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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for the ‘OracleDB’ Category

Generating a million sequential numbers on the fly in a Firebird query – some solutions and speed measurements

Posted by jpluimers on 2018/07/19

The testing was done with Firebird 2.5.x x86 on Windows 7 x64.

Where other relational database platforms have plenty of opportunities to generate sequences (see for instance the below links on Oracle and SQL Server), with Firebird you can use a WITH RECURSIVE query construct that normally is being used to manage tree structures ([WayBackPkLab – Firebird: Tree data mangement with recursive CTE).

However, that uses query stack which has a depth limit of 1024 levels. When you reach the limit, Firebird gives you an error like this:

with 
  recursive 
  sequence(n) as (        
    -- When you select more than 1024 values, this error occurs:
    -- Error while fetching data:  Too many concurrent executions of the same request    
    select 0 -- start
    from rdb$database
    union all
    select sequence.n + 1
    from sequence
    where sequence.n < 1023 -- finish
  )
select sequence.n 
from sequence
--where sequence.n in (24, 38) 
order by sequence.n

It however is a pretty quick and CU bound solution: on my system ([WayBackAMD A8-7600 @ 3.1 Ghz), it runs 1000 records within ~0.1 seconds.

In such a short time, it’s hard to see how the speed is bound, so I wanted to go for some orders of magnitude more. In ~0.1 seconds, the processor executes about 0.3 * 10^9 cycles generating 1000 numbers which is ~ 300-thousand cycles per number. That sounds like a lot of cycles for so few numbers. Would this become a better ratio for more numbers?

Read the rest of this entry »

Posted in Database Development, Development, Firebird, OracleDB, SQL, SQL Server | Leave a Comment »

Visual Representation of SQL Joins – CodeProject

Posted by jpluimers on 2017/08/02

I thought I posted a reference to this a long time ago, but didn’t.

It’s one of the things I show when explaining joins to people. Sometimes I need it myself too (:

The article explains these in greater detail:

  • INNER JOIN
  • LEFT JOIN
  • RIGHT JOIN
  • OUTER JOIN
  • LEFT JOIN EXCLUDING INNER JOIN
  • RIGHT JOIN EXCLUDING INNER JOIN
  • OUTER JOIN EXCLUDING INNER JOIN

Note:

  • the opposite of INNER JOIN is not OUTER JOIN. It’s OUTERJOIN EXCLUDING INNER JOIN
  • the opposite of OUTER JOIN is empty set.

But the diagram is usually speaks for itself.

–jeroen

Source: Visual Representation of SQL Joins – CodeProject

Read the rest of this entry »

Posted in Access, Database Development, DB2, Development, Firebird, InterBase, MySQL, OracleDB, PostgreSQL, SQL, SQL Server | Leave a Comment »

Common Table Expressions: no nesting, but consecutively usage – via Stack Overflow

Posted by jpluimers on 2016/09/28

Common table expressions are awesome. They work in at least Oracle and SQL Server.

You cannot nest them, but you can use them consecutively. Thanks spender for explaining that:

WITH
x AS
(
SELECT * FROM MyTable
),
y AS
(
SELECT * FROM x
)
SELECT * FROM y

–jeroen

via: sql – Can you create nested WITH clauses for Common Table Expressions? – Stack Overflow.

Posted in Database Development, Development, OracleDB, SQL Server, SQL Server 2005, SQL Server 2008, SQL Server 2008 R2, SQL Server 2012, SQL Server 2014 | Leave a Comment »

Oracle Cross database queries: you need a “database link” for that

Posted by jpluimers on 2016/09/27

I recently learned that you can do cross database queries in Oracle using database links.

You need to prefix your objects with the right schema/owner (for instance dbo.) and suffix with an @ sign followed by the database link name.

This query finds all configured database links:

select * from all_db_links;

–jeroen

via: Ask Tom “database link”.

Posted in Database Development, Development, OracleDB | Leave a Comment »

sql – Oracle to_date() to parse input to datetime values – via Stack Overflow

Posted by jpluimers on 2016/09/07

The Oracle to_date() function has saved me numerous times parsing date time strings into datetime values like this:

to_date('29/07/2015 13:18:00', 'DD/MM/YYYY HH24:MI:SS')

One gotcha is that when you query back the resulting date-time values, they might be formatted differently because of NLS_DATE_FORMAT settings

In addition to the query for ‘nls_date_format’ mentioned in sql – Oracle to_date() incorrect output – Stack Overflow, I usually use this one:

select v.*
from v$parameter v
where name like 'nls_%'
order by lower(v.name)
;

–jeroen

Posted in Database Development, Development, OracleDB, Software Development | Leave a Comment »

 
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