Suppressing repeating values in SQL

Sometimes you my find a need to suppress repeating values in SQL. One case is when your reporting tool does not have such a feture or you just cannot find it fast enough. That happened to me with a report that was to be converted to APEX from Oracle Reports the other week.

I could not find an option in APEX to suppress repeating values, and I did not want to make them all control breaks as that would chop up the report too much.

Let’s begin with a real simple SQL that shows the departments different employees work in.

select a.dname
,b.ename
from dept a
inner join emp b
on a.deptno = b.deptno
order by 1,2

This of course gives a list like this:

ACCOUNTING CLARK
ACCOUNTING KING
ACCOUNTING MILLER
RESEARCH ADAMS
RESEARCH FORD
RESEARCH JONES
RESEARCH SCOTT
RESEARCH SMITH
SALES ALLEN
SALES BLAKE
SALES JAMES
SALES MARTIN
SALES TURNER
SALES WARD

Suppose I want to remove the department name when it is the same as on the previous row. SQL does not give a way to just mention a keyword to have it done and APEX does not allow for it as far as I can tell.

The first step is to add a rownumber to the result table for the above SQL.
select c.*
,rownum rn
from (select a.dname
,b.ename
from dept a
inner join emp b
on a.deptno = b.deptno
order by 1,2) c

Now each row has an number starting from 1 and ending with 14. We wrap this in a prefactor construct (i.e. wrap it in a with so the select can use the data).

with rpt as(select c.*
,rownum rn
from (select a.dname
,b.ename
from dept a
inner join emp b
on a.deptno = b.deptno
order by 1,2) c)
select case
when a.dname = b.dname then ' '
else a.dname
end dname
,a.ename
from rpt a left outer join rpt b
on a.rn = b.rn + 1
order by a.rn;

Here we have the select creating the list with the rownumber in a with that the select that follows can reference. We have now also added a selfjoin so we join each row with its predecessor in rownumber order.

This allows us to show data from the prefactored SQl using a to display data and b to check data on the previous row.

In this case we check the department and when it is the same as it was on the previous row we display a single space. if it is not the same, we display the name of the department. Thus, the case performs the supressing of repåeating department names.

The result of this SQL is the following report.

ACCOUNTING CLARK
KING
MILLER
RESEARCH ADAMS
FORD
JONES
SCOTT
SMITH
SALES ALLEN
BLAKE
JAMES
MARTIN
TURNER
WARD

That is it. Suppressing isn’t too hard to do, but it requires a little bit of setup in the SQL.

October 11th, 2011 by