Use the obsoletes tag in the specfile. This will cause “package-oldname” to be uninstalled before installing “package-renamed”.


Name: package-renamed
Obsoletes: package-oldname


An example of using dd to create a 1GB file.

dd if=/dev/zero of=testfile bs=1M count=1024



SET FOREIGN_KEY_CHECKS = 0;
SELECT @@FOREIGN_KEY_CHECKS;
SET FOREIGN_KEY_CHECKS = 1;


Alexander Stetsenko blog has more info about it.

If you try to open a .eml file in Outlook 2007, it will create a new message containing the .eml file as an attachment.

In order to read .eml files from Outlook 2007, you need to pass the /eml argument to outlook.exe . This Microsoft kb article describes how to tweak the windows registry to accomplish this.

Essentially you need to edit the HKEY_CLASSES_ROOT/Microsoft Internet Mail Message\shell\open\command default value to :
“%ProgramFiles%\Microsoft Office\Office12\OUTLOOK.EXE” /eml “%1″

To revert back to using Outlook Express, set the value of the same key to:
“%ProgramFiles%\Outlook Express\msimn.exe” /eml:%1

A little code snippet to show files ordered by size:

find . -type f -printf "%k %p\n" | sort -rn | head

Not immediately obvious, but if you’re looking for an rpm to install the Apache2::Request module you need perl-libapreq2 (this works in CentOS 5 and I imagine others as well).

yum install perl-libapreq2

List available repositores
yum repolist
Search by package name (case insensitive)
yum list "*package*"
List files inside a repository package (regardless if that package has been installed)
repoquery --list <PACKAGE_NAME>

Add the following to the /etc/sudoers file:

<USERNAME> ALL= NOPASSWD: ALL

This is my commonly used rpm command reference

Install/Upgrade a package
rpm -Uvh package.rpm
Install an old version of a package
rpm -Uvh --oldpackage oldpackage.rpm
Query which package installed a given file
rpm -qf /bin/cp
To force a package to overwrite conflicting files
rpm -Uvh --replacefiles package.rpm
To list files owned by an installed package
rpm -ql package
List scripts inside rpm file (pre, post scriptlets)
rpm -qp --scripts /tmp/ods-dsc-1.0-18.13201_centos53.noarch.rpm

I came across an interesting post at Xaprb.com on selecting the first and last row of a group in MySQL.  This solution builds on a comment in that post about using GROUP_CONCAT to achieve this.

As a practical example of the usage of the first and last aggregate functions, consider an application converting financial data between different timeframes.

The table structure might look like this:

CREATE TABLE EURUSD_3600 (
    datetime datetime NOT NULL,
    open float NOT NULL,
    low float NOT NULL,
    high float NOT NULL,
    close float NOT NULL,
    PRIMARY KEY (datetime)
);

Assuming data in the hourly  timeframe (3600 is the number of seconds in one hour):

INSERT INTO EURUSD_3600 SELECT '2001-01-03 00:00:00',0.9507,0.9505,0.9509,0.9506;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 01:00:00',0.9506,0.9492,0.951,0.9496;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 02:00:00',0.9496,0.9495,0.9509,0.9505;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 03:00:00',0.9504,0.9498,0.9508,0.9499;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 04:00:00',0.9499,0.9499,0.9507,0.9503;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 05:00:00',0.9503,0.9503,0.9506,0.9506;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 06:00:00',0.9506,0.9505,0.9507,0.9505;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 07:00:00',0.9505,0.9503,0.9513,0.9509;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 08:00:00',0.951,0.9468,0.951,0.9489;
INSERT INTO EURUSD_3600 SELECT '2001-01-03 09:00:00',0.949,0.9489,0.9543,0.9539;

SQL is a suitable tool to group this data into a more compact timeframe, for instance, the 2-hour timeframe. By using a formatting function on the datetime field, a standard GROUP BY SQL statment can be used to achieve this:

CONCAT(
    year(datetime), '-',
    month(datetime), '-',
    day(datetime), ' ',
    floor(hour(datetime) / 2) * 2, ':00:00'
)

Calculating the low and high of the period is easy, using the standard min/max aggregate functions:


SELECT
    CAST(
        CONCAT(
            year(datetime), '-',
            month(datetime), '-',
            day(datetime), ' ',
            floor(hour(datetime) / 2) * 2, ':00:00'
        ) AS DATETIME) AS datetime,
    MIN(low) AS low,
    MAX(high) AS high
FROM EURUSD_3600
GROUP BY CAST(
        CONCAT(
            year(datetime), '-',
            month(datetime), '-',
            day(datetime), ' ',
            floor(hour(datetime) / 2) * 2, ':00:00'
        ) AS DATETIME)


GROUP_CONCAT can be used for the opening(first) and close(last), the trick is to be sure to use the ORDER BY clause in the group concatenation, otherwise the returned first and last records might not be as expected:


SUBSTRING_INDEX(
GROUP_CONCAT(CAST(open AS CHAR) ORDER BY datetime),
',',
1
) as open

SUBSTRING_INDEX(
GROUP_CONCAT(CAST(close AS CHAR) ORDER BY datetime),
',',
-1
) as close


GROUP_CONCAT returns a comma separated string of all groups values, then SUBSTRING_INDEX is used to return the first and last values of the group. This works as long as data is properly sorted and the resulting string fits in the GROUP_CONCAT buffer.

Note however that there is a problem with the preceding code when calculating the close value. SUBSTRING_INDEX looks from the end of the string, but the GROUP_CONCAT buffer has a limited size, and the it will be truncated if the concatenated rows exceed the buffer size, in which case the returned value will not be the last in the group as desired.

An easy solution is to reverse the sort order and fetch the first element instead:


SUBSTRING_INDEX(
GROUP_CONCAT(CAST(close AS CHAR) ORDER BY datetime DESC),
',',
1
) as close


MySQL will still throw a warning about truncated group_concat when there are too many rows in the source table, but this can be safely ignored for the purpose of this example.

The final query turns out as:


SELECT
    CAST(
        CONCAT(
            year(datetime), '-',
            month(datetime), '-',
            day(datetime), ' ',
            floor(hour(datetime) / 2) * 2, ':00:00'
        ) AS DATETIME) AS datetime,
    SUBSTRING_INDEX(
        GROUP_CONCAT(CAST(open AS CHAR) ORDER BY datetime),
        ',',
        1
    ) as open,
    MIN(low) AS low,
    MAX(high) AS high,
    SUBSTRING_INDEX(
        GROUP_CONCAT(CAST(close AS CHAR) ORDER BY datetime DESC),
        ',',
        1
    ) as close
FROM EURUSD_3600
GROUP BY CAST(
        CONCAT(
            year(datetime), '-',
            month(datetime), '-',
            day(datetime), ' ',
            floor(hour(datetime) / 2) * 2, ':00:00'
        ) AS DATETIME);