如何从mysql读取并在csv文件中明智地编写列？

Question

I have different tables in a database(say mysql). i need to extract some columns from different table and write it in a csv file. consider table 1

A 1
B 2
C 3
table 2       table 3
X 7           AB A1
Y 8           BC B2
Z 10          CD C3
U 11          DE D4
V 12          
W 13

i want to write 1st column from table 1,2nd col from table 2, and 1st col from table 3 in a csv file such that empty rows are made null.

output: 
A,7,AB
B,8.BC
C,10,CD
null,11,DE
null,12,null
null,13,null 

i can do the basic reading and writing from mysql to csv, need help in the logic or code to get the above output. "Looking for a generic solution for say 'n' number of columns from 'n' number of tables". above is jus a example.

Answer 1

Since you just want the logic here is some pseudo code that may help you in whatever language you are using. Since I don't know how you are exporting to csv I made it pretty generic.

arr1 = select column1 from table1;
arr2 = select column2 from table2;
arr3 = select column1 from table3;
max = isBigger(arr1.length, arr2.length);
max = isBigger(max, arr3.length);
for(i=0; i<max; i++)
{
 if(arr1[i]=="") arr1[i]=null;
 if(arr2[i]=="") arr2[i]=null;
 if(arr3[i]=="") arr3[i]=null;
 print arr1[i] + "," + arr2[i] + "," + arr3[i];
}

Answer 2

I do not know how you read your database but assume you do it with JDBC:

    ResultSet tableAReusltSet= null;
    ResultSet tableBReusltSet= null;
    ResultSet tableCReusltSet= null;
    List<PseudeContainer> container = new ArrayList<>();
    while (tableAReusltSet.next()) { 
        PseudeConteiner ps = new PseudoContainer();
        ps.col1 = tableAReusltSet.getString("ColumnName");
        container.add(ps);
    }

    int i = 0;
    while (tableBReusltSet.next()) { 
        if(container.size() <= i){
             container.add(new PseudeContainer());
        }
        container.get(i).col2 = tableBReusltSet.getString("ColumnName");
    }

    i = 0;
    while (tableBReusltSet.next()) { 
        if(container.size() <= i){
             container.add(new PseudeContainer());
        }
        container.get(i).col2 = tableBReusltSet.getString("ColumnName");
    }

//.. now you have a collection to work with which you can write

public PseudeContainer {
    String col1 = null;
    String col2 = null;
    String col3 = null;

}

Above should work.. still pseudo code...

Answer 3

Recommend 3rd party libraries to handle CSV files:

Apache Commons CSV

Open CSV