Sql Statement cutting Rows in half

Question

Here is my current sql query

        SELECT 
    w.city, t.tmc, t.date, t.time, w.event, 
    ROUND(AVG(w.Pave), 0) surface_temperature, 
    ROUND(AVG(w.Air), 0) air_temperature, 
    ROUND(AVG(a.material_rate),0) material_rate, 
    w.intensity, t.avg 
    FROM winter2010.traffictest t 
    LEFT OUTER JOIN winter2010.application a ON a.DATE = t.DATE AND a.tmc = t.tmc AND a.tmc = t.tmc AND Left(a.time,2) = Left(t.time, 2) 
    LEFT OUTER JOIN winter2010.weather w ON t.DATE = w.DATE AND t.tmc = w.tmc AND  Left(t.time, 2) =  Left(w.time, 2) 
    GROUP BY tmc, t.time 
    INTO OUTFILE 'c:/sass2.csv' FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' LINES 
TERMINATED BY '\n';

I'm creating this csv file from the following sql statement and it works great except for one thing, its only taking 1/2 the rows!

There are a total of 2661 rows in traffictest And when I outfile the data into the csv its only putting 1331.

I've changed the size of traffictest and this holds true except when I had only 3 rows it actually pulled all three rows.

Any help figuring this out would be much appreciated.

Answer 1

When you use GROUP BY MySql returns only one row per group, You have rows with similar tmc-time and MySql is returning only one row for all such rows that's the reason for decrease in row number.