Firebird 5.0 Language Reference

Distinct subsets of SQL apply to different areas of activity. The SQL subsets in Firebird’s language implementation are:

Dynamic SQL is the major part of the language which corresponds to Part 2 (SQL/Foundation) of the SQL specification. DSQL represents statements passed by client applications through the public Firebird API and processed by the database engine.

Procedural SQL augments Dynamic SQL to allow compound statements containing local variables, assignments, conditions, loops and other procedural constructs. PSQL corresponds to Part 4 (SQL/PSM) of the SQL specifications. PSQL extensions are available in persistent stored modules (procedures, functions and triggers), and in Dynamic SQL as well (see EXECUTE BLOCK).

Embedded SQL is the SQL subset supported by Firebird gpre, the application which allows you to embed SQL constructs into your host programming language (C, C++, Pascal, Cobol, etc.) and preprocess those embedded constructs into the proper Firebird API calls.

Interactive ISQL refers to the language that can be executed using Firebird isql, the command-line application for accessing databases interactively. As a regular client application, its native language is DSQL. It also offers a few additional commands that are not available outside its specific environment.

Both DSQL and PSQL subsets are completely presented in this reference. Neither ESQL nor ISQL flavours are described here unless mentioned explicitly.

For ISQL, consult the manual Firebird Interactive SQL Utility.

SQL dialect is a term that defines the specific features of the SQL language that are available when accessing a database. SQL dialects can be defined at the database level and specified at the connection level. Three dialects are available:

Processing of every SQL statement either completes successfully or fails due to a specific error condition. Error handling can be done both client-side in the application, or server-side using PSQL.

The 16-bit SMALLINT data type is for compact data storage of integer data for which only a narrow range of possible values is required. Numbers of the SMALLINT type are within the range from -2¹⁶ to 2¹⁶ - 1, that is, from -32,768 to 32,767.

The INTEGER — or INT — data type is a 32-bit integer. Numbers of the INTEGER type are within the range from -2³² to 2³² - 1, that is, from -2,147,483,648 to 2,147,483,647.

BIGINT is a 64-bit integer data type, available only in Dialect 3.

Numbers of the BIGINT type are within the range from -2⁶³ to 2⁶³ - 1, or from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807.

INT128 is a 128-bit integer data type. This type is not defined in the SQL standard.

Numbers of the INT128 type are within the range from -2¹²⁷ to 2¹²⁷ - 1.

Constants of integer types can be specified in a hexadecimal format by means of 1 to 8 digits for INTEGER, 9 to 16 hexadecimal digits for BIGINT, and 10 to 32 hexadecimal digits for INT128. Hex representation for writing to SMALLINT is not explicitly supported, but Firebird will transparently convert a hex number to SMALLINT if necessary, provided it falls within the ranges of negative and positive SMALLINT.

The usage and numerical value ranges of hexadecimal notation are described in more detail in the discussion of number constants in the chapter entitled Common Language Elements.

The hexadecimal INTEGERs in the above example are automatically cast to BIGINT before being inserted into the table. However, this happens after the numerical value is determined, so 0x80000000 (8 digits) and 0x080000000 (9 digits) will be stored as different BIGINT values.

Approximate floating-point values are stored in an IEEE 754 binary format that comprises sign, exponent and mantissa. Precision is dynamic, corresponding to the physical storage format of the value, which is exactly 4 bytes for the FLOAT type and 8 bytes for DOUBLE PRECISION.

Considering the peculiarities of storing floating-point numbers in a database, these data types are not recommended for storing monetary data. For the same reasons, columns with floating-point data are not recommended for use as keys or to have uniqueness constraints applied to them.

For testing data in columns with floating-point data types, expressions should check using a range, for instance, BETWEEN, rather than searching for exact matches.

When using these data types in expressions, extreme care is advised regarding the rounding of evaluation results.

Decimal floating-point values are stored in an IEEE 754 decimal format that comprises sign, exponent and coefficient. Contrary to the approximate floating-point data types, precision is either 16 or 34 decimal digits.

Further to the explanation above, Firebird will store NUMERIC data according the declared precision and scale. Some more examples are:

The storage format in the database for DECIMAL is similar to NUMERIC, with some differences that are easier to observe with the help of some more examples:

The DATE data type in Dialect 3 stores only date without time. The available range for storing data is from January 01, 1 to December 31, 9999.

In Dialect 1, DATE is an alias for TIMESTAMP. Dialect 1 has no “date-only” type.

