如何浏览MySQL更新查询中的所有记录？

Question

I have a java code where I need to read many lines from a text file and I need to update records with the read lines. For example, the text file contains: aaa, bbb, ccc, .. etc (the comma means new line), so, I want to update col4 in record1 with the value aaa, record2 with the value bbb, etc.

How can I make update statement that makes the update for every record automatically ??

This is my Java code:

counter=0;
        while((fileLine=in.readLine())!=null) 
        { 
            System.out.println("Line read is: "+fileLine);

                //execute db insertion
                try {
                    //insert in the database
                    String Query= "update db.table set col4=?";    //database
                    preparedStmt3 = DBConnection.con.prepareStatement(Query); 
                    preparedStmt3.setString (1, fileLine);
                    preparedStmt3.executeUpdate();
                    System.out.println("Complete update statement for row: "+counter);
                    counter++;
                } catch (Exception e) {
                    System.out.println("DB_Error:_"+ e.toString());
                }

        } //end while loop 

Answer 1

Note: As Andreas and Frank noted, your update statement looks a little incorrect. You seem to be missing a where clause from your update statement. This is because, you're trying to set only one parameter to your PreparedStatement. Ideally an update statement looks something like this:

UPDATE table_name
SET column1=?, column2=?,...
WHERE some_column=?

ie you at least need to have one or more columns in your where clause to identify which record or records that needs be updated. If you omit the WHERE clause, all records will be updated ( which is something that you might not want to do )

Also, as a performance improvement for huge data set, consider updating in batch. So here's what you would do:

• Create a batch size.
• For each line that you read from your file, add it to batch. This you can do by calling the addBatch() method of PreparedStatement
• Once you reach the batch size, execute the batch by calling executeBatch() . You would then clear you batch ( clearBatch() )and continue the process till you're done with reading all the lines from your file.

Something like this:

PreparedStatement preparedStmt3 = null;
try{
    counter=0;
    int batchCutoff = 1000, currentBatchSize = 0;

    Query= "update db.table set col4=?";    //database
    preparedStmt3 = DBConnection.con.prepareStatement(Query); 

    while((fileLine=in.readLine())!=null) 
    { 
        System.out.println("Line read is: "+fileLine);
        //execute db insertion
        try {
            //insert in the database
            preparedStmt3.setString (1, fileLine);
            preparedStmt3.addBatch();
            preparedStmt3.clearParameters();
            currentBatchSize++;

            if(currentBatchSize >= batchCutoff){
                preparedStmt3.executeBatch();
                preparedStmt3.clearBatch();
                System.out.println("Complete update statement for: "+currentBatchSize+" row(s)");
                currentBatchSize = 0;
            }
            counter++;
        } catch (Exception e) {
            System.out.println("DB_Error:_"+ e.toString());
        }
    } //end while loop 
    //In case the cut-off has not been reached and some statements in the batch are remaining
    try{
        preparedStmt3.executeBatch();
        preparedStmt3.clearBatch();
    }catch (Exception e) {
            System.out.println("DB_Error:_"+ e.toString());
    }finally{
        System.out.println("Total of: "+counter+" row(s) updated");
    }
}finally{
    if(preparedStmt3 != null){
        try{
            preparedStmt3.close();
        }catch(Exception exe){
        }
    }
}

Answer 2

Since you are using a counter you can use it in where clause

----->>counter=1;
    while((fileLine=in.readLine())!=null) 
    { 
        System.out.println("Line read is: "+fileLine);

            //execute db insertion
            try {
                //insert in the database
                String Query= "update db.table set col4=? where id=?";    //database
                preparedStmt3 = DBConnection.con.prepareStatement(Query); 
                preparedStmt3.setString (1, fileLine);
       ---->>   preparedStmt3.setInt(2,counter);
                preparedStmt3.executeUpdate();
                System.out.println("Complete update statement for row: "+counter);
                counter++;
            } catch (Exception e) {
                System.out.println("DB_Error:_"+ e.toString());
            }

    } //end while loop 

I have inserted the arrow where I have made changes