Question

I have a table mytable in ORACLE database having only one column Roll_no like following:

Roll_no
----------
CD01 0001
CD01 0002
CD01 0004
CD01 0005
CD01 0008

here CD01 is fixed, after CD01 one space is there and then numbers are written 0001 and onwards.

Please see here that CD01 0003, CD01 0006, CD01 0007 are missing and these are my required output.

Required output:

Roll_no
----------
CD01 0003
CD01 0006
CD01 0007

How can I write a SQL query to find out these missing alphanumeric characters? Any ideas please

Answer 1

You could generate the full list of number and then with a NOT EXISTS (or NOT IN) filter the existing records out. For example:

SELECT new_roll_no
  FROM (SELECT 'CD01 ' || to_char(rownum, 'fm0000') new_roll_no FROM dual 
        CONNECT BY LEVEL <= (SELECT to_number(MAX(substr(roll_no, 6))) FROM T))
 WHERE new_roll_no NOT IN (SELECT roll_no FROM T)
 ORDER BY 1

Here’s how it works:

• CONNECT BY LEVEL is a bit of a trick query, it will generate rows up to the level you indicate (in this case I chose the max from your table). I think this query first appeared on the askTom site and is now quite widespread.
• I’ve generated 8 rows, I will next use the TO_CHAR function to make a column look like your primary key (see the format model documentation of the TO_CHAR function).
• I then compare these generated values to your actual table value (NOT IN is an ANTI-JOIN)