What are some ways that these two databases can [INSERT INTO] Between each other or use commands or display the data in the table of the two databases ?
Not directly, but there are ways...
What you're asking about is called "cross database queries".
Each database in PostgreSQL has its own system tables and ways of keeping itself organized. Queries between two databases can break this, even for databases hosted by the same database server.
But there are ways to achieve what you want
Single database, multiple schemas
Instead of two databases, you can run one database with two schemas. The keeps the tables, views, etc separated and easier to maintain, and allows queries between the two. It also allows security and data isolation for users who are only allowed to access one of the schemas.
You're actually already using a schema in PostgreSQL called "public"; adding more schemas simply extends this.
See the documentation.
Foreign Data Wrappers
Foreign Data Wrappers (fdw) allow you to "link" a schema (or just tables, if you prefer) in another database. See the documentation for CREATE SERVER, and this seems like a pretty clear blog post on the subject.
Note that Foreign Data Wrappers will allow you to link to other databases than just PostgreSQL e.g. Oracle, SQL Server, MySQL, and lots more. See here.
Related
SQL Server and Oracle terminology -
In SQL Server If I have two applications and want to keep the database completely separate, I could simply create 1 database for each application therefore I end up with 2 databases.
If I wanted to do the same thing in oracle, what do I need to create?
- create a new "Databases"? "Instance", "Schema", or "Tablespace" per application?
(Note, these two applications is the same application used by two different companies, that do not share data!)
Reference: http://www.codeproject.com/Tips/492342/Concept-mapping-between-SQL-Server-and-Oracle
Having worked with SQL Server a lot in the past, I have sympathy with trying to figure out how Oracle organizes things as I struggled with the same thing. My comments below are from SQL Server 2000 and 2003 so forgive me if things have changed since then.
Previous responders have been helpful. I think one problematic assumption here is that there is an exact "level" equivalency between SQL Server and Oracle. What I mean by "level" is something that occupies the same space in the hierarchies that you have diagrammed above (and which, btw, I think are a good place to start but might need a bit of editing in a couple of places, for example how you have diagrammed "user" and "schema" in the Oracle hierarchy, I might put them side-by-side.) I do not think these concept "levels" match exactly between the DB platforms.
A schema in Oracle is somewhat equivalent to a separate database in SQL Server but not entirely.
I would say that the "walls" -- not an exact technical term but oh well -- between databases in SQL server are a bit higher than the "walls" between schemas in Oracle. Others might disagree but here is my reasoning:
a. A schema in Oracle is a purely logical construct. It denotes who has ownership of objects. It has nothing to do with the physical location or layout of the objects. A tablespace (orthagonal concept, as noted by a previous poster) indicates the physical location of objects. A tablespace can hold objects that are in multiple schemas and vice versa. In SQL Server these two concepts are sort of merged into one -- a database is both tablespace and schema, more or less, although in some respects within a DB in SQL Server you then have multiple owners with various object ownership. This can get a bit confusing because as I remember (it's been a couple of years) if not using NT Authentication the users are defined at the server level and then have to "link" to the users in the individual DBs.
b. I remember finding it easier, or at least a bit simpler, to assure myself that users to two separate DBs in SQL Server had no access to the relative other user's DB than I have found it in Oracle.
c. Because a DB in SQL server represents both physical storage and logical ownership, you can detach the DB and move it to another SQL Server Instance and attach it. You can't do this with a schema in Oracle. I mean, you can datapump the data out or back it up or whatever to another server and another schema, but that all takes at least some scripting and such or at least a fair amount of clicking in Enterprise Manager. It doesn't give you the one-click "Detach DB" option that you have in SQL Server which makes it a lot easier to get the idea that SQL Server DBs are units that you can more-or-less move back and forth between databases.
To sum things up, I think either option would work. That is, 1) Create two separate instances of Oracle with one schema in each instance for each application, or 2) Create two separate schemas in one Oracle instance.
