An explicit local connection string consists of the path + filename specification in the native format of the filesystem used on the server machine, for example

Many clients also allow relative path strings (e.g. “..\examples\empbuild\employee.fdb”) but you should use them with caution, as it's not always obvious how they will be expanded. Getting an error message is annoying enough, but applying changes to another database than you thought you were connected to may be disastrous.

Instead of a file path, the local connection string may also be a database alias that is defined in aliases.conf, as mentioned earlier. The format of the alias depends only on how it's defined in the aliases file, not on the server filesystem. Examples are:

A TCP/IP connection string consists of:

Examples:

Notice how the aliased connection strings don't give any clue about the server OS. And they don't have to, either: you talk to a Linux Firebird server just like you talk to a Windows Firebird server. In fact, specifying an explicit database path is one of the rare occasions where you have to be aware of the difference.

Please note that some third-party client programs may have different requirements for the composition of connection strings. Refer to their documentation or online help to find out.

Firebird ships with a text-mode client named isql (Interactive SQL utility). You can use it in several ways to connect to a database. One of them, shown below, is to start it in interactive mode. Go to the bin subdirectory of your Firebird installation and type isql (Windows) or ./isql (Linux) at the command prompt.

[In the following examples, ↵ means “hit Enter”]

C:\Program Files\Firebird\Firebird_2_0\bin>isql↵
Use CONNECT or CREATE DATABASE to specify a database
SQL>CONNECT ..\examples\empbuild\employee.fdb user SYSDBA password masterkey;↵

At this point, isql will inform you that you are connected:

Database:  ..\examples\empbuild\employee.fdb, User: sysdba
SQL>

You can now continue to play about with the employee.fdb database. With isql you can query data, get information about the metadata, create database objects, run data definition scripts and much more.

To get back to the command prompt, type:

SQL>QUIT;↵

You can also type EXIT instead of QUIT, the difference being that EXIT will first commit any open transactions, making your modifications permanent.

GUI client tools usually take charge of composing the CONNECT string for you, using server, path (or alias), user name and password information that you type into prompting fields. Use the elements as described in the preceding topic.

To create a database interactively using the isql command shell, get to a command prompt in Firebird's bin subdirectory and type isql (Windows) or ./isql (Linux):

C:\Program Files\Firebird\Firebird_2_0\bin>isql↵
Use CONNECT or CREATE DATABASE to specify a database

Now you can create your new database interactively. Let's suppose that you want to create a database named test.fdb and store it in a directory named data on your D drive:

SQL>CREATE DATABASE 'D:\data\test.fdb' page_size 8192↵
CON>user 'SYSDBA' password 'masterkey';↵

The database will be created and, after a few moments, the SQL prompt will reappear. You are now connected to the new database and can proceed to create some test objects in it.

But to verify that there really is a database there, let's first type in this query:

SQL>SELECT * FROM RDB$RELATIONS;↵

Although you haven't created any tables yet, the screen will fill up with a large amount of data! This query selects all of the rows in the system table RDB$RELATIONS, where Firebird stores the metadata for tables. An “empty” database is not really empty: it contains a number of system tables and other objects. The system tables will grow as you add more user objects to your database.

To get back to the command prompt type QUIT or EXIT, as explained in the section on connecting.

Firebird accords with the SQL standard by truncating the result (quotient) of an integer/integer calculation to the next lower integer. This can have bizarre results unless you are aware of it.

For example, this calculation is correct in SQL:

If you are upgrading from an RDBMS which resolves integer/integer division to a float quotient, you will need to alter any affected expressions to use a float or scaled numeric type for either dividend, divisor, or both.

For example, the calculation above could be modified thus in order to produce a non-zero result:

In SQL, NULL is not a value. It is a condition, or state, of a data item, in which its value is unknown. Because it is unknown, NULL cannot behave like a value. When you try to perform arithmetic on NULL, or involve it with values in other expressions, the result of the operation will almost always be NULL. It is not zero or blank or an “empty string” and it does not behave like any of these values.

Below are some examples of the types of surprises you will get if you try to perform calculations and comparisons with NULL.

The following expressions all return NULL:

You might have expected 6 from the first expression and “Home sweet” from the third, but as we just said, NULL is not like the number 0 or an empty string – it's far more destructive!

The following expression:

will return NULL if either FirstName or LastName is NULL. Otherwise it will nicely concatenate the two names with a space in between – even if any one of the variables is an empty string.

Now let's examine some PSQL (Procedural SQL) examples with if-constructs:

if (a is not distinct from b) then
  MyVariable = 'Equal';
else
  MyVariable = 'Not equal';

if (a is distinct from b) then
  MyVariable = 'Not equal';
else
  MyVariable = 'Equal';