SELECT COUNT(*), WEEK(mydate)

FROM mytable

GROUP BY WEEK(mydate);

The problem here is, that the output is totally meaningless to me as I do not have a clue what does week 36 mean. What I really want is to have on the screen is a nice, human readable date interval – the beginning and ending date of any given week.

Surprisingly, this turns out the be a major pain in the ass. There is no simple function that will yield a week interval (or start/end date of a week) given a week number. You have to find these dates manually. Here is how I did it:

SELECT COUNT(*) AS reports_in_week, DATE_ADD(mydate, INTERVAL( 1-DAYOFWEEK(mydate) ) DAY), DATE_ADD(mydate, INTERVAL( 7-DAYOFWEEK(mydate) ) DAY)

FROM mytable

GROUP BY WEEK(mydate)

How does it work? The DAYOFWEEK() function returns an integer ranging from 1 (Sunday) to 7 (Saturday). So if mydate happens to be Tuesday we get the following statements:

DATE_ADD(mydate, INTERVAL -2 DAY)

which essentially means "subtract 2 days from mydate (which is that week’s Sunday) and also:

DATE_ADD(mydate, INTERVAL 4 DAY)

which yields the date of that week’s Friday.

Just today I wanted to see who in our company does not have an assigned laptop. I have two tables – one is user table which contains names and info of all the employees. The other one is computer table which contains service tags of all the machines we own. You can join these two on username and you figure out which computer is assigned to who. But I wanted the opposite thing – which is a logical intersection of the two tables on username. Which is usually obtained via MINUS operator.

Of course MySQL does not have MINUS, so I spent 5 minutes staring at the screen and typing mangled SQL queries trying to extract this info. Then I decided to google for it. Here is the solution:

SELECT users.username
FROM users
LEFT JOIN computers ON computers.username = user.username
WHERE computers.username IS NULL