Category: Daily Commute - Standard hints/tips

Useful links

By admin on Feb 16, 2010 | In Daily Commute - Standard hints/tips, Formula 1 - More advanced stuff | Send feedback »

Sorry, I've been away for a while. Hope to write a useful post fairly soon. In the meantime, here are a couple of things I've found out today. To look for useful twitter posts regarding Mysql, search on #Mysql (I know, fairly basic, but I'm not yet a Twitter person).
Second I found mysql commands you may not know
It's in Spanish, and if I get some time I'll translate some of the entries. Very interesting!

Tags: twitter

A quick look at using dates in Mysql

By admin on Jul 24, 2009 | In Daily Commute - Standard hints/tips | Send feedback »

The manipulation of dates in sql is often involved, and Mysql is no exception. I'm going to go over some of the more common requirements when playing with dates in Mysql, but advise that a lot more detail is available in the manual (link :-
date_time_functions_link )

The two main data types you'll probably deploy in Mysql (although there are others) are DATE and DATETIME. If you need to record time of day as well as the date then you'll probably be using DATETIME. If you then want to select records for a certain day accessing a DATETIME field then the function DATE can be very useful:-

Code:

mysql> select dateposted from entries where date(dateposted) = '2008-04-14';

Remember always that when inserting (or selecting from) Mysql DATE fields that the default format is 'YYYY-MM-DD'; when importing from a csv file created by Excel remember to first change the format accordingly on the initial Excel field. Another point (which upsets sql purists) is that if the date value being inserted into a DATETIME field is invalid, often the date will be set to '0000-00-00', rather than the insert being rejected. Here's what the mysql manual used to say on the subject:

The MySQL server only performs basic checking on the validity of a date: days 00-31, months 00-12, years 1000-9999. Any date not within this range will revert to 0000-00-00. Please note that this still allows you to store invalid dates such as 2002-04-31. It allows web applications to store data from a form without further checking. To ensure a date is valid, perform a check in your application.

To show the year part of the date use YEAR(

), to show the month (in digits) use MONTH(), and for the day you can use either DAY() or DAYNAME()

select curdate() will return the current date in yyyy-mm-dd format whereas
select now() will return the current date in a longer datetime format.

For various possibilities in formatting the date output, you can use DATE_FORMAT, for example:

Code:

mysql> select DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y');
+------------------------------------------------+
| DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y') |
+------------------------------------------------+
| Sunday October 2009                            |
+------------------------------------------------+

Other options include %m (month numeric), %b (abbreviated month name), %e (day of the month numeric), %D (day of month with suffix 1st, 2nd etc). Here's another sql example of DATE_FORMAT:

Code:

mysql> select DATE_FORMAT('2009-10-04 22:23:00', '%e %b %y');
+------------------------------------------------+
| DATE_FORMAT('2009-10-04 22:23:00', '%e %b %y') |
+------------------------------------------------+
| 4 Oct 09                                       |
+------------------------------------------------+

The DATE_FORMAT function can be very useful for finding the first day of a month eg:

Code:

SELECT DATE_FORMAT('2004-01-20' ,'%Y-%m-01');

Once you have this, you can find out the last day of the month, first day of following month etc by using DATE_ADD and DATE_SUB (see below).

To find out the date 50 days from now, use the ADDDATE function, normally using format ADDDATE( date , Number_of_days ), as follows:

Code:

mysql> select curdate(), adddate(curdate(), 50);
+------------+------------------------+
| curdate()  | adddate(curdate(), 50) |
+------------+------------------------+
| 2009-07-21 | 2009-09-09             |
+------------+------------------------+

You can use PERIOD_DIFF (p1, p2) to find the number of months between 2 dates p1 and p2, so long as p1 and p2 are in format YYMM or YYYYMM.

Code:

mysql> select period_diff(200907,200901);
+----------------------------+
| period_diff(200907,200901) |
+----------------------------+
|                          6 |
+----------------------------+

select period_diff(date_format(now(), '%Y%m'), date_format(hiredate, '%Y%m')) as months from emp;

It has to be said however, that the normal method of date arithmetics in Mysql is done with the functions date_sub and date_add.

Code:

mysql> select date_sub('2005-01-01 00:05:00', INTERVAL 1 DAY);
+-------------------------------------------------+
| date_sub('2005-01-01 00:05:00', INTERVAL 1 DAY) |
+-------------------------------------------------+
| 2004-12-31 00:05:00                             |
+-------------------------------------------------+

Code:

mysql> select DATE_ADD('2010-12-31 23:58:00', INTERVAL 1 DAY);
+-------------------------------------------------+
| DATE_ADD('2010-12-31 23:58:00', INTERVAL 1 DAY) |
+-------------------------------------------------+
| 2011-01-01 23:58:00                             |
+-------------------------------------------------+
Tags: dates, datetime, excel

Bumper tricks

By admin on Jul 2, 2009 | In Daily Commute - Standard hints/tips, Formula 1 - More advanced stuff | Send feedback »

I'm going to list on this thread, those little tricks and tips that I've come across over time that really help when using Mysql.
I will probably add to this thread as I find out more.

For example, if you're in command line Mysql, you can show the DDL for a table quite easily:

Code:

show create table table_name;

A useful tip when running sql queries (again in command line mode) is to terminate a query with \G switch instead of a semi-colon. This displays records in row format rather than columns

Normally Mysql queries are case insensitive - if you search for a field containing '%king%', the query will bring back results where the field contains 'King', 'kinG' or even 'KING'
To try and force Mysql to be case sensitive use the 'binary' keyword - for example

Code:

Select mainfield from T1Name where binary mainfield like '%King%'

If you are performing date operations on a datetime field, use the date function, for example:

Code:

select dateposted from entries where date(dateposted) = '2008-04-14' \G

Lastly, if you want to break down the number of orders recorded against different countries into discrete groupings of price, the following code is very useful:

Code:

SELECT 5*FLOOR(productprice/5),
  SUM(CASE WHEN cnty = 'UK' THEN 1 ELSE 0 END) as UK,
  SUM(CASE WHEN cnty = 'FR' THEN 1 ELSE 0 END) as FR
FROM orders o
WHERE o.cnty IN ('UK','FR')
GROUP BY 5*FLOOR(productprice/5);

...More tips to follow soon

... and here's one.
If you want to change the order of a sort (perhaps sort by 1, 4, 3 ,2 instead of 1, 2, 3,4) you can use the field function within the Order by clause.
For example

Order By Field(sort_field,1,4,3,2)

Read more about this extremely useful function HERE Of course you can always use the CASE statement in the ORDER BY clause, but it's always good to have a choice!

Sometimes it's good to know the range of values in your table fields - what the maximum value is, the minimum length stored etc - especially when you're working out what kind of index to implement. In this scenario, the procedure analyse is very useful. In fact it examines the result from a query and returns an analysis of the results. To use it append PROCEDURE ANALYSE to the end of a SELECT statement eg:

Code:

select * from orders procedure analyse(0,0)\G

There's a very useful article on using this when
indexing text columns in mysql here

Tags: command line, dates, tips
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