Tuesday, October 06, 2015

Sorting by Day of the Week in DB2

Today's blog will walk you through a sorting "trick" that you can use when you are dealing with days of the week. A relatively common problem involves sorting a column that contains a day-of-the-week not in alphabetical order by in the order it appears in the week...

Assume that you have a table containing transactions, or some other type of interesting facts. The table has a CHAR(3) column containing the name of the day on which the transaction happened; let’s call this column DAY_NAME. Now, let’s further assume that we want to write queries against this table that orders the results by DAY_NAME. We’d want Sunday first, followed by Monday, Tuesday, Wednesday, and so on. How can this be done?

Well, if we write the first query that comes to mind, the results will obviously be sorted improperly:


The results from this query would be ordered alphabetically; in other words
  1. FRI
  2. MON
  3. SAT
  4. SUN
  5. THU
  6. TUE
  7. WED

And it would rare to want this type of data sorted in that order, right? The more common need would be to sort the data the way it appears on the calendar.

One solution would be to design the table with an additional numeric or alphabetic column that would allow us to sort that data properly. By this I mean that we could add a DAY_NUM column that would be 1 for Sunday, 2 for Monday, and so on. Then we could SELECT DAY_NAME but sort by DAY_NUM.

But this is a bad fix. Not only does it requires a database design change, this "fix" also introduces the possibility for the DAY_NUM value and DAY_NAME value to get out of sync... unless we are very careful, or perhaps do not allow DAY_NUM to be changed other than via an INSERT trigger that automatically populates the correct number. But requiring a trigger adds even more complexity to this "fix" which really should indicate to us that it is not a very good proposal.

A better solution uses just SQL and requires no change to the database structures. All you need is an understanding of SQL and SQL functions – in this case, the LOCATE function.  Here is the SQL:


The trick here is to understand how the LOCATE function works: it returns the starting position of the first occurrence of one string within another string. So, in our example, LOCATE finds the position of the DAY_NAME value within the string 'SUNMONTUEWEDTHUFRISAT', and returns the integer value of that position.

So,if DAY_NAME is WED, the LOCATE function in the above SQL statement returns 10... and so on. To summarize, Sunday would return 1, Monday 4, Tuesday 7, Wednesday 10, Thursday 13, Friday 16, and Saturday 19. This means that our results would be in the order we require.

(Note: Some other database management systems have a function similar to LOCATE called INSTR.) 

Of course, you can go one step further if you’d like. Some queries may need to actually return the day of week. You can use the same technique with a twist to return the day of week value given only the day’s name. To turn this into the appropriate day of the week number (that is, a value of 1 through 7), we divide by three, use the INT function on the result to return only the integer portion of the result, and then add one:


Let’s use our previous example of Wednesday again. The LOCATE function returns the value 10. So, INT(10/3) = 3 and add 1 to get 4. And sure enough, Wednesday is the fourth day of the week. 

You can monkey with the code samples here to modify the results to your liking. For example, if you prefer Monday to be the first day of the week and Sunday the last day of the week, simply change the text string in the LOCATE function as follows:


My intent with this blog posting is actually twofold. Firstly, I wanted to share a method of sorting days-of-the-week... but secondly, I hope to also have piqued your interest in using SQL functions to achieve all types of different processing requirements. DB2 offers quite a lot of functions and it seems to me that many programmers fail to use functions to their fullest.

So if you are ever face-to-face with a difficult query request, step back and take a moment to consider how DB2 functions might be able to help!


Sergey Zhidelyov said...

Ineresting approach.
But where is day 7 - see picture? Is't trick?

Craig S. Mullins said...

I hadn't noticed that the calendar graphic I chose was missing the 7th day of the month! I guess I better inspect the clip art I choose more closely!