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Accessing SQL Data from Your Host Program
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After you establish the connection handle, you must explicitly connect to the database within the context of the handle. To do this, you must use the SQLConnect( ) function, which takes seven arguments: the connection handle, the target SQL server, the length of the server name, the connection user, the length of the username, the connection password, and the length of the password. For C strings, you can use SQL_NTS in place of length arguments to indicate that a length does not have to be specified for the preceding string, as shown in the following example:
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SQLConnect ( hdbc, server01, SQL_NTS, SaleMngr, SQL_NTS, SalesPW, SQL_NTS );
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As you can see, the function specifies the hdbc connection handle and the server01 SQL server. The connection will be established using the SaleMngr user account and the SalesPW password. Instead of specifying the length of any of the strings, SQL_NTS is used. Once you ve connected to the database, you can create statement handles and execute SQL statements.
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Establishing a Statement Handle
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In order to execute an SQL statement from within your host program, you must create a statement handle within the context of your connection handle. As with other types of handles, you can use the AllocHandle( ) function to establish the statement handle. As you have seen, the AllocHandle( ) function requires three arguments. In the case of a statement handle, those arguments are SQL_HANDLE_STMT, the host variable that identifies the connection handle, and the host variable that identifies the statement handle. The host variable that identifies the statement handle is preceded by an ampersand, as shown in the following example:
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SQLAllocHandle ( SQL_HANDLE_STMT, hdbc, &hstmt );
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In this function, the connection handle referenced is hdbc, and the variable identifying the statement handle is hstmt.
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Executing SQL Statements
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Now that you ve established your allocation handles and connected to the database, you can set up functions that allow you to execute the SQL statements. The CLI model supports two methods that you can use to execute SQL statements. The first is direct execution and the second is preparing the statement for later execution.
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Using the ExecDirect( ) Function
The first method you can use to execute an SQL statement is the ExecDirect( ) function. The function takes three arguments. The first argument is the name of the statement handle in whose context you ll be executing the statement. The second argument is the actual SQL statement, enclosed in double quotation marks. The third argument is the length of the statement. In a C program, you generally use SQL_NTS to indicate that you do not have to specify the length of the string.
SQL: A Beginner s Guide
Let s take a look at an example of the ExecDirect( ) function to demonstrate how you can use it to execute an SQL statement. The following function references the hstmt statement handle and defines a DELETE statement:
SQLExecDirect ( hstmt, "DELETE FROM CD_INVENTORY WHERE CD_ID = 5731", SQL_NTS );
As you can see, the SQL statement is passed as an argument to the CLI routine. In this case, any rows with a CD_ID value of 5731 will be deleted from the CD_INVENTORY table. Notice that the SQL_NTS value is used to indicate that you do not have to specify the length of the string (the actual SQL statement).
Using the Prepare( ) and Execute( ) Functions
Another method that you can use to execute a statement is to first prepare the statement and then later execute it. You would use this method if you need to execute your statement more than one time. The first function that you use in the two-step process is the Prepare( ) function, which requires the same three arguments as the ExecDirect( ) function, as shown in the following example:
SQLPrepare ( hstmt, "DELETE FROM CD_INVENTORY WHERE CD_ID = 5731", SQL_NTS );
Notice that we reference the same statement handle and define the same SQL statement as in the preceding ExecDirect( ) example. The only difference is that, in the case of the Prepare( ) function, the statement isn t actually executed, but is instead prepared for execution. When a statement must be executed multiple times, this process saves on overhead because the statement has to be analyzed and optimized only once. Once you ve prepared the statement, you can then use the Execute( ) function to execute the statement. The Execute( ) function takes only one argument the statement handle that contains the prepared statement, as shown in the following example:
SQLExecute ( hstmt );
Because the statement was already prepared, you merely need to reference the statement handle to execute the statement. You can execute the statement as often as necessary simply by invoking the Execute( ) function and specifying the statement handle.
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