Surprising timing stats for sql queries

I have two queries whose timing parameters I want to analyze.

The first query is taking much longer than the second one while in my opinion it should be the other way round.Any Explanations?

First query:

select mrn
from EncounterInformation
limit 20;

Second query:

select enc.mrn, fl.fileallocationid
from EncounterInformation as enc inner join 
     FileAllocation as fl
     on enc.encounterIndexId = fl.encounterid
limit 20;

The first query runs in 0.760 seconds on MYSQL while second one runs in 0.509 seconds surprisingly.

Solution

There are many reasons why measured performance between two queries might be different:

The execution plans for the queries (the dominant factor)
The size of the data being returned (perhaps mrn is a string that is really long for the result set in the first query but not the second)
Other database activity, that locks tables and indexes
Other server activity
Data and index caches that are pre-loaded -- either in the database itself or in the underlying OS components

Your observation is correct. The first should be faster than the second. More importantly though is the observation that this simply does not make sense for your simple query:

The first query runs in 0.760 seconds
select mrn
from EncounterInformation
limit 20;

The work done for this would typically be to load one data page (or maybe a handful). That would only consistently take 0.760 seconds if:

You had really slow data storage (think "carrier pigeons").
EncounterInformation is a view and not a table.
You don't understand timings.

If the difference is between 0.760 milliseconds and 0.509 milliseconds, then the difference is really small and likely due to other issues -- warm caches, other activity on the server, other database activity.

It is also possible that you are measuring elapsed time and not database time, so network congestion could potentially be an issue.

If you are querying views, all bets are off without knowing what the views are. In fact, if you care about performance you should be including the execution plan in the question.

I can't explain the difference. What I can say is that your observation is reasonable, but your question lacks lots of information that suggests you need to learn more about how to understand timings. I would suggest that you start with learning about explain.