20 March 2017

Generate Rows based on a Column Value

My requirement is the following:

In a database table the activity, start time and the number of repetitions is stored, but for the report this needs to be expanded. The number of repetitions dictates the number of rows per activity, each incremented by five minutes from the start time.
To get things going a simplified table is created, as well as some sample data.
create table activities
(activity      varchar2(10)
,activity_date date 
,no_of_reps    number

insert into activities values ('X', to_date ('20-03-2017 12:00', 'dd-mm-yyyy hh24:mi'), 2);
insert into activities values ('Y', to_date ('20-03-2017 12:30', 'dd-mm-yyyy hh24:mi'), 4);
insert into activities values ('Z', to_date ('20-03-2017 13:00', 'dd-mm-yyyy hh24:mi'), 3);


The sample data looks like the following output; Activity X was started at 12:00 o'clock and repeated twice; Activity Y was started at 12:30 and repeated four times; Activity Z was started at 13:00 and repeated three times.

select a.activity
      ,to_char (a.activity_date, 'dd-mm-yyyy hh24:mi') start_time
  from activities a

---------- ---------------- ----------
X          20-03-2017 12:00          2
Y          20-03-2017 12:30          4
Z          20-03-2017 13:00          3

New in Oracle 12c is the LATERAL join making it very easy to generate the required number of rows for each row in the table.

select a.activity
      ,to_char (a.activity_date, 'dd-mm-yyyy hh24:mi') start_time
  from activities a
      ,lateral (select 1 from dual connect by level <= a.no_of_reps)
---------- ----------------
X          20-03-2017 12:00
X          20-03-2017 12:00
Y          20-03-2017 12:30
Y          20-03-2017 12:30
Y          20-03-2017 12:30
Y          20-03-2017 12:30
Z          20-03-2017 13:00
Z          20-03-2017 13:00
Z          20-03-2017 13:00

 9 rows selected 
In the example above the LATERAL join with the "Connect By Level" trick is used to generate rows.

For the last part of the requirement, increment the time with five minutes for each repetition, the Analytic Function ROW_NUMBER is used.
The ROW_NUMBER (minus one) is multiplied by a five minute interval which is added to the original start time of the activity.

select a.activity
      ,to_char (a.activity_date, 'dd-mm-yyyy hh24:mi') start_time
      ,to_char (a.activity_date 
                 + (row_number() over (partition by a.activity_date
                                           order by null
                                      ) - 1)
                 * to_dsinterval ('0 00:05:00')
               ,'dd-mm-yyyy hh24:mi') as calculated_time
  from activities a
      ,lateral (select 1 from dual connect by level <= a.no_of_reps)

---------- ---------------- ----------------
X          20-03-2017 12:00 20-03-2017 12:00
X          20-03-2017 12:00 20-03-2017 12:05
Y          20-03-2017 12:30 20-03-2017 12:30
Y          20-03-2017 12:30 20-03-2017 12:35
Y          20-03-2017 12:30 20-03-2017 12:40
Y          20-03-2017 12:30 20-03-2017 12:45
Z          20-03-2017 13:00 20-03-2017 13:00
Z          20-03-2017 13:00 20-03-2017 13:05
Z          20-03-2017 13:00 20-03-2017 13:10

 9 rows selected 


03 January 2017

APEX: Display the Page Alias on every page - without modifying the Template

Having a Page Alias shown on the page can make communication with your end users a little bit easier. Instead of directing them to the URL and asking them for the second value shown after the "f?p", it is a little bit simpler to direct them to the location of the Page Alias, e.g. bottom left corner of your screen.
For my projects I tend to use the Page Alias as a link between my APEX front-end and my database code back-end.
Until now I always used an Application Item, a Computation and a change to the Page Template to display the Page Alias. Using this method would require changes to the Page Template, which was not a big deal.
In order to make changes to the Page Template you would have to copy the template or make changes to the Master Template. I haven't tried the latter, but there is a risk with copying the Page Template:
Unsubscribed templates are editable and will not get overwritten by a theme refresh.

Going through the Oracle APEX documentation, I stumbled upon a Substitution String that I haven't seen before, APP_PAGE_ALIAS. I don't know when this was introduced, but it makes it a bit easier to include the Page Alias on the page. My first thought was that I could use this in the Page Template, but that would still mean that I would have to copy the Page Template and unsubsribe it from the Master Template.
Placing the APP_PAGE_ALIAS Substitution String in the Version Attribute (at Application Level, under Edit Application Definition) would overcome this. It will display the Page Alias on every page, right next to the release number of the application.
No changes to the Page Template, no Application Item, no Computation... easy peasy.


Like stated before: I didn't know when this Substitution String was introduced, but Peter Raganitsch does.


11 October 2016

OTN Appreciation Day: Analytic Functions

This is my contribution to the OTN Appreciation Day, which was initiated by Tim Hall.

One of my favorite features of the Oracle Database are Analytic Functions. They were introduced with Oracle Database 8.1.6 Enterprise Edition, and have been in the Standard Edition since version 9.

With analytic functions you can add inter-row calculations, aggegrate over multiple dimensions, rank assignments based on values. All this without a GROUP BY clause.
The syntax might take some getting used to, but you'll find that it's not that hard.

While I was still working at AMIS, I followed their 7UP training (all new features starting from Oracle 7 onwards) and that's when I first learned about Analytic Functions.
A little bit after that training, while doing consulting work for T-Mobile, I was triggered by a colleague to come up with an analytic function solution to the problem at hand. That was the moment that I really fell in love with Analytic Functions. It provided a very elegant and extremely performant solution.
The blog I wrote back then is still available at the AMIS-site: Analytic Power


07 October 2016

Iconic Breadcrumbs in APEX

Normally when you use a Breadcrumb in an APEX application you enter the data and that's it. The breadcrumb will look something like this:

Or when you reduce the screen, the Breadcrumb will look slightly different:

It's trivial to replace the text that you enter in the Breadcrumb with a little bit of markup and show a Font Awesome (or with APEX 5.1 Font APEX) icon.

