MYSQL: Doing a where on a own defined parameter in SELECT

Question

When I do a SELECT statement with an AS definement and i want to filter on that it give the folowing error: #1054 - Unknown column 'TimeInSeconds' in 'where clause'

Code:

Select Id, time/10000000 as TimeInSeconds FROM Results WHERE
TimeInSeconds > 5 AND
TimeInSeconds < 36000

I understand the error that TimeInSeconds is not part of the original table, but are there ways to work with these defined collumns?

Answer 1

You can also use HAVING clause to filter out the results of custom aliases

Select Id, `time`/10000000 as TimeInSeconds 
FROM Results 
HAVING TimeInSeconds > 5 AND TimeInSeconds < 36000

Or repeat the expression using WHERE clause

Select Id, `time`/10000000 as TimeInSeconds 
FROM Results 
WHERE (`time`/10000000) > 5 AND (`time`/10000000) < 36000

Answer 2

One option is to put the results in a subquery:

Select Id, timeinseconds
From (Select Id, time/10000000 as TimeInSeconds FROM Results) t
Where TimeInSeconds > 5 AND TimeInSeconds < 36000

Answer 3

The most efficient way to do this is:

select id, time/10000000 TimeInSeconds
from results
where time > 5 * 10000000
and time < 36000 * 10000000

The reason is the your field is being compared directly to values. The subquery method selects the whole darn table before filtering the results. Doing math on a field in the where clause, like using formulas, tends to be slow.

This is even more significant if the field in question is indexed.