How to use i for if condition in a for i in loop?

Question

Hi friends,
I have a question on how to use i for IF condition in a loop, in order to get an efficient programming and results. Here is outlined SQL:
cursor is
select c1,c2,c3 from table1; -- 100 rows returned.
open cursor
loop
fetch c1,c2,c3 into v1,v2,v3;
for i in 1..3 loop
if 'v'||i between 90 and 100 then
v_grade := 'Excellent';
elsif 'v'||i between 80 and 89 then
elsif 'v'||i between 50 and 59 then
end if;
end loop;
close cursor;
This way, I don't need to use a lot of if..then for hard code v1,v2,v3,.... actually I have more v..
But Oracle gave an error of this usage of 'if 'v'||i' or 'v'||to_char(i).
Thanks for any advice in advance!

Answer 1

user508774 wrote:
Thanks for comments and advices. But I didn't get your inputs clearly. Are you saying I don't need to use PL/SQL to achieve this?Correct. Ronel and John showed you the basic approaches. SQL is not a mere I/O language for making read and write calls. It is a very capable, flexible and powerful language. One can solve a problem with a few lines of SQL code, that will take 100's of lines of PL/SQL or Java code.
So do not underestimate what can be done in SQL.
v_cmd := 'UPDATE parts_categ_counts SET w1='||v1||', w2='||v2||...||v9||' WHERE seq='||vseq||';
EXECUTE IMMEDIATE v_cmd;This code is also wrong. Besides the fact that there is no need for dynamic SQL, this approach creates a brand new SQL statement each loop iteration.
SQL is source code. It needs to be parsed (compiled). The end result is an executable program that is called a cursor. This cursor needs to be stored in server memory (the SQL Shared Pool in the SGA).
The problem with your code is that it is slow and expensive - it generates lots of unique SQL statements that need CPU for parsing and server memory for storage.
These add up to a very significant performance overhead. That is the wrong approach. The correct approach is the same one that you would use in any other programming language.
Let's say you need to use Java to process a bunch of CSV files - exact same CSV layout used by each file. A file needs to be read, processed, and a log file created.
Will you write a Java program that loops through the files, for each file found, write a Java program for processing that file, compile it, then execute it?
Or would you write a Java program that takes the name of the file as input, and then process that file and writes the log file?
The 2nd approach provides a program that can process any of those CSV files - one simply needs to pass the filename as an input parameter.
Your code and approach use the 1st method. Not the 2nd. And that is why it is wrong.
To create a SQL program with parameters is done by using bind variables. Instead of
v_cmd := 'UPDATE parts_categ_counts SET w1='||v1||', w2='||v2||...||v9||' WHERE seq='||vseq||';
The following SQL source code should be created:
v_cmd := 'UPDATE parts_categ_counts SET w1=:v1, w2=:v2 ..., w9=:v9 WHERE seq= :vseq';
The tokens with the colon prefix (such as :v1), are bind variables. Think of these as the parameters to the SQL cursor.
The server parses this SQL into a cursor. You can now execute the same cursor over and over again, using different bind variables. (just like the 2nd approach above that one would use in Java)
In PL/SQL, this is made even easier as you can code native SQL code with PL/SQL code and use PL/SQL variables in it. The PL/SQL compiler is clever enough to do the SQL parsing, variable binding, and cursor execution for you. So in PL/SQL, you would use:
UPDATE parts_categ_counts SET w1=v1, w2=v2 ..., w9=v9 WHERE seq= vseq;
Where v1 and the others are PL/SQL variables.
That all said - PL/SQL is only used for data crunching, when the processing of data is too complex for the SQL language to deal with. And this is very seldom the case.
The main reason for using PL/SQL it to provide processing flow control, conditional processing and error handling, for SQL code. As the SQL language does not have these features.