How to write junit test cases for if and else statements in Java?

Question

Here assuming this to be my simple method I want to write unit test cases covering if and else statement both ..

 public String getdata() {
        String existsQuery = "select ID from DEF where " +
        " NAME = ? "  ; 
        if (data) {
            if (existsQuery.indexOf("?") > 1) {
                existsQuery = existsQuery + " and ";
            } else {
                existsQuery = existsQuery + " where ";
            } 
            existsQuery = existsQuery + "ID = ?";
        }
        return existsQuery;
    }

I thought to call the call by creating object and assert but this is not the right way I think can someone help me with this ..Thanks in advance

import org.junit.Test;

import com.planlytx.gson.pojo.FilterDef;

import static org.junit.Assert.*;

public class FilterdefUnitTest {

    @Test
    public void testConcatenate() {
        Myunit myUnit = new Myunit();

        String result = myUnit.existSQL();

        assertEquals("select ...................", result);
    }
}