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Customer Accounts, Serial Numbers once assigned have an infinite lifetime. They never can be reused. If you delete them from the current database they should still exist in your data warehouse
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If your sample data has a 3, your query returns 7 not 6. 
 
This one only hits the table once: 

<pre><code class="code-1 nolinks" id="code-20-42998076-1">select min(rn) from (
	select num, row_number() over(order by num) rn from tab1
)
where num!=rn
;
</code></pre>
 
To show all missing numbers, this is what I came up with: 

<pre><code class="code-10 nolinks" id="code-20-42998076-2">with max_num as (
	select max(num) max_num from tab1
),
all_numbers as (
	select rownum rn from max_num connect by level &lt;= max_num
),
combined as (
	select rn, num
	from all_numbers a left join tab1 t1 on a.rn = t1.num
)
select rn from combined where num is null order by rn
;
</code></pre>
 
There are many other options on the web.
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<div class="content wysiwyg-content">
Hello, 
 
How to find a missing sequence Using oracle 8i from a table ? &nbsp;the purpose I need to assign available sequences automatically for customer accounts, as below example : 
 
Account_no 
1 
2 
4 
5 
8 
 
results: 
1) The result of the query is 3 , when (3) inserted in the table, query should get 6..etc. 
2) Also how can I find all missing sequences , result : 3,6,7 using one select? 
 
Thank You
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Hello, 

How to find a missing sequence Using oracle 8i from a table ?  the purpose I need to assign available sequences automatically for customer accounts, as below example :

Account_no
1
2
4
5
8

results:
1) The result of the query is 3 , when (3) inserted in the table, query should get 6..etc.
2) Also how can I find all missing sequences , result : 3,6,7 using one select?

Thank You

Find missing serials

Databases are organized collections of data, most commonly accessed through management systems including schemas, tables, queries and processes that allow users to enter and manipulate the information or utilize it in other fashions, such as with web applications or for reporting purposes.

Databases

Oracle is an object-relational database management system. It supports a large number of languages and application development frameworks. Its primary languages are SQL, PL/SQL and Java, but it also includes support for C and C++. Oracle also has its own enterprise modules and application server software.

Oracle Database

SQL (Structured Query Language) is designed to be used in conjunction with relational database products as of a means of working with sets of data. SQL consists of data definition, data manipulation, and procedural elements. Its scope includes data insert, query, update and delete, schema creation and modification, and data access control.