A statement is most easily thought of as an SQL statement, such as SELECT * FROM Employee. However, a statement is more than just an SQL statement - it consists of all of the information associated with that SQL statement, such as any result sets created by the statement and parameters used in the execution of the statement. A statement does not even need to have an application-defined SQL statement. For example, when a catalog function such as SQLTables is executed on a statement, it executes a predefined SQL statement that returns a list of table names.
Each statement is identified by a statement handle. A statement is associated with a single connection, and there can be multiple statements on that connection. Some drivers limit the number of active statements they support; the SQL_MAX_CONCURRENT_ACTIVITIES option in SQLGetInfo specifies how many active statements a driver supports on a single connection. A statement is defined to be active if it has results pending, where results are either a result set or the count of rows affected by an INSERT, UPDATE, or DELETE statement, or data is being sent with multiple calls to SQLPutData.
Within a piece of code that implements ODBC (the Driver Manager or a driver), the statement handle identifies a structure that contains statement information, such as:
The statement's state
The current statement-level diagnostics
The addresses of the application variables bound to the statement's parameters and result set columns
The current settings of each statement attribute
Statement handles are used in most ODBC functions. Notably, they are used in the functions to bind parameters and result set columns (SQLBindParameter and SQLBindCol), prepare and execute statements (SQLPrepare, SQLExecute, and SQLExecDirect), retrieve metadata (SQLColAttribute and SQLDescribeCol), fetch results (SQLFetch), and retrieve diagnostics (SQLGetDiagField and SQLGetDiagRec). They are also used in catalog functions (SQLColumns, SQLTables, and so on) and a number of other functions.
Statement handles are allocated with SQLAllocHandle and freed with SQLFreeHandle.
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