There are pros and cons for each option. Option 1 is probably going to be more work to set up and configure but will also give you more separation, independence, ability to have separate hardware, etc., for each DB. Option 2 will be quite a bit simpler but gives you less separation between the data and greater risk of configuration screw-ups or other things allowing users of one schema to access the other. It also means you have to be a bit more careful that someone writing a query accessing data in one schema doesn't use all the CPU and IO resources and starve a user on the other schema.
Also, yes, you could use pluggable databases in 12c. However, given the fact that you need to ask these questions (no shame, just pointing out where you're at) makes me hesitant about recommending what can easily be a more complex setup.
TL;DR -- SQL Server isn't Oracle and Oracle isn't SQL Server. Either option works and there are pros and cons to each.
If you're using 12.1 or later with the multitenant option, you could create separate pluggable databases in a single container database. The other option, which works in any version of Oracle, would be to create a separate schema. It would be possible, as well, to create a separate database, though that is generally not the preferred approach unless you have a particular need to do things like upgrade the database that one application is using without affecting the other.
Creating a Database
If you create a separate database, you'd end up with complete separate memory structures (i.e. the SGA and PGA for each database would be separate) as well as a completely separate set of background processes (each database would have its own log writer process(es) for example). That is a very heavyweight option-- you can't have too many databases on a single server before you start having a lot of contention for RAM, for scheduling all the background processes, etc. It does provide for the maximum separation between different applications-- each database can be running a different version of Oracle with a different set of initialization parameters-- but this also tends to increase the complexity of managing the environment. This generally only makes sense when you have third party applications that require a specific version of the database or a specific set of initialization parameters.
Creating a Schema
If you create a separate schema, you still have a single database so the two schemas are sharing the same memory structures (competing with each other for space in the SGA's buffer cache, for example), initialization parameters, etc. You have to exercise a modicum of planning to ensure that that the two don't interfere with each other-- you'd probably want to make sure that nether application creates public synonyms or at least that they won't wan to create the same public synonym as the other application-- but this is generally pretty trivial.
Creating a Pluggable Database
This only works in 12.1 and only if you have the multitenant option. This is the most similar to the SQL Server concept of creating a new database for each application.
You should create a new instance (schema) on the same database, where the schema in oracle is the same as the SQL server database
I'm building a new NHibernate 3.3 application that must connect to a legacy system in order to look up some information about my users. There's a separate, read-only, database that holds course enrollments that I'd like to use to populate a collection on my Student entity. These would be components in NHibernate-speak, consisting of a department code and course and section numbers, like "MTH101 sec. 2"
The external database has a surrogate key, the student number, which corresponds to a property in my User entity, but it's not the primary key of a Student.
These databases are on separate servers. I can't change the legacy database,
Do I have a hope of mapping the enrollments collection as NHibernate components?
Two Options
When you have multiple databases or multiple database servers that you're trying to link together in a single domain model using NHibernate, you basically have two options.
Leverage the database server's capabilities (linked servers, etc.) to join the data so that NHibernate only has to worry about connecting to one database. In your NHibernate mappings, you fully specify the table attribute so that the database server knows to query against the other database server. For your "surrogate key, ... not the primary key", you could map this using <many-to-one property-ref="...">.
Use multiple NHibernate session factories, one for each database. You would be responsible for coordinating what gets loaded from which database. You configure each session factory for just the tables that exist in that database and with the appropriate connection string. Then, to load the data, you execute two queries, one against one database, and another against the other database.
Which one?
Which is the right choice? It depends...
Available features
If your database server doesn't have any features to support #1, or if there are other things preventing you from using those features, then you obviously have to use #2.
Cross-DB where Clauses
#1 gives you more flexibility when writing queries - you could specify where clauses that span both databases if you needed to, though you need to be careful that the query you write doesn't require database A to fetch tons of data from database B. With method #2 you execute a second query to get what you need from database B, which forces you to be more conscious about exactly what data you have to fetch from each database to get the job done.
Unenforced relationship
There won't be any foreign keys enforcing the relationship because the data lives in two different databases. NHibernate (very reasonably) assumes that database relationships are enforced by foreign keys. Since there's a chance these two databases could be out of sync, #1 will require you to resort to things like not-found="ignore", which has performance implications.
