Question

The problem

I’m running a query using Workbench 5.2.35 and a MySQL server 5.5 and I have the error “Error Code: 2013. Lost connection to MySQL server during query” after 600.516 seconds also after little changes in the query. The query has two roles:

1. select a particular type of records characterised by having ‘value1’ in ‘col1’ (pass from Stage A to Stage B)

2. remove the records where the value in ‘col2’ is the same as the value in ‘col2’ of the next result (pass from Stage B to Stage C)

   Stage A             Stage B             Stage C
   ***************     ***************     ***************
   *ID *col1*col2*     *ID *col1*col2*     *ID *col1*col2*
   ***************     ***************     ***************
   *1  * A  * a  *     *3  * C  * a  *     *3  * C  * a  *
   *2  * B  * a  *     *7  * C  * f  *     *7  * C  * f  *
   *3  * C  * a  *     *8  * C  * f  *     *16 * C  * b  *
   *4  * S  * a  *     *9  * C  * f  *     *18 * C  * c  *
   *5  * B  * a  *     *16 * C  * b  *
   *6  * A  * g  *     *17 * C  * b  *
   *7  * C  * f  *     *18 * C  * c  *
   *8  * C  * f  *
   *9  * C  * f  *
   *10 * A  * f  *
   *11 * B  * f  *
   *12 * D  * f  *
   *13 * S  * f  *
   *14 * F  * f  *
   *15 * F  * f  *
   *16 * C  * b  *
   *17 * C  * b  *
   *18 * C  * c  *
   

and is a generalisation of:
MySQL, select rows where a parameter value depends on the value that it has in a different row

The query is:

SELECT t.id, t.col2, t.col3, t.col4, t.col5 FROM tablename t
WHERE t.id < 1000000
    AND t.col1 = 'value1' 
    AND t.col2 <> 
    (SELECT col2 FROM tablename
        WHERE col1 = 'value1' 
        AND id > t.id 
        LIMIT 1);

Reason for the error

Now, from this post
https://serverfault.com/questions/29597/what-does-mysql-error-2013-mean
the causes of this error can be:

1. Someone KILLed the query
2. Network problems caused the connection to die
3. The server crashed/died
4. Your connection was idle for wait_timeout and was killed
5. The client wasn’t pulling data fast enough for net_wait_timeout and was killed

but since the query stops at 600.516 seconds, I guess that the problem in this case is number 4 (timeout).

Possible problems and solutions

The first idea would be to increase the wait_timeout time, but I think that this is triggered by a previous error: the query doesn’t return anything but continues to run. The limit t.id < 1000000 is used exactly to test the query in a reasonably limited subset (the database has about 200 millions of entries)
So, I’m thinking that there is some problem in the query, in particular in the passage between Stage B and Stage C (the previous step is trivial)

Any idea for the error or for the query will be much appreciated.

Thanks

---

The solution

this is the working code, inspired by the best answer. The command DISTINCT work, but finally I used GROUP BY and ORDER BY in order to present the results in a better way.

SELECT id, col1, col2, ..., coln FROM tablename
    WHERE col1 = 'value1' 
    AND col2 = 'value2'
    ... 
    AND coln = 'valuen'
    GROUP BY col2
    ORDER BY id;

Answer 1

SELECT DISTINCT Par FROM table_name

http://www.w3schools.com/sql/sql_distinct.asp