从两个表中选择相同的列值

Question

I am doing one project on struts in which i want to compare two table's row values and want to get matched columns as result set and from result set i want to take out no of matched columns. I am doing the following coding but i always gets 3 as columnsNumber. Please help me to get exact result.

public class ResultDAO {


 public int correctAnswer() throws Exception {

     int columnsNumber=0;

     Connection con=null;       
     PreparedStatement pstmt = null;

     ResultSet rs = null;
     String query="SELECT DISTINCT q1,q2,q3 FROM test WHERE (q1,q2,q3)  IN (SELECT q1,q2,q3 FROM result)ORDER BY q1,q2,q3 ASC";
     try {

         con=DatabaseConnection.getConnection();

         pstmt = con.prepareStatement(query);

         rs = pstmt.executeQuery();

         ResultSetMetaData rsmd = (ResultSetMetaData) rs.getMetaData();

         columnsNumber = rsmd.getColumnCount();
         System.out.println(columnsNumber);


    } 
     catch (Exception e) {
         System.out.println("exception in DAO");
    }
    return columnsNumber;


 }
}

Table1 : test

columns: id, q1, q2, q3 values : 1,A,C,C

Table2 : result

columns: id, q1, q2, q3 values : 1,A,B,C

I am using this code but getting value as null

public class ResultDAO {

 public String correctAnswer() throws Exception {

     String columnsNumber = null;

     Connection con=null;       
     PreparedStatement pstmt = null;

     ResultSet rs = null;
     String query="SELECT test.id,IF (test.q1 = result.q1, test.q1, NULL) as q1,IF (test.q2 = result.q2, test.q2, NULL) as q2,IF (test.q3 = result.q3, test.q3, NULL) as q3,(test.q1 = result.q1) + (test.q2 = result.q2) + (test.q3 = result.q3) as matchedColumns FROM test INNER JOIN result USING (id) ORDER BY q1,q2,q3";
     try {

         con=DatabaseConnection.getConnection();

         pstmt = con.prepareStatement(query);

         rs = pstmt.executeQuery();

         System.out.println(rs.next());
         while(rs.next()){
        columnsNumber=rs.getString("matchedColumns");


         }
         System.out.println(columnsNumber);


    } 
     catch (Exception e) {
         System.out.println("exception in DAO");
    }
    return columnsNumber;


 }

}

Answer 1

Please try this sqlFiddle

SELECT test.id,
       IF (test.q1 = result.q1, test.q1, NULL) as q1,
       IF (test.q2 = result.q2, test.q2, NULL) as q2,
       IF (test.q3 = result.q3, test.q3, NULL) as q3,
       (test.q1 = result.q1) + (test.q2 = result.q2) + (test.q3 = result.q3) as matchedColumns
FROM test
INNER JOIN result
USING (id)
ORDER BY q1,q2,q3

edit: to get unmatched columns, just change the = sign to != like this and add another created column named unmatchedColumns sqlFiddle

SELECT test.id,
       IF (test.q1 = result.q1, test.q1, NULL) as q1,
       IF (test.q2 = result.q2, test.q2, NULL) as q2,
       IF (test.q3 = result.q3, test.q3, NULL) as q3,
       (test.q1 = result.q1) + (test.q2 = result.q2) + (test.q3 = result.q3) as matchedColumns,
       (test.q1 != result.q1) + (test.q2 != result.q2) + (test.q3 != result.q3) as unmatchedColumns
FROM test
INNER JOIN result
USING (id)
ORDER BY q1,q2,q3

Answer 2

I am assuming id is foreign key in result table

 select tst.id,tst.q1,tst.q2,tst.q3,
    r.id,r.q1,r.q2,r.q3
     from test as tst
    LEFT JOIN result AS r
    ON tst.id = r.id