAND() or ORDER BY SUM()?

Answer 1

ORDER will work with any value you can put in your results (but doesn’t have to be one of the values in the results). This can be a column in any of the source tables or calculated using a function. For example, you could use ORDER UPPER(name) for a case-insensitive sort.

If you ORDER BY RAND() you’re ordering by a random number generated for each row in the results, i.e. returning the rows in a random order. If you’re ordering by SUM() you’ve probably got a GROUP BY in there too so you could order customers by total calculated invoice total for example.

Ideally you want to use a column from an index as this will be much faster.