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1.4. Accessing a Database

Once you have created a database, you can access it by:

  • Running the PostgreSQL interactive terminal program, called psql, which allows you to interactively enter, edit, and execute SQL commands.

  • Using an existing graphical frontend tool like PgAccess or ApplixWare (via ODBC) to create and manipulate a database. These possibilities are not covered in this tutorial.

  • Writing a custom application, using one of the several available language bindings. These possibilities are discussed further in The PostgreSQL Programmer's Guide.

You probably want to start up psql, to try out the examples in this tutorial. It can be activated for the mydb database by typing the command:

$ psql mydb

If you leave off the database name then it will default to your user account name. You already discovered this scheme in the previous section.

In psql, you will be greeted with the following message:

Welcome to psql, the PostgreSQL interactive terminal.
 
Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help on internal slash commands
       \g or terminate with semicolon to execute query
       \q to quit
 
mydb=>

The last line could also be

mydb=#

That would mean you are a database superuser, which is most likely the case if you installed PostgreSQL yourself. Being a superuser means that you are not subject to access controls. For the purpose of this tutorial this is not of importance.

If you have encountered problems starting psql then go back to the previous section. The diagnostics of psql and createdb are similar, and if the latter worked the former should work as well.

The last line printed out by psql is the prompt, and it indicates that psql is listening to you and that you can type SQL queries into a work space maintained by psql. Try out these commands:

mydb=> SELECT version();
                            version
----------------------------------------------------------------
 PostgreSQL 7.2devel on i586-pc-linux-gnu, compiled by GCC 2.96
(1 row)

mydb=> SELECT current_date;
    date
------------
 2001-08-31
(1 row)

mydb=> SELECT 2 + 2;
 ?column?
----------
        4
(1 row)

The psql program has a number of internal commands that are not SQL commands. They begin with the backslash character, "\". Some of these commands were listed in the welcome message. For example, you can get help on the syntax of various PostgreSQL SQL commands by typing:

mydb=> \h

To get out of psql, type

mydb=> \q

and psql will quit and return you to your command shell. (For more internal commands, type \? at the psql prompt.) The full capabilities of psql are documented in the Reference Manual. If PostgreSQL is installed correctly you can also type man psql at the operating system shell prompt to see the documentation. In this tutorial we will not use these features explicitly, but you can use them yourself when you see fit.

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