AUTOCOMMIT 事务和 ODBC 事务

AUTOCOMMIT 连接属性控制 INSERT、ALTER、COPY 和其他数据处理语句是否在完成后自动提交。默认情况下，AUTOCOMMIT 已启用，所有语句将在执行后提交。这通常不是最佳设置，因为效率较低。此外，您通常想要控制是否作为一个整体提交一组语句，而非逐个提交语句。例如，您可能只想在所有插入都成功时才提交一系列插入。如果 AUTOCOMMIT 已禁用，则当其中一个语句失败时，您可以回退事务。

如果 AUTOCOMMIT 已打开，则会在执行语句之后立即提交这些语句的结果。不能回退在 AUTOCOMMIT 模式下执行的语句。

例如，如果 AUTOCOMMIT 已打开，将自动提交以下单个 INSERT 语句：

ret = SQLExecDirect(hdlStmt, (SQLCHAR*)"INSERT INTO customers VALUES(500,"
    "'Smith, Sam', '123-456-789');", SQL_NTS);

如果 AUTOCOMMIT 已关闭，您需要在执行语句之后手动提交事务。例如：

ret = SQLExecDirect(hdlStmt, (SQLCHAR*)"INSERT INTO customers VALUES(500,"
    "'Smith, Sam', '123-456-789');", SQL_NTS);
// Other inserts and data manipulations
// Commit the statements(s)
ret = SQLEndTran(SQL_HANDLE_DBC, hdlDbc, SQL_COMMIT);

只有在调用 SQLEndTran() 时，才会提交插入的行。提交事务之前，您可以随时回退 INSERT 和其他语句。

以下示例演示了关闭 AUTOCOMMIT 以及执行插入和手动提交事务。

// Some standard headers
#include <stdio.h>
#include <stdlib.h>
// Only needed for Windows clients
// #include <windows.h>
// Standard ODBC headers
#include <sql.h>
#include <sqltypes.h>
#include <sqlext.h>
int main()
{
    // Set up the ODBC environment
    SQLRETURN ret;
    SQLHENV hdlEnv;
    ret = SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &hdlEnv);
    if(!SQL_SUCCEEDED(ret)) {
        printf("Could not allocate a handle.\n");
        exit(EXIT_FAILURE);
    } else {
        printf("Allocated an environment handle.\n");
    }
    // Tell ODBC that the application uses ODBC 3.
    ret = SQLSetEnvAttr(hdlEnv, SQL_ATTR_ODBC_VERSION,
        (SQLPOINTER) SQL_OV_ODBC3, SQL_IS_UINTEGER);
    if(!SQL_SUCCEEDED(ret)) {
        printf("Could not set application version to ODBC3.\n");
        exit(EXIT_FAILURE);
    } else {
        printf("Set application to ODBC 3.\n");
    }
    // Allocate a database handle.
    SQLHDBC hdlDbc;
    ret = SQLAllocHandle(SQL_HANDLE_DBC, hdlEnv, &hdlDbc);
    if(!SQL_SUCCEEDED(ret)) {
        printf("Could not allocate database handle.\n");
        exit(EXIT_FAILURE);
    } else {
        printf("Allocated Database handle.\n");
    }
    // Connect to the database
    printf("Connecting to database.\n");
    const char *dsnName = "ExampleDB";
    const char* userID = "dbadmin";
    const char* passwd = "password123";
    ret = SQLConnect(hdlDbc, (SQLCHAR*)dsnName,
        SQL_NTS,(SQLCHAR*)userID,SQL_NTS,
        (SQLCHAR*)passwd, SQL_NTS);
    if(!SQL_SUCCEEDED(ret)) {
        printf("Could not connect to database.\n");
        exit(EXIT_FAILURE);
    } else {
        printf("Connected to database.\n");
    }
    // Get the AUTOCOMMIT state
    SQLINTEGER  autoCommitState;
    SQLGetConnectAttr(hdlDbc, SQL_ATTR_AUTOCOMMIT, &autoCommitState, 0, NULL);
    printf("Autocommit is set to: %d\n", autoCommitState);


    // Disable AUTOCOMMIT
    printf("Disabling autocommit.\n");
    ret = SQLSetConnectAttr(hdlDbc, SQL_ATTR_AUTOCOMMIT, SQL_AUTOCOMMIT_OFF,
        SQL_NTS);
    if(!SQL_SUCCEEDED(ret)) {
        printf("Could not disable autocommit.\n");
        exit(EXIT_FAILURE);
    }

    // Get the AUTOCOMMIT state again
    SQLGetConnectAttr(hdlDbc, SQL_ATTR_AUTOCOMMIT, &autoCommitState, 0, NULL);
    printf("Autocommit is set to: %d\n", autoCommitState);

    // Set up a statement handle
    SQLHSTMT hdlStmt;
    SQLAllocHandle(SQL_HANDLE_STMT, hdlDbc, &hdlStmt);


    // Create a table to hold the data
    SQLExecDirect(hdlStmt, (SQLCHAR*)"DROP TABLE IF EXISTS customers",
        SQL_NTS);
    SQLExecDirect(hdlStmt, (SQLCHAR*)"CREATE TABLE customers "
        "(CustID int, CustName varchar(100), Phone_Number char(15));",
        SQL_NTS);


    // Insert a single row.
    ret = SQLExecDirect(hdlStmt, (SQLCHAR*)"INSERT INTO customers VALUES(500,"
        "'Smith, Sam', '123-456-789');", SQL_NTS);
    if(!SQL_SUCCEEDED(ret)) {
        printf("Could not perform single insert.\n");
    } else {
        printf("Performed single insert.\n");
    }


    // Need to commit the transaction before closing, since autocommit is
    // disabled. Otherwise SQLDisconnect returns an error.
    printf("Committing transaction.\n");
    ret =  SQLEndTran(SQL_HANDLE_DBC, hdlDbc, SQL_COMMIT);
    if(!SQL_SUCCEEDED(ret)) {
        printf("Error committing transaction.\n");
        exit(EXIT_FAILURE);
    }

    // Clean up
    printf("Free handles.\n");
    ret = SQLDisconnect(hdlDbc);
    if(!SQL_SUCCEEDED(ret)) {
        printf("Error disconnecting from database. Transaction still open?\n");
        exit(EXIT_FAILURE);
    }
    SQLFreeHandle(SQL_HANDLE_STMT, hdlStmt);
    SQLFreeHandle(SQL_HANDLE_DBC, hdlDbc);
    SQLFreeHandle(SQL_HANDLE_ENV, hdlEnv);
    exit(EXIT_SUCCESS);
}

运行以上代码后，将生成以下输出：

Allocated an environment handle.
Set application to ODBC 3.
Allocated Database handle.
Connecting to database.
Connected to database.
Autocommit is set to: 1
Disabling autocommit.
Autocommit is set to: 0
Performed single insert.
Committing transaction.
Free handles.