We will migrate the database from mysql to postgresql in our product(through java). So we need to change the mysql query to postgresql query in java application. How to create the table i.e., databasename.tablename in postgresql.
For mysql, we can directly create the table e.g create table information.employee.
Here database name is "information" and table name is "employee" . Is it possible to achieve same query in postgresql.
I searched google it says cross database reference is not possible. Please help me.
I saw pg_class table it contains the table names in the specific database, like wise databse and tables relationships are stored in any other table.
This is normally done using schemas rather than databases, which is more or less like how MySQL organizes it anyway.
Instead of
create database xyz
use
create schema xyz
When you create tables, create them:
create table xyz.myTable
you will need to update your search path to see them on the psql command line tool, or if you want to query them without using the schema explicitly. The default schema is public, so when you create a table without a schema name, it ends up in public. If you modify your search_path as below, the default schema becomes the first in the list: xyz.
set search_path=xyz,public,pg_catalog;
and you must not have spaces in that statement. You can do it globally for a user/role too:
alter role webuser set search_path=xyz,public,pg_catalog;
Also, don't forget that postgresql string matches are case sensitive by default (this one catches people out a lot).
If you want to have different physical locations for the files for each schema, you can do that with tablespaces. If you have a look at the postgresql documentation page, they have info on how to do it, it's pretty easy.
database in MySQL == schema in PostgreSQL. So you will most probably want to migrate all your mysql dbs into one postgres db. Then you will be able to do "cross-database" queries.
See my answer to this question: Relationship between catalog, schema, user, and database instance
Related
I have access to a remote database, and I would like to dump the schema and data of several views onto my local machine and load this into my local database as tables in a quick and easy way.
I lack the user privileges to run CREATE TABLE AS (SELECT * FROM target_view), otherwise this would be trivial to solve. In other words, I want to retrieve and recreate the "composite" schema of target_view as if it were a table.
I do not want the output of SHOW CREATE VIEW, as this only shows a complex SELECT statement with joins to various tables on remote I have limited ability to access. And a problem I'm seeing in MySQL 8.x is when I run SHOW CREATE TABLE on the view, this command simply acts as an alias of SHOW CREATE VIEW (which is reasonable).
Frustratingly, I can run DESCRIBE and see the schema of these views as they were tables. I really just need to convert this information into a CREATE TABLE statement without actually being able to run CREATE TABLE.
In case it weren't obvious, the key is to avoid manual reconstruction of these views' tabular schemas (as they may change in the future). I also want to avoid the solution of reverse engineering a generic table construction of 20-30 generic VARCHAR or TEXT columns from a CSV dump.
I don't know of any way to display the metadata of a result set in CREATE TABLE syntax.
What I would do given your circumstance is first create on your local MySQL instance the base table and the view, then you can use the CREATE TABLE AS SELECT ... syntax to produce a concrete table to match the metadata of the view result set.
Is there a difference between a schema and a database in MySQL? In SQL Server, a database is a higher level container in relation to a schema.
I read that Create Schema and Create Database do essentially the same thing in MySQL, which leads me to believe that schemas and databases are different words for the same objects.
As defined in the MySQL Glossary:
In MySQL, physically, a schema is synonymous with a database. You can substitute the keyword SCHEMA instead of DATABASE in MySQL SQL syntax, for example using CREATE SCHEMA instead of CREATE DATABASE.
Some other database products draw a distinction. For example, in the Oracle Database product, a schema represents only a part of a database: the tables and other objects owned by a single user.
Depends on the database server. MySQL doesn't care, its basically the same thing.
Oracle, DB2, and other enterprise level database solutions make a distinction. Usually a schema is a collection of tables and a Database is a collection of schemas.
Refering to MySql documentation,
CREATE DATABASE creates a database with the given name. To use this
statement, you need the CREATE privilege for the database. CREATE
SCHEMA is a synonym for CREATE DATABASE as of MySQL 5.0.2.
PostgreSQL supports schemas, which is a subset of a database:
https://www.postgresql.org/docs/current/static/ddl-schemas.html
A database contains one or more named schemas, which in turn contain
tables. Schemas also contain other kinds of named objects, including
data types, functions, and operators. The same object name can be used
in different schemas without conflict; for example, both schema1 and
myschema can contain tables named mytable. Unlike databases, schemas
are not rigidly separated: a user can access objects in any of the
schemas in the database they are connected to, if they have privileges
to do so.
Schemas are analogous to directories at the operating system level, except that schemas cannot be nested.
In my humble opinion, MySQL is not a reference database. You should never quote MySQL for an explanation. MySQL implements non-standard SQL and sometimes claims features that it does not support. For example, in MySQL, CREATE schema will only create a DATABASE. It is truely misleading users.
This kind of vocabulary is called "MySQLism" by DBAs.
in MySQL schema is synonym of database.
Its quite confusing for beginner people who jump to MySQL and very first day find the word schema, so guys nothing to worry as both are same.
When you are starting MySQL for the first time you need to create a database (like any other database system) to work with so you can CREATE SCHEMA which is nothing but CREATE DATABASE
In some other database system schema represents a part of database or a collection of Tables, and collection of schema is a database.
Yes, people use these terms interchangeably with regard to MySQL. Though oftentimes you will hear people inappropriately refer to the entire database server as the database.
