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Add Input Parameters to Your Procedures
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The NEW_AGE_CDS procedure that we looked at in the previous examples can be very handy because it saves you having to create an SQL statement each time you want to view information about New Age CDs. However, in order to return information about other types of CDs, such as Blues or Country, you must create a new query or set up a procedure for the specific type of music. But there is another alternative. You can create a procedure that does not specifically define the music type but instead allows you to enter that type whenever you call that procedure. That way, you need only one procedure to check any desired type of music. To support this type of procedure, you must declare a parameter within the procedure definition that allows the procedure to accept input values when you call it. Let s return to the CD_INVENTORY table and CD_TYPES table shown in Figure 13-1. If we modify the
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Creating SQL-Invoked Routines
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language of the procedure we created earlier, we can create a new procedure that includes the necessary input parameter, as shown in the following CREATE PROCEDURE statement:
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CREATE PROCEDURE CDS_BY_TYPE ( IN p_CD_Type CHAR(20) ) SELECT CD_TITLE, CD_STOCK FROM CD_INVENTORY i, CD_TYPES t WHERE i.CD_TYPE_ID = t.CD_TYPE_ID AND CD_TYPE_NAME = p_CD_Type;
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In the first line of code, a parameter is defined after the CREATE PROCEDURE clause. The parameter declaration includes the IN keyword, the name of the parameter (p_CD_Type), and the data type for that parameter (CHAR(20)), all of which are enclosed in parentheses.
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The p_ convention used to name the parameters is not necessary. However, I like to use some type of naming convention to set parameters apart, making them easier to pick out in the code.
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SQL supports three types of parameters: input, output, and input/output. The three types are represented by the parameter mode keywords IN, OUT, and INOUT, respectively. Input parameters allow you to provide values when you invoke a procedure. Those values are then used within the routine body when the SQL statements are executed. Output parameters allow your procedure to provide values as a result of invoking the procedure. Input/output parameters are those that provide the functionality of both input and output parameters. You do not have to specify one of the parameter mode keywords when you define your parameters. However, if you don t specify one of the keywords, SQL assumes that you re defining an input parameter.
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As with many other aspects of the CREATE PROCEDURE statement, parameter declarations can vary from product to product. In SQL Server, for example, parameter names must be preceded by the at (@) symbol, as in @p_CD_Type, the parameter declarations are not enclosed in parentheses, and the IN keyword is not used. Oracle, on the other hand, does not require the at symbol and does use parentheses. Oracle also uses the IN keyword, but it is positioned after the name of the parameter, as in p_CD_Type IN CHAR (20).
Now let s return to the CDS_BY_TYPE procedure that is defined in the previous CREATE PROCEDURE statement. Once you define your input parameter, you ll want to use it in some meaningful way within the routine body. In this case, the p_CD_Type parameter is used in the second predicate in the WHERE clause (CD_TYPE_NAME = p_CD_Type). This means that the value you enter when you invoke the procedure is compared to the CD_TYPE_NAME values of the CD_TYPES table when the SELECT statement is executed. As a result, your query results will include CD information about the specified music type.
SQL: A Beginner s Guide
Once you create your procedure, you can invoke it by using a CALL statement that specifies a value for the parameter. For example, if you want to return information about Folk Rock CDs, you can use the following CALL statement:
CALL CDS_BY_TYPE('Folk Rock');
Notice that you include the value for the parameter in parentheses after the name of the procedure. The value must conform to the data type assigned to the parameter, which in this case is CHAR(20). As with any other instance in which you re working with character string values, you must enclose the value in single quotes. When you invoke this procedure, the Folk Rock value is inserted into the predicate in the WHERE clause and the procedure returns the following query results:
CD_TITLE -------------------Famous Blue Raincoat CD_STOCK -------19
As you can see, you now have a procedure that you can use to return CD information on any music type. You simply provide the name of the music type when you call the procedure. However, procedures are not limited to only one parameter. You can include multiple parameters in any procedure definition. For example, suppose you want to modify the preceding procedure definition to allow you to enter an amount. You want to use that amount to return CD information for only those CDs with a CD_STOCK value that exceeds the specified amount. At the same time, you still want to return CD information for only the specified music type. As a result, you need to define two parameters, as shown in the following CREATE PROCEDURE statement:
CREATE PROCEDURE CDS_BY_TYPE ( IN p_CD_Type CHAR(20), IN p_Amount INT ) SELECT CD_TITLE, CD_STOCK FROM CD_INVENTORY i, CD_TYPES t WHERE i.CD_TYPE_ID = t.CD_TYPE_ID AND CD_TYPE_NAME = p_CD_Type AND CD_STOCK > p_Amount;
Notice that the parameter declaration clause now includes two input parameters: p_CD_ Type and p_Amount. The p_Amount parameter is configured with the INT data type. The p_Amount parameter, like the p_CD_Type parameter, is used in a predicate in the WHERE clause (CD_STOCK > p_Amount). As a result, the rows returned by the procedure must include CD_STOCK values greater than the amount specified when calling the procedure. Once you ve created the procedure, you can call it by using a CALL statement that includes values for both parameters, as shown in the following example:
CALL CDS_BY_TYPE('New Age', 5);
Now your CALL statement includes two values (separated by a comma) within the parentheses. The values must be listed in the order in which the parameters are defined in the CREATE PROCEDURE statement. When you invoke this statement, the p_C_Type is
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