Sometimes the requirements of a particular application dictate that data needs to be sorted using some irregular collating sequence. These odd needs sometimes cause developers to sit and scratch their heads for hours, searching for ways to make DB2 do something that seems “unnatural.” But often you can create an answer just by understanding the problem and applying some creative SQL.

At this point, some of you might be asking “What the heck is he talking about?” Fair enough. Let’s look at an example to clarify the issue.

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:

SELECT   DAY_NAME, COL1, COL2 . . .
FROM     TXN_TABLE
ORDER BY DAY_NAME;

The results from this query would be ordered alphabetically; in other words:

FRI
MON
SAT
SUN
THU
TUE
WED

This is what I mean by an “irregular sorting requirement.” The example gives a commonly occurring requirement, but many businesses have similar requirements for which the business needs dictate a different sort order than strictly alphabetical or numeric. So what is the solution here?

Of course, one solution would be to design the table with an additional numeric or alphabetic column that would sort 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. But this requires a database design change, and it becomes possible for the DAY_NUM and DAY_NAME to get out of sync.

There is another solution that is both elegant and does not require any change to the database. To implement this solution, all you need is an understanding of SQL and SQL functions — in this case, the LOCATE function. Here is the SQL:

SELECT   DAY_NAME, COL1, COL2 . . .
FROM     TXN_TABLE
ORDER BY LOCATE('SUNMONTUEWEDTHUFRISAT',DAY_NAME);

The trick here is to understand how the LOCATE function works. The LOCATE function returns the starting position of the first occurrence of one string within another string.

So, in our example SQL, 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 previously mentioned SQL statement returns 10. (Note: Some other database systems have a similar function called INSTR.) 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.

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:

INT(LOCATE('SUNMONTUEWEDTHUFRISAT',DAY_NAME)/3) + 1;

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.

Summary

With a sound understanding of the features of DB2 SQL – and a little imagination – many “irregular” requirements are achievable using nothing more than SQL!

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Craig Mullins is an independent consultant and president of Mullins Consulting, Inc. Craig has extensive experience in the field of database management having worked as an application developer, a DBA, and an instructor with multiple database management systems including DB2, Sybase, and SQL Server. Craig is also the author of the DB2 Developer’s Guide, the industry-leading book on DB2 for z/OS, and Database Administration: Practices and Procedures, the industry’s only book on heterogeneous DBA procedures. You can contact Craig via his web site at http://www.craigsmullins.com.