Simply if you are thinking or discussing about Mysql. Then take a simple answer
"SCHEMA & DATABASE are exactly the same thing, just a synthetic
sugar in mysql."
Just add some more info:
MongoDB also distinguish schema from database.
schema represent the tables, which means the structure of database.
Microsoft SQL Server for instance, Schemas refer to a single user and is another level of a container in the order of indicating the server, database, schema, tables, and objects.
For example, when you are intending to update dbo.table_a and the syntax isn't full qualified such as
UPDATE table.a the DBMS can't decide to use the intended table. Essentially by default the DBMS will utilize myuser.table_a
not like Postgres, SQL server schema is set of database have same thing
but in mysql schema and database it is the same
MySQL does not support the concept of schema. In MySQL, schema and schemas are synonyms for database and databases.
When a user connects to MySQL, they don't connect to a specific database. Instead, they can access any table they have permissions for
I have a dataset with a lot of columns I want to import into a MySQL database, so I want to be able to create tables without specifying the column headers by hand. Rather I want to supply a filename with the column labels in it to (presumably) the MySQL CREATE TABLE command. I'm using standard MySQL Query Browser tools in Ubuntu, but I didn't see in option for this in the create table dialog, nor could I figure out how to write a query to do this from the CREATE TABLE documentation page. But there must be a way...
A CREATE TABLE statement includes more than just column names
Table name*
Column names*
Column data types*
Column constraints, like NOT NULL
Column options, like DEFAULT, character set
Table constraints, like PRIMARY KEY* and FOREIGN KEY
Indexes
Table options, like storage engine, default character set
* mandatory
You can't get all this just from a list of column names. You should write the CREATE TABLE statement yourself.
Re your comment: Many software development frameworks support ways to declare tables without using SQL DDL. E.g. Hibernate uses XML files. YAML is supported by Rails ActiveRecord, PHP Doctrine and Perl's SQLFairy. There are probably other tools that use other format such as JSON, but I don't know one offhand.
But eventually, all these "simplified" interfaces are no less complex to learn as SQL, while failing to represent exactly what SQL does. See also The Law of Leaky Abstractions.
Check out SQLFairy, because that tool might already convert from files to SQL in a way that can help you. And FWIW MySQL Query Browser (or under its current name, MySQL Workbench) can read SQL files. So you probably don't have to copy & paste manually.
I got a case where I have several databases running on the same server. There's one database for each client (company1, company2 etc). The structure of each of these databases should be identical with the same tables etc, but the data contained in each db will be different.
What I want to do is keep a master db that will contain no data, but manage the structure of all the other databases, meaning if I add, remove or alter any tables in the master db the changes will also be mirrored out to the other databases.
Example: If a table named Table1 is created in the master DB, the other databases (company1, company2 etc) will also get a table1.
Currently it is done by a script that monitors the database logs for changes made to the master database and running the same queries on each of the other databases. Looked into database replication, but from what I understand this will also bring along the data from the master database, which is not an option in this case.
Can I use some kind of logic against database schemas to do it?
So basicly what I'm asking here is:
How do I make this sync happen in the best possible way? Should I use a script monitoring the logs for changes or some other method?
How do I avoid existing data getting corrupted if a table is altered? (data getting removed if a table is dropped is okay)
Is syncing from a master database considered a good way to do what I wish (having an easy maintainable structure across several datbases)?
How will making updates like this affect the performance of the databases?
Hope my question was clear and that this is not a duplicate of some other thread. If more information and/or a better explantion of my problem is needed, let me know:)
You can get the list of tables for a given schema using:
select TABLE_NAME from information_schema.tables where TABLE_SCHEMA='<master table name>';
Use this list for a script or stored procedure ala:
create database if not exists <name>;
use <name>;
for each ( table_name in list )
create table if not exists <name>.table_name like <master_table>.table_name;
Now that Im thinking about it you might be able to put a trigger on the 'information_schema.tables' db that would call the 'create/maintain' script. Look for inserts and react accordingly.
I would like to select data from a second MySQL database in order to migrate data from one server to another.
I'm looking for syntax like
SELECT * FROM username:password#serverip.databaseName.tableName
Is this possible? I would be able to do this in Microsoft SQL Server using linked servers, so I'm assuming it's possible in MySQL as well.
You can create a table using FEDERATED storage engine:
CREATE TABLE tableName (id INT NOT NULL, …)
ENGINE=FEDERATED
CONNECTION='mysql://username:password#serverip/databaseName/tableName'
SELECT *
FROM tableName
Basically, it will serve as a view over the remote tableName.
There are generally two approaches you can take, although neither of them sound like what you're after:
Use replication and set up a master/slave relationship between the two databases.
Simply dump the data (using the command line mysqldump tool) from the 1st database and import it into the 2nd.
However, both of these will ultimately migrate all of the data (i.e.: not a subset), although you can specify specific table(s) via mysqldump. Additionally, if you use the mysqldump approach and you're not using InnoDB you'll need to ensure that the source database isn't in use (i.e.: has integrity) when the dump is created.
You can't do this directly, but as someone else alluded to in a comment, you can use mysqldump to export the contents of a table as a SQL script.
At that point you could run the script on the new server to create the table, or if more manipulation of the data is required, import that data into a table with a different name on the new server, then write a query to copy the data from there.