我创建了一个在 MySQL Workbench 中可以正常工作的查询，但是当我在 Eclipse 中运行它时却无法工作。与数据库的连接工作正常，因为通过更简单的查询，我得到了正确的结果。
java代码：
String queryAlter = "SELECT count(*) ";
queryAlter += "INTO @exist ";
queryAlter += "FROM information_schema.columns ";
queryAlter += "WHERE table_schema = database() ";
queryAlter += "and COLUMN_NAME = 'question" + (100 + number + 1)+"' ";
queryAlter += "AND table_name = '"+ tableName+"'; ";
queryAlter += "set @query = IF(@exist <= 0, 'alter table "+ tableName+" add column question" + (100 + number + 1)+" varchar(2048) NULL', ";
queryAlter += "'select \\'Column Exists\\' status'); ";
queryAlter += "use joomla30; ";
queryAlter += "prepare stmt from @query; ";
queryAlter += "EXECUTE stmt;";
我收到这条消息：
发生错误。可能用户/密码无效 com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException：您的 SQL 语法有错误；检查与您的 MySQL 服务器版本相对应的手册，了解在 'set @query = IF(@exist <= 0, 'alter table n0x3c_twoquestions_table add column qu' at line 1
附近使用的正确语法)查询字符串是：
SELECT count(*) INTO @exist FROM information_schema.columns WHERE table_schema = database() and COLUMN_NAME = 'question101' AND table_name = 'n0x3c_twoquestions_table'; set @query = IF(@exist <= 0, 'alter table n0x3c_twoquestions_table add column question101 varchar(2048) NULL', 'select \'Column Exists\' status'); use joomla30; prepare stmt from @query; EXECUTE stmt;
我也在java中尝试过这个，我从MySQL查询在PhpMyAdmin中工作，但在JAVA Eclipse中不工作：
String s1 = "SELECT count(*) INTO @exist FROM information_schema.columns WHERE table_schema = database() and COLUMN_NAME = 'question" + (100 + number + 1)+"' AND table_name = '"+ tableName+"'; ";
String s2 = "set @query = IF(@exist <= 0, 'alter table "+ tableName+" add column question" + (100 + number + 1)+" varchar(2048) NULL', 'select \\'Column Exists\\' status'); ";
String s3 = "use joomla30; ";
String s4 = "prepare stmt from @query; ";
stmt.addBatch(s1);
stmt.addBatch(s2);
stmt.addBatch(s3);
stmt.addBatch(s4);
stmt.executeBatch();
conn1.commit();
但是我有这个错误：
java.sql.BatchUpdateException：无法通过executeUpdate()发出SELECT。
您可以使用两个不同的查询。
首先是检查该列是否存在：
String countQuery = "select count(*) from information_schema.columns WHERE table_schema = database() and COLUMN_NAME = 'question106' and table_name='n0x3c_twoquestions_table'";
然后如果
countQueryalterQueryString alterQuery = "alter table n0x3c_twoquestions_table add column question106 varchar(2048) NULL";
如果你不想使用两个不同的查询，你可以通过异常捕获来处理：
String alterQuery = "alter table n0x3c_twoquestions_table add column question106 varchar(2048) NULL";
try (Connection conn = ds.getConnection(); PreparedStatement ps = conn.prepareStatement(alterQuery)) {
ps.execute();
} catch (SQLException e) {
if (e.getErrorCode() == 1060) {
columnExists = true;
} else {
// TODO handle exception
}
}
我找到了错误代码MySQL服务器错误代码。
@WebServlet("/uploadServlet")
@MultipartConfig(maxFileSize = 160000) // upload file's size up to 16MB /
// 16000000 byte
public class FileUploadToDataBaseServlet extends HttpServlet {
/*
* FileUploadToDataBaseServlet: files upload to database with Servlet, JSP
* and MySQL. Creating MySQL database table. Coding upload form page. Coding
* file upload servlet. Coding message page. Testing the application and
* verifying file stored in database.
*/
private String databaseURL = "jdbc:mysql://localhost:3306/MyDatabase";
private String databaseUser = "root";
private String databasePassWord = "2888";
/*
* This method is called by servlet service method to handle the POST
* request from client. The HTTP POST method allows the client to send data
* of unlimited length to the Web server a single time and is useful when
* posting information to the server.
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// gets values of text fields
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
String phone = request.getParameter("phone");
String email = request.getParameter("email");
String country = request.getParameter("country");
String photo = request.getParameter("photo");
// InputStream's are used for reading byte based data
InputStream inputStream = null; // input stream of the upload file
// obtains the upload file part in this multipart request
Part filePart = request.getPart("photo");
if (filePart != null) {
// prints out some information for debugging
System.out.println(filePart.getName());
System.out.println(filePart.getSize());
System.out.println(filePart.getContentType());
// obtains input stream of the upload file
inputStream = filePart.getInputStream();
}
Connection connection = null; // connection to the database
String message = null; // message will be sent back to client
try {
// connects to the database
DriverManager.registerDriver(new com.mysql.jdbc.Driver());
connection = DriverManager.getConnection(databaseURL, databaseUser, databasePassWord);
// constructs SQL statement
String sql = "INSERT INTO registration (first_name, last_name, phone, email, country, photo )"
+ "values (?, ?, ?, ?, ?, ?)";
// The performance of the application will be faster if you use
// PreparedStatement,
// which gives better performance than Statement class.
PreparedStatement statement = connection.prepareStatement(sql);
statement.setString(1, firstName);
statement.setString(2, lastName);
statement.setString(3, phone);
statement.setString(4, email);
statement.setString(5, country);
if (inputStream != null) {
// bring input stream of the upload file for the blob column
statement.setBlob(6, inputStream);
}
// sends the statement to the database server
int row = statement.executeUpdate();
if (row > 0) {
message = "File uploaded and saved into database";
}
} catch (SQLException exception) {
message = "ERROR: " + exception.getMessage();
exception.printStackTrace();
} finally {
if (connection != null) {
// closes the database connection
try {
connection.close();
} catch (SQLException exception) {
exception.printStackTrace();
}
}
// sets the message in request scope
request.setAttribute("Message", message);
// forwards message to the page
getServletContext().getRequestDispatcher("/Alert.html").forward(request, response);
}
}