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Databases - Practical PostgreSQL
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Using Functions

To use a function in a SQL statement, type the function's name, followed by its list of parameters (called arguments ), if any. The arguments passed to a function are enclosed in parentheses. There are two general styles of entering arguments: the standard SQL92 functions are generally implemented so that they accept their arguments delimited by special SQL keywords, such as FROM, FOR, and USING. PostgreSQL-style functions, on the other hand, accept arguments delimited by commas (which you might expect if you have experience with a programming language such as C).

Arguments may be constants, valid identifiers, or expressions. The particular arguments you need to pass to a function will depend completely on the function being used, and its requirements: especially with regards to data types. With a couple of exceptions, all functions require the open and closing parentheses following the function name, even if no arguments are passed.

 ( { 
 } [...] )
 [, ...] )

Note: The exceptions to the parenthetical function syntax are the SQL92 functions current_date, current_time, and current_timestamp. These lack parentheses to remain compatible with the SQL92 specification.

A powerful use of functions is that they may be nested, provided that the data type returned by a nested function is compatible with the argument accepted by the function it is nested within. Functions may be nested to any depth:

 [, ...] ) [, ...] )

PostgreSQL defines a rich set of functions for its built-in data types. To view a complete list of functions available, execute the \df slash command within psql . PostgreSQL also supports extensibility of its function set through the CREATE FUNCTION command. See Chapter 7 for more on this topic.

Note: The default name for a column that is described by a function in the target list will be the name of the function, without trailing parentheses, or arguments (e.g., to_char).

Databases - Practical PostgreSQL
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