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In Transact-SQL, a statement block has this simple structure:
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/* Transact-SQL block of statements */ begin /* Sequence of SQL statements appears here */ . . . end
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The sole function of the BEGIN END pair is to create a statement block; they do not impact the scope of local variables or other database objects. The Transact-SQL procedure definition, conditional execution and looping constructs, and others, are all designed to operate with single SQL statements, so statement blocks are frequently used in each of these contexts to group statements together as a single unit. In Informix SPL, a statement block includes not only a statement sequence, but may optionally declare local variables for use within the block and exception handlers to handle errors that may occur within the block. Here is the structure of an Informix SQL statement block:
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/* Informix SPL block of statements */ /* Declaration of any local variables */ define . . . /* Declare handling for exceptions */ on exception . . . /* Define the sequence of SQL statements */ begin. . . end
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The variable declaration section is optional; we have already seen an example of it in the Informix stored procedure body in Figure 20-5. The exception-handling section is also optional; its role is described later in the Handling Error Conditions section. The BEGIN END sequence performs the same function as it does for Transact-SQL. Informix also allows a single statement to appear in this position, if the block consists of just the other two components and a single SQL or SPL statement. The Informix SQL language structures don t require the use of statement blocks as often as the Transact-SQL structures. In the Informix dialect, the looping conditional execution statements each include an explicit termination (IF END IF, WHILE END WHILE, FOR END FOR). Within the structure, a single SQL statement or a sequence of statements (each ending with a semicolon) may appear. As a result, an explicit block structure is not always needed simply to group together a sequence of SQL statements.
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The Oracle PL/SQL block structure has the same capabilities as the Informix structure. It offers the capability to declare variables and exception conditions, using this format:
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/* Oracle PL/SQL statement block */ /* Declaration of any local variables */ declare . . . /* Specify the sequence of statements */ begin . . . /* Declare handling for exceptions */ exception . . . end;
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All three sections of the block structure are optional. It s common to see the structure used with only the BEGIN END sequence to define a statement sequence, or with a DECLARE BEGIN END sequence to declare variables and a sequence of statements. As with Informix, the Oracle structures that specify conditional execution and looping have a self-defining end-of-statement marker, so sequences of statements within these structures do not necessarily need an explicit BEGIN END statement block structure.
Returning a Value
In addition to stored procedures, most SPL dialects support a stored function capability. The distinction is that a stored function returns a value while a stored procedure does not. Here s a simple example of a stored function. Assume you want to define a stored procedure that, given a customer number, calculates the total current order amount for that customer. If you define the procedure as a function, the total amount can be returned as its value. Figure 20-7 shows an Oracle stored function that calculates the total amount of current orders for a customer, given the customer number. Note the RETURNS clause in the procedure definition, which tells the DBMS the data type of the value being returned. In most DBMS products, if you enter a function call via the interactive SQL capability, the function value is displayed in response. Within a stored procedure, you can call a stored function and use its return value in calculations or store it in a variable. Many SPL dialects also allow you to use a stored function as a user-defined function within SQL value expressions. This is true of the Oracle PL/SQL dialect, so this use of the function defined in Figure 20-7 within a search condition is legal:
SELECT FROM WHERE AND COMPANY, NAME CUSTOMERS, SALESREPS CUST_REP = EMPL_NUM GET_TOT_ORDS(CUST_NUM) > 10000.00
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