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SQL-Invoked Procedures and Functions
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As I mentioned earlier, an SQL-invoked routine can be either an SQL-invoked procedure or an SQL-invoked function (or, in the case of user data types, an SQL-invoked method). SQL-invoked procedures and functions are similar in many ways, although there are some basic differences. Table 13-1 provides an overview of the main differences and similarities. The easiest way to distinguish between SQL-invoked procedures and functions is to think of a procedure as a set of one or more stored SQL statements, similar to how a view stores a SELECT statement (as described in 5) and to think of a function as a type of operation that returns a value, similar to set functions such as SUM or AVG (as described in 10).
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There are many similarities between the syntax used to create procedures and that used to create functions. In fact, they re defined as one syntactic element in SQL:2006. In addition, the syntax
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Invoked from SQL statements, not from a programming language. Can be written in SQL or another programming language. Invoked by using the CALL statement. Support input and output parameters, although neither is required.
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Functions
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Invoked from SQL statements, not from a programming language. Can be written in SQL or another programming language. Invoked as a value in an expression. Support input parameters, although none are required. You cannot define output or input /output parameters for a function. The function returns a single output value.
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Table 13-1 Differences and similarities of SQL procedures and functions
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is, at its most basic level, similar to how procedures are created in most SQL implementations. Let s take a look at the syntax for each one to better understand their basic elements.
Using the CREATE PROCEDURE Statement
The first syntax we ll look at is that for creating a procedure. At its most basic, the CREATE PROCEDURE statement looks like the following: CREATE PROCEDURE <procedure name> ( [ <parameter declaration> [ { , <parameter declaration> } . . . ] ] ) [ <routine characteristic> . . . ] <routine body> As you can see, you must provide a name for the procedure in the CREATE PROCEDURE clause followed by zero or more parameter declarations, which are enclosed in parentheses. If no declarations are defined, you must still provide the parentheses. If more than one declaration is defined, you must separate them using commas. Following the parameter declarations, you have the option of defining one or more routine characteristics. For example, you can specify whether the routine is an SQL routine or one written in another language such as C or Java.
NOTE
The type of routine characteristics that you can define vary greatly among the SQL implementations, not only in terms of which options are supported, but also with regard to how they re defined. Consequently, I will keep my discussion of these options short, so be sure to check the product documentation for more information. For example, the procedural extensions in Oracle are defined using a language that Oracle calls PL/SQL, while in SQL Server and Sybase, the procedural extensions are part of a language called Transact-SQL, both of which are significantly different from the SQL standard. On the other hand, MySQL and DB2 generally follow the SQL standard in defining functions and stored procedures.
13:
Creating SQL-Invoked Routines
After you ve defined the procedure s characteristics, you re ready to add the SQL statements, which are represented by the <routine body> placeholder. Many of the statements you ll use in this section will be similar to those you ve already seen in this book. However, the SQL/PSM standard introduced new language elements that make procedures more dynamic. As we continue through this chapter, we ll look at many of these elements and how they re used to extend the functionality of SQL-invoked procedures.
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