The operator to concatenate in SQL Server is '+', but this operator is also used to add values.
Being the same operator for both, the query analyzer makes a concatenation or a sum based on the type of data fields being treated. If the fields are of 'string' type, the operator concatenate, and if it has numeric fields, two integers, for example, it add the two values.
In many cases, especially in ETL processes or data loading to a data warehouse, for example, we would rather use a single sentence or the checking of the existence of a register and its updating. If register doesn't exist we would want to insert it. This combination has been nicknamed UPSERT, although in SQL there is a specific statement to make it, which is MERGE.
In a LinkedIn debate about how to update from a select with summarized registers in SQL Server, Sergio Romero has shared an interesting tip about new summarize functions on Oracle 11g R2.
A very powerful way to do a table's update in a SQL Server database is to link it with another table with a join, and updating the fields of everyone of its registers using the field's values of the registers linked with the another one. This technique had been discussed in the forum post Update with join or update from select in SQL Server..
The operation of databases from different manufacturers is similar, but there are many differences between them, even at level of SQL syntax.
For example, if you knows Oracle, and one day you have to do a date conversion with MySQL, you will wonder the following:
- How to do a TO_DATE () / TO_CHAR () with MySQL?
I open this blog entry to discuss different ways of doing things with each database type.
Here I link the SQL Tips Bristle Software , which explains quite well how to do some things with MySQL, Oracle and SQL Server, and I find especially useful the chapter on differences between Oracle and SQL Server
In order to access to the same database where are you working outside the database server you must activate the service called listener, it has to be listening.
It can happens that the database is properly raised and can not connect from other servers, which are also set correctly (correct TNSNAMES, etc.)..
In these cases could be that the listener has a problem, or simply has not been initiated.
To Check the status, start or stop it is very simple. Just open a command line session (console terminal, etc..) with the user that has installed the database, and run the lsnrctl command with the following parameters:
Check your state:
> lsnrctl status
Stop the listener:
> lsnrctl stop
Start the listener:
> lsnrctl start
Keep in mind that when you stop the listener, the connections that are already in the database won't be closed, so a short stop is not very traumatic, only connections trying to enter while the listener is stopped are rejected, should not affect anyone who already has an opened session.
If you have the system user who installed the database you can enter SQL Plus as DBA user, without entering a password as follows:
1. Enters the system with this user.
2. From the command line, go into SQLPlus typing:
> sqlplus "/as sysdba"
If you need to enter using this way because you forgot the password of a user, you can easily change it:
SQL> alter user user_name identified by new_password;
It can be more than one Database installed on the server, so you have to validate that the environment variables of the Oracle's user are pointing to your database.
For verifying that you has login into the correct database you can execute this statement:
SQL> select name from v$database;
The easiest way to access from an Oracle database objects from another Oracle database is using a DBLINK (being the easiest does not mean that it is always the most desirable, the abuse of DBLINKS can create many problems, both of performance and safety)
To do this it's necessary a user with CREATE DATABASE LINK privilege, and create a DBLINK in the source database (A) by a simple statement such as:
Create database link LNK_from_A_to_B connect to USER identified by PASSWORD USING 'B';
'LNK_from_A_to_B' is the name of the link, 'USER' and 'PASSWORD' are the IDs of the user who will use the link to connect, which will inherit the permissions of all access through the link, and B is the name of the database's instance.
Using the DBLINK we can connect to the objects with the remote database's permissions that user has been provided in the creation statement.
To reference an object from the remote database should indicate the name of the object, concatenated with the character '@' and the name that we had given to the DBLINK.
select * from TABLA@LNK_from_A_to_B
Great video showing through the use of social media an original story of Jesus' birth at Christmas.
Besides being fun, it's a good way to make a quick tour of the main features of Facebook, Twitter, YouTube, Google, Wikipedia, Google Maps, GMail, Foursquare or Amazon ...
How times change:)