如何获取Oracle SQL表中未使用/可用的主键（主键范围从1到999）的列表？

Question

I have employee table where primary key empl_no ranges from 1 to 999. Now I want to know what primary_key are still available for adding new employee(s). For example, below query returns the value 993. That means there are still 6 empl_no availavle which have not been used yet. How can I get the list of those 6 available empl_no using Oracle SQL? Thanks in advance for you help :)

SELECT count(empl_no)
FROM emplpyee
WHERE empl_no BETWEEN 1 AND 999;

Answer 1

WITH cteNumbers AS (
    SELECT LEVEL AS NUM
    FROM DUAL
    CONNECT BY LEVEL <= 999
)

SELECT n.NUM
FROM
    cteNumbers n
    LEFT JOIN emplpyee e
    ON n.Num = e.empl_no 
WHERE
    e.empl_no IS NULL

Again don't re-use primary keys! But if you really want to know what is missing use a numbers table. In ORACLE they make it easy using CONNECT BY . Then LEFT JOIN back to your table and look for when the empl_no is missing.

here is a link that shows oracle's ability to create a numbers table on the fly: http://rextester.com/CZDKC69208

Answer 2

select level from dual connect by level <= 999
minus
select empl_no from emplpyee 

Answer 3

Subtract all IDs found in the table from all available IDs. You create all available IDs by determining the number range first ( number(3) ie 0 to 999 in your case) and then recursively create all numbers in that range.

In standard SQL (and in Oracle as of version 11.2) you'd use a recursive CTE for this.

with all_ids(id) as
(
  select power(10, data_precision) - 1 as id from all_tab_cols 
    where owner = 'HR' and table_name = 'EMPLOYEES' and column_name = 'EMPLOYEE_ID'
  union all
  select id - 1 from all_ids where id > 0
)
select id from all_ids
minus
select employee_id from employees;

Answer 4

I am assuming that your table consists some columns X,Column Y other than empl_no.So to find an available primary key you need to find the of rows which are NULL for column X and Y (that means they are empty).Below query might help..

SELECT empl_no FROM emplpyee WHERE columnX IS NULL and columnY IS NULL