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SQL: A Beginner s Guide
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The v_CD_ID variable in the WHERE clause is an input host variable, and the v_CD_ NAME and v_IN_STOCK variables in the INTO clause are output host variables. The only difference is in how they are used by the SQL statement they are defined in exactly the same manner within the host language.
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Handling Null Values
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In 4, I discussed null values and how they re used to represent unknown or unavailable data. As you ll recall from that discussion, most SQL columns, by default, permit null values, although you can override the default by defining a NOT NULL constraint on the column. However, if you don t override the default and null values are permitted, you can run into a problem with the host language because most application programming languages do not support null values. To work around this issue, SQL allows you to declare indicator host variables. An indicator host variable is a type of variable that accompanies a regular host variable, also referred to as a data host variable. For simplicity, I refer to these two types of host variables as just indicator variables and data variables. The indicator variable contains a value that specifies whether or not the value in the associated data variable is null. Indicator variables are declared in the same way as other host variables. Let s take a look at an example of indicator variables to illustrate how they work. In the following embedded SELECT statement, an indicator variable has been added to each of the data variables in the INTO clause:
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EXEC SQL SELECT INTO FROM WHERE CD_NAME, IN_STOCK :v_CD_NAME :ind_CD_NAME, :v_IN_STOCK :ind_IN_STOCK CD_INVENTORY CD_ID = :v_CD_ID;
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The INTO clause includes two indicator variables: ind_CD_NAME and ind_IN_STOCK. Notice that each indicator variable follows the associated data variable. The placement of the indicator variable is the only indication the SQL implementation has that a particular host variable is an indicator variable. There is nothing in the variable declaration or naming that distinguishes indicator variables from data variables. When the implementation sees that one variable follows the other and that no comma separates the two, the implementation assigns a value of 0 to the indicator variable if the associated variable contains a real value (is not null). If the associated variable contains a null value, the implementation assigns a value of -1 to the indicator variable. The host program then takes the appropriate action based on this information. It is essential that the host program test the indicator variables for null values before attempting to use the values in the host variable because when nulls are encountered, the value of the corresponding host variable is not changed and thus could contain leftover values from a previous statement or unknown data that programmers call garbage.
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Accessing SQL Data from Your Host Program
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When you declare an indicator variable, be sure to use a numeric data type that supports the 0 and -1 values. The value 0 is logically a false condition because all bits will be set to a binary 0, while the value -1 is logically a true condition because all bits will be set to a binary 1.
Error Handling
When you embed SQL statements into your host language, you should provide a way to take specific actions if you receive error or warning messages when you try to access data. SQL provides a relatively straightforward method that you can use to monitor errors and warnings and take actions depending on the results of that monitoring. By embedding WHENEVER statements in your host language, you can provide your program with an effective level of error handling that works alongside your other embedded statements. The WHENEVER statement includes two sets of options, as shown in the following syntax: WHENEVER { SQLEXCEPTION | SQLWARNING | NOT FOUND } { CONTINUE | GOTO <target> } As you can see, you must first specify the WHENEVER keyword and then specify the necessary options. The first set of options indicates the condition to which the WHENEVER statement applies. If that condition is met, a specified action is taken. A WHENEVER statement can include one of three conditions:
SQLEXCEPTION The condition is met whenever an SQL statement generates an exception. For example, an exception might be generated when you try to insert invalid data into a column. SQLWARNING The condition is met whenever an SQL statement generates a warning. For example, a statement might generate a warning if a number has been rounded off. NOT FOUND The condition is met whenever a SELECT statement cannot return data in its query results. This can apply to a singleton SELECT statement or to a FETCH statement at the end of the cursor s result set.
Once you specify a condition in your WHENEVER statement, you must specify an action. The WHENEVER statement supports two actions:
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