For a bare TIME, WITHOUT TIME ZONE is assumed.

The TIME data type is available in Dialect 3 only. It stores the time of day within the range from 00:00:00.0000 to 23:59:59.9999.

If you need to get the time-part from DATE in Dialect 1, you can use the EXTRACT function.

See also the EXTRACT() function in the chapter entitled Built-in Scalar Functions.

For a bare TIMESTAMP, WITHOUT TIME ZONE is assumed.

The TIMESTAMP data type is available in Dialect 3 and Dialect 1. It comprises two 32-bit integers — a date-part and a time-part — to form a structure that stores both date and time-of-day. In Dialect 1, DATE is an alias for TIMESTAMP.

The EXTRACT function works equally well with TIMESTAMP as with the Dialect 1 DATE type.

As the name implies, the session time zone can be different for each database attachment. It can be set explicitly in the DPB with the item isc_dpb_session_time_zone; otherwise, by default, it uses the same time zone as the operating system of the Firebird server process. This default can be overridden in firebird.conf, setting DefaultTimeZone.

Subsequently, the time zone can be changed to a given time zone using a SET TIME ZONE statement or reset to its original value with SET TIME ZONE LOCAL or ALTER SESSION RESET.

A time zone is specified as a string, either a time zone region (for example, 'America/Sao_Paulo') or a displacement from GMT in hours:minutes (for example, '-03:00').

A time/timestamp with time zone is considered equal to another time/timestamp with time zone if their conversions to UTC are equivalent. For example, time '10:00 -02:00' and time '09:00 -03:00' are equivalent, since both are the same as time '12:00 GMT'.

The method of storing date and time values makes it possible to involve them as operands in some arithmetic operations. In storage, a date value or date-part of a timestamp is represented as the number of days elapsed since “date zero” — November 17, 1858 — whilst a time value or the time-part of a timestamp is represented as the number of deci-milliseconds (100 microseconds) since midnight.

An example is to subtract an earlier date, time or timestamp from a later one, resulting in an interval of time, in days and fractions of days.

DATEADD, DATEDIFF

Firebird provides a number of features to discover time zone information.

Most current development tools support Unicode, implemented in Firebird with the character sets UTF8 and UNICODE_FSS. UTF8 comes with collations for many languages. UNICODE_FSS is more limited and was previously used mainly by Firebird internally for storing metadata. Keep in mind that one UTF8 character occupies up to 4 bytes, thus limiting the size of CHAR fields to 8,191 characters (32,767/4).

The UTF8 character set implemented in Firebird supports the latest version of the Unicode standard, thus recommending its use for international databases.

While working with strings, it is essential to keep the character set of the client connection in mind. If there is a mismatch between the character sets of the stored data and that of the client connection, the output results for string columns are automatically re-encoded, both when data are sent from the client to the server and when they are sent back from the server to the client. For example, if the database was created in the WIN1251 encoding but KOI8R or UTF8 is specified in the client’s connection parameters, the mismatch will be transparent.

The character set NONE is a special character set in Firebird. It can be characterized such that each byte is a part of a string, but the string is stored in the system without any clues about what constitutes any character: character encoding, collation, case, etc. are simply unknown. It is the responsibility of the client application to deal with the data and provide the means to interpret the string of bytes in some way that is meaningful to the application and the human user.

Data in OCTETS encoding are treated as bytes that may not be interpreted as characters. OCTETS provides a way to store binary data. The database engine has no concept of what it is meant to do with a string of bytes in OCTETS, other than store and retrieve it. Again, the client side is responsible for validating the data, presenting them in formats that are meaningful to the application and its users and handling any exceptions arising from decoding and encoding them. CHAR and VARCHAR with character set OCTETS have synonyms BINARY and VARBINARY.

Each character set has a default collation (COLLATE) that specifies the collation order (or, collation sequence, or collating sequence). Usually, it provides nothing more than ordering based on the numeric code of the characters and a basic mapping of upper- and lower-case characters. If some behaviour is needed for strings that is not provided by the default collation and a suitable alternative collation is supported for that character set, a COLLATE collation clause can be specified in the column definition.

A COLLATE collation clause can be applied in other contexts besides the column definition. For comparison operations, it can be added in the WHERE clause of a SELECT statement. If output needs to be sorted in a special alphabetic sequence, or case-insensitively, and the appropriate collation exists, then a COLLATE clause can be included with the ORDER BY clause when rows are being sorted on a character field and with the GROUP BY clause in case of grouping operations.