Complexity of Deployment
Inter-database relationships make deploying to various environments (DEV, QA, PROD) difficult. You can't just deploy the application and database, and make sure the application's connection strings are pointing at the correct databases; instead you also have to make sure that any references inside the databases to other databases are pointing to the correct places.
Given all of the above factors, I usually lean towards option #2, but there are some situations where #1 is just so much more convenient.
I'm planing a webproject, containing 4 websites build in MVC3. As a databaseserver I'm going to use the ms sql server.
Each of this websites will have something arround 40 tables. But some of the tables are shared between the websites:
Contact, Cities, Postalcodes, Countries...
How to handle this? should I put all the tables of each database into a common database (so that the database of website 1,2,3 and website 4 are in one databse together). Or should I create one database containing shared datase?
But then I think I'm getting problems with the data consitency, because I think there is no way to point from one database to an other (linking for example the citytable in database one to the buldingtable in databse 2).
Any ideas?
Thanks a lot!
What I like about splitting it out into separate databases is that if each web site has its own database, and one of those web sites gets extremely popular, it is very easy to just move their database to a different, more powerful database server and not much has to change except (a) you need to reference the central "control" data remotely (or replicate/mirror/etc), and (b) you point that web site at a different database server. Another benefit is that if two web sites have the same types of tables (e.g. Patients), you don't have to have tables like Patients_WebSite1, Patients_WebSite2, with different stored procedures that are identical except for table names (or ugly dynamic SQL procedures that paste the table name in). Separated out you can have the exact same schema and the exact same codebase without having to combine everyone's data into a single table.
If you mix the data within a single database, data consistency is easier, and the whole setup is slightly simpler, but splitting it out when you grow is a lot tougher. If you split it out into different databases, no you won't be able to enforce referential integrity using standard DRI (foreign keys). You can accomplish this in other ways if it is important (triggers, validation before insert/update, etc).
Is there a way to make a schema diagram from an SQL Server database using the stored procedures of this database?
I don't mind if I must use an external software.
You could try playing around with CodeSmith Generator. It's SchemaExplorer Schema Discovery API allows you to programmatically access database elements for a given database and do something creative with it. However, it will still be logically hard to reverse-engineer a schema/diagram this way.
You can build a SQLCLR procedure which uses the Scripter Class from the SMO library.
update: more info on the question reveals the idea is to generate a table schema with dependencies based on the content of the stored procedures.
The approach would be to generate the table structure from the information_schema views and then parse the contents of the syscomments table to figure out the relations. This will always be approximate as it is very hard to establish the one-to-many relationships purely from the SQL Statements. I think you can make a guess based on the field which is referenced more.
If you can't see the tables then you can not generate the schema.
That is, you can't if you have permissions on stored procedures only.
At least two reasons:
the stored proc may JOIN and use several tables
you can't see constraints, indexes, keys etc even if you had table names
Basically, you can only:
see what you have permissions on in SSMS etc
see the internals if you have VEIW DEFINITION rights
Edit, after clarification
There is no way to script implied aspects (such as missing foreign keys) of the schema from code
I have two applications using two nearly identical MySQL databases within the same cluster. Some tables must contain separate data, but others should hold identical contents (i.e. all writes and rows in db1.tbl should be accessible in db2.tbl and vice versa).
What's the proper way to go about this? Note that the applications use hardcoded table (but not database) names, so simply telling application 2 to access db1.tbl is not an option.
What you need to do is set up replication for the tables that you need. See http://dev.mysql.com/doc/refman/5.0/en/replication.html for the documentation on setting up replication in MySQL.
For databases on different mysqld processes
You should check the official manual for replicating individual tables:
http://dev.mysql.com/doc/refman/5.1/en/replication-options-slave.html#option_mysqld_replicate-do-table
You can setup an Master-Master relation between the two mysql processes just keep in mind to be carefull and have uniqueness on your Primary Key.
For databases residing on the same server & mysqld service
IMHO design wise you should consider the idea of moving all your shared tables under a different DB.
This way you will avoid all the overkill of triggers for updating them.