<span class="fa fa-home"></span>
Now the text will be replaced with a little house.

And resized, it will look like this

If you want to use this, keep in mind that this solution is not screen-reader-friendly. Personally I would only use it for "top-level" Breadcrumbs, keeping the same images that you use in the Navigation Menu.

26 September 2016

Celebrate Batman Day with SQL

Because today is Batman Day, it calls for a celebration of the SQL type:
SQL> select listagg (b)
  2        within group (order by null) ||' Batman'
  3    from (select 0f/0 b
  4        from dual
  5      connect by level <= 14
  6      );

NanNanNanNanNanNanNanNanNanNanNanNanNanNan Batman

01 September 2016

RegExp: Constraint to prevent spaces at the beginning or end.

Even though a space is a regular character, the client didn't want spaces at the beginning or end of a string. Any spaces in the middle were fine.
Of course this could be handled by the application, but it must also be implemented in the database. Using a check constraint with a regular expression will prevent the end user from entering unwanted data.

To try things out, let's just start with a simple table with a single column.

   create table test
   (str varchar2(10));

Now the magic part: add a check constraint using a regular expression

   alter table test
   add constraint str_chk check (
      regexp_like (str, '^[^[:space:]].+[^[:space:]]$')

The regular expression reads: The string should start (the first caret) with any character which is not in the character class of [:space:], followed by one or more characters (the period) and it should end with (the dollar) any character as long as it's not in the character class of [:space:].

UPDATE - 07-Sept-2016

The expression that is used in the above constraint will also prevent from a single allowed character or double character to be entered. This omission has been corrected by David Grimberg. The correct regular expression should be:


To test the constraint, the following insert statement were used.

   insert into test values ('hello');
   insert into test values ('hel lo');
   -- Not allowed:
   --   starting with a space
   insert into test values (' hello');
   --   ending with a space
   insert into test values ('hello ');
   --   just a space
   insert into test values (' ');
   --   multiple spaces
   insert into test values ('   ');
   --   Tab
   insert into test values (chr(9));
   --   Line feed
   insert into test values (chr(10));
   --   Carrige Return
   insert into test values (chr(13));

31 May 2016

Top N- queries: using the 12c syntax.

One of the new features with Oracle database 12c is the new syntax for Top N queries and pagination. Did we really need this? Should you choose for the new syntax over the way we used to do it, with an inline view? I think so, it simply adds syntactic clarity to the query, and in this blogpost I will show the difference between the "old" and the "new".

For the examples I will use my all time favourite demo data: the EMP table.

SQL> select ename
  2        ,sal
  3    from emp
  4   order by sal desc
  5  /

ENAME             SAL
---------- ----------
KING             5000
FORD             3000
SCOTT            3000
JONES            2975
BLAKE            2850
CLARK            2450
ALLEN            1600
TURNER           1500
MILLER           1300
WARD             1250
Widlake          1250
ADAMS            1100
JAMES             950
SMITH             800

14 rows selected.
As you can tell from the output above, KING has the highest salary and FORD and SCOTT have the same salary which is the second highest.

If you wanted to write a Top N query before Oracle database 12c, let's say the Top 2 of most earning EMP, you would probably have written something with an inline view.

SQL> select ename
  2        ,sal
  3    from (select ename
  4                ,sal
  5            from emp
  6           order by sal desc
  7         )
  8   where rownum <= 2
  9  /

ENAME             SAL
---------- ----------
KING             5000
SCOTT            3000
This query might do the job most of the time, but the results might not be what you were looking for. In this case the requirement is "the Top 2 of most earning EMP", SCOTT and FORD should have both been in the results as they have the same salary.
To resolve this, you would have to rewrite your query using an Analytic Ranking Function in the inline view:
SQL> select ename
  2        ,sal
  3    from (select ename
  4                ,sal
  5                ,rank() over (order by sal desc) rn
  6            from emp
  7         )
  8   where rn <= 2
  9  /

ENAME             SAL
---------- ----------
KING             5000
SCOTT            3000
FORD             3000

Using the new Top N syntax in Oracle database 12c, the query is a lot easier to understand.

SQL> select ename
  2        ,sal
  3    from emp
  4   order by sal desc
  5   fetch first 2 rows with ties
  6  /

ENAME             SAL
---------- ----------
KING             5000
SCOTT            3000
FORD             3000
On line 4 the results are sorted based on the salary (highest on top) and line 5 instructs to get the first two rows. Because of the addition "with ties" both SCOTT and FORD are shown in the results.
To see what happens under the covers, an explain plan is created for this query.
SQL> explain plan for
  2  select ename
  3        ,sal
  4    from emp
  5   order by sal desc
  6   fetch first 2 rows with ties
  7  /


SQL> select *
  2    from table (dbms_xplan.display())
  3  /

Plan hash value: 3291446077

| Id  | Operation                | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT         |      |    14 |   644 |     4  (25)| 00:00:01 |
|*  1 |  VIEW                    |      |    14 |   644 |     4  (25)| 00:00:01 |
|*  2 |   WINDOW SORT PUSHED RANK|      |    14 |   154 |     4  (25)| 00:00:01 |
|   3 |    TABLE ACCESS FULL     | EMP  |    14 |   154 |     3   (0)| 00:00:01 |

Predicate Information (identified by operation id):

   1 - filter("from$_subquery$_002"."rowlimit_$$_rank"<=2)

16 rows selected.
The output above shows that the Analytic Function RANK is used.