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Working with Host Variables
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In the preceding examples, the SQL statements that we executed were relatively straightforward because no host variables were used in the statement. However, if you plan to pass host variable values into or out of an SQL statement, you must take an extra step to bind those host variables to the SQL statement. For example, suppose you want to set up a DELETE statement that takes an input variable identifying the row to be deleted. Your Prepare( ) function would be similar to the following:
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SQLPrepare ( hstmt, "DELETE FROM CD_INVENTORY WHERE CD_ID = ", SQL_NTS );
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Accessing SQL Data from Your Host Program
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Notice that a question mark is used to indicate the position of the variable. The question mark is used in place of the host variable. This is often called a placeholder. Once you ve prepared your SQL statement, you must now bind the host variable to the statement. To do so in a C program, you must use a BindParameter( ) function that identifies the statement handle, the position of the host variable within the SQL statement, the name of the host variable, and a number of other arguments, as shown in the following example:
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SQLBindParameter ( hstmt, 1, SQL_PARAMETER_MODE_IN, SQL_INT, SQL_INT, 4, 0, &v_CD_ID, 4, &ind_CD_ID );
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As you can see, the BindParameter( ) function takes 10 arguments. Table 17-2 lists the arguments used in the preceding example and provides a description of each of those arguments. If more than one host variable is included in your SQL statement, a BindParameter( ) function statement should be defined for each variable, and the position (the second argument) should be incremented by one for each additional variable. Once you bind the host variables to your SQL statement, you can execute the statement by using the Execute( ) function.
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Example
hstmt 1 SQL_PARAMETER_MODE_IN SQL_INT SQL_INT 4 0 &v_CD_ID 4 &ind_CD_ID
Description
Identifies the statement handle that provides the context for the SQL statement execution Identifies the position of the host variable in the SQL statement Specifies whether the host variable is an in, out, or in/out variable Identifies the data type of the value supplied Identifies the data type of the host variable Specifies the column size of the host variable Specifies the number of digits to the right of the decimal required by the host variable Identifies the name of the host variable, as declared in the host program Specifies the length in octets (bytes) of the host variable Identifies the name of the indicator variable, as declared by the host program
Table 17-2 Arguments Used in the BindParameter( ) Function
SQL: A Beginner s Guide
Retrieving SQL Data
Up to this point, the SQL statements that we ve executed in the CLI environment have not returned any data. However, you ll often run into situations where your program will need to query the database and process the values that are returned by that query. As a result, you ll need some sort of mechanism to bind the output from your query to variables that you declared in the host language. For example, suppose that you want to execute the following SELECT statement:
SQLExecDirect ( hstmt, "SELECT CD_NAME, IN_STOCK FROM CD_INVENTORY", SQL_NTS );
As you can see, the statement will return a list of CD_NAME values and IN_STOCK values from the CD_INVENTORY table. In order to deal with those values, you must bind them to the applicable host variables. To do this in a C program, you should use the BindCol( ) function. The BindCol( ) function is a little simpler than the BindParameter( ) function and takes only six arguments, as shown in the following example:
SQLBindCol ( hstmt, 1, SQL_CHAR, &v_CD_NAME, 60, &ind_CD_NAME ); SQLBindCol ( hstmt, 2, SQL_INT, &v_IN_STOCK, 5, &ind_IN_STOCK );
Table 17-3 lists the arguments used in the first statement of this example and provides a description of each of those arguments. Notice that two function statements have been defined, one for each column retrieved by the SELECT statement. You must define a function statement for each column that is listed in the SELECT clause of the SELECT statement. Once you bind the column values to the host variables, you can use those variables in the host program to process the data within the program as necessary.
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