The maximum length for an index key equals one quarter of the page size, i.e. from 1,024 — for page size 4,096 — to 8,192 bytes — for page size 32,768. The maximum length of an indexed string is 9 bytes less than that quarter-page limit.

The table below shows the maximum length of an indexed string (in characters), according to page size and character set, calculated using this formula.

CREATE DATABASE, Collation, SELECT, WHERE, GROUP BY, ORDER BY

The SQL-compliant BOOLEAN data type (8 bits) comprises the distinct truth values TRUE and FALSE. Unless prohibited by a NOT NULL constraint, the BOOLEAN data type also supports the truth value UNKNOWN as the null value. The specification does not make a distinction between the NULL value of this data type, and the truth value UNKNOWN that is the result of an SQL predicate, search condition, or Boolean value expression: they may be used interchangeably to mean the same thing.

As with many programming languages, the SQL BOOLEAN values can be tested with implicit truth values. For example, field1 OR field2 and NOT field1 are valid expressions.

The optional SUB_TYPE parameter specifies the nature of data written to the column. Firebird provides two pre-defined subtypes for storing user data:

It is also possible to add custom data subtypes, for which the range of enumeration from -1 to -32,768 is reserved. Custom subtypes enumerated with positive numbers are not allowed, as the Firebird engine uses the numbers from 2-upward for some internal subtypes in metadata. Custom subtype aliases can be inserted into the RDB$TYPES table by users with the system privilege CREATE_USER_TYPES.

The maximum size of a BLOB field depends on the page size of the database, whether the blob value is created as a stream blob or a segmented blob, and if segmented, the actual segment sizes used when populating the blob. For most built-in functions, the maximum size of a BLOB field is 4 GB, or data beyond the 4 GB limit is not addressable. For a page size of 4 KB (4096 bytes) the maximum size is slightly less than 4 GB.

Text BLOBs of any length and any character set — including multi-byte — can be operands for practically any statement or internal functions. The following operators are fully supported:

As an efficient alternative to concatenation, you can also use BLOB_APPEND() or the functions and procedures of system package RDB$BLOB_UTIL.

Partial support:

FILTER, DECLARE FILTER, BLOB_APPEND(), RDB$BLOB_UTIL

By default, dimensions are 1-based — subscripts are numbered from 1. To specify explicit upper and lower bounds of the subscript values, use the following syntax:

A new dimension is added using a comma in the syntax. In this example we create a table with a two-dimensional array, with the lower bound of subscripts in both dimensions starting from zero:

The database employee.fdb, found in the ../examples/empbuild directory of any Firebird distribution package, contains a sample stored procedure showing some simple work with arrays:

If the features described are enough for your tasks, you might consider using arrays in your projects. Currently, no improvements are planned to enhance support for arrays in Firebird.

The SQL_NULL type holds no data, but only a state: NULL or NOT NULL. It is not available as a data type for declaring table fields, PSQL variables or parameter descriptions. This data type exists to support the use of untyped parameters in expressions involving the IS NULL predicate.

An evaluation problem occurs when optional filters are used to write queries of the following type:

After processing, at the API level, the query will look like this:

This is a case where the developer writes an SQL query and considers :param1 as though it were a variable that they can refer to twice. However, at the API level, the query contains two separate and independent parameters. The server cannot determine the type of the second parameter since it comes in association with IS NULL.

The SQL_NULL data type solves this problem. Whenever the engine encounters an “? IS NULL” predicate in a query, it assigns the SQL_NULL type to the parameter, which will indicate that parameter is only about “nullness” and the data type or the value need not be addressed.

The following example demonstrates its use in practice. It assumes two named parameters — say, :size and :colour — which might, for example, get values from on-screen text fields or drop-down lists. Each named parameter corresponds with two positional parameters in the query.

Explaining what happens here assumes the reader is familiar with the Firebird API and the passing of parameters in XSQLVAR structures — what happens under the surface will not be of interest to those who are not writing drivers or applications that communicate using the “naked” API.

The application passes the parameterized query to the server in the usual positional ?-form. Pairs of “identical” parameters cannot be merged into one, so for the two optional filters in the example, four positional parameters are needed: one for each ? in our example.

After the call to isc_dsql_describe_bind(), the SQLTYPE of the second and fourth parameters will be set to SQL_NULL. Firebird has no knowledge of their special relation with the first and third parameters: that responsibility lies entirely on the application side.

Once the values for size and colour have been set (or left unset) by the user, and the query is about to be executed, each pair of XSQLVARs must be filled as follows:

In other words: The value compare parameter is always set as usual. The SQL_NULL parameter is set the same, except that sqldata remains null at all times.

The CAST function enables explicit conversion between many pairs of data types.

See also CAST() in Chapter Built-in Scalar Functions.

Implicit data conversion is not possible in Dialect 3 — the CAST function is almost always required to avoid data type clashes.

In Dialect 1, in many expressions, one type is implicitly cast to another without the need to use the CAST function. For instance, the following statement in Dialect 1 is valid:

The string literal will be cast to the DATE type implicitly.

In Dialect 3, this statement will raise error 35544569, “Dynamic SQL Error: expression evaluation not supported, Strings cannot be added or subtracted in dialect 3” — a cast will be needed:

Or, with a datetime literal:

In Dialect 1, mixing integer data and numeric strings is usually possible because the parser will try to cast the string implicitly. For example,

will be executed correctly.

In Dialect 3, an expression like this will raise an error, so you will need to write it as a CAST expression:

The exception to the rule is during string concatenation.

A domain definition has required and optional attributes. The data type is a required attribute. Optional attributes include:

Explicit Data Type Conversion for the description of differences in the data conversion mechanism when domains are specified for the TYPE OF and TYPE OF COLUMN modifiers.

While defining a column using a domain, it is possible to override some attributes inherited from the domain. Table 17 summarises the rules for domain override.

A domain is created with the DDL statement CREATE DOMAIN.

CREATE DOMAIN in the Data Definition (DDL) Statements chapter.

The scalar data types are simple data types that hold a single value. For reasons of organisation, the syntax of BLOB types are defined separately in BLOB Data Types Syntax.

The BLOB data types hold binary, character or custom format data of unspecified size. For more information, see Binary Data Types.

If the SUB_TYPE and CHARACTER SET clauses are absent, then subtype BINARY (or 0) is used. If the SUB_TYPE clause is absent and the CHARACTER SET clause is present, then subtype TEXT (or 1) is used.

The array data types hold multiple scalar values in a single or multi-dimensional array. For more information, see Array Types

A literal — or constant — is a value that is supplied directly in an SQL statement, not derived from an expression, a parameter, a column reference nor a variable. It can be a string or a number.

SQL operators comprise operators for comparing, calculating, evaluating and concatenating values.

A conditional expression is one that returns different values according to how a certain condition is met. It is composed by applying a conditional function construct, of which Firebird supports several. This section describes only one conditional expression construct: CASE. All other conditional expressions apply internal functions derived from CASE and are described in Conditional Functions.

NULL is not a value in SQL, but a state indicating that the value of the element either is unknown or it does not exist. It is not a zero, nor a void, nor an “empty string”, and it does not act like any value.

When you use NULL in numeric, string or date/time expressions, the result will always be NULL. When you use NULL in logical (Boolean) expressions, the result will depend on the type of the operation and on other participating values. When you compare a value to NULL, the result will be unknown.

Consult the Firebird Null Guide for more in-depth coverage of Firebird’s NULL behaviour.

A subquery is a special form of expression that is a query embedded within another query. Subqueries are written in the same way as regular SELECT queries, but they must be enclosed in parentheses. Subquery expressions can be used in the following ways:

A condition — or Boolean expression — is a statement about the data that, like a predicate, can resolve to TRUE, FALSE or NULL. Conditions consist of one or more predicates, possibly negated using NOT and connected by AND and OR operators. Parentheses may be used for grouping predicates and controlling evaluation order.

A predicate may embed other predicates. Evaluation sequence is in the outward direction, i.e. the innermost predicates are evaluated first. Each “level” is evaluated in precedence order until the truth value of the ultimate condition is resolved.

A comparison predicate consists of two expressions connected with a comparison operator. There are six traditional comparison operators:

For the complete list of comparison operators with their variant forms, see Comparison Operators.

If one of the sides (left or right) of a comparison predicate has NULL in it, the value of the predicate will be UNKNOWN.

This group of predicates includes those that use subqueries to submit values for all kinds of assertions in search conditions. Existential predicates are so called because they use various methods to test for the existence or non-existence of some condition, returning TRUE if the existence or non-existence is confirmed or FALSE otherwise.