Combine Two Tables

Solution For Combine Two Tables

The Combine Two Tables problem on Leetcode is a SQL problem that requires you to combine and present data from two tables. The problem statement is as follows:

“Table: Person

| Column Name | Type |
| PersonId | int |
| FirstName | varchar |
| LastName | varchar |
PersonId is the primary key column for this table.

Table: Address

| Column Name | Type |
| AddressId | int |
| PersonId | int |
| City | varchar |
| State | varchar |
AddressId is the primary key column for this table.

Write a SQL query for a report that provides the following information for each person in the Person table, regardless if there is an address for each of those people:

FirstName, LastName, City, State”

Now let’s see how we can solve this problem.


To combine data from two separate tables, we will use the LEFT JOIN clause in SQL. This will allow us to display all the records from the Person table and matching records from the Address table. However, these may not have corresponding records in the Address table.

To obtain the desired output, we will use the SELECT statement to retrieve the required columns from both tables and use the LEFT JOIN clause to join them. The final SQL query will look like this:

SELECT FirstName, LastName, City, State
FROM Person
ON Person.PersonId = Address.PersonId;

The above query will return all the rows from the Person table along with matching rows from the Address table. If there is no matching record in the Address table, the City and State fields will be null.

Note: In this example, Person.PersonId is the foreign key that connects the two tables.

The above SQL query will output the desired results. You can test this query on Leetcode’s SQL editor and the output will match the expected output.

In conclusion, combining data from multiple tables in SQL requires the use of JOIN clauses. In the Combine Two Tables problem on Leetcode, we use the LEFT JOIN clause to obtain data from both tables.

Step by Step Implementation For Combine Two Tables

 * This is the solution for the leetcode problem combine-two-tables

// import the required packages
import java.sql.*;

public class Solution {
    public void combineTables(int id, String name, int age, Date dob, String address) {
        // JDBC driver name and database URL
        final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
        final String DB_URL = "jdbc:mysql://localhost/leetcode";

        //  Database credentials
        final String USER = "root";
        final String PASS = "root";
        Connection conn = null;
        Statement stmt = null;
           //STEP 2: Register JDBC driver

           //STEP 3: Open a connection
           System.out.println("Connecting to database...");
           conn = DriverManager.getConnection(DB_URL,USER,PASS);

           //STEP 4: Execute a query
           System.out.println("Creating statement...");
           stmt = conn.createStatement();
           // create the insert query
           String sql = "INSERT INTO table1 " +
                        "VALUES (?, ?, ?, ?, ?)";
           // create the prepared statement
           PreparedStatement ps = conn.prepareStatement(sql);
           // set the values for the prepared statement
           ps.setInt(1, id);
           ps.setString(2, name);
           ps.setInt(3, age);
           ps.setDate(4, dob);
           ps.setString(5, address);
           // execute the insert query
           // close the prepared statement
           // close the statement
           // close the connection
        }catch(SQLException se){
           //Handle errors for JDBC
        }catch(Exception e){
           //Handle errors for Class.forName
           //finally block used to close resources
           }catch(SQLException se2){
           }// nothing we can do
           }catch(SQLException se){
           }//end finally try
        }//end try
    }//end main
}//end FirstExample
# SQL query to find the names of all students who are taking at least one course 
FROM Student s, Enroll e 
WHERE s.ID = e.StudentID
 * @param {number} n
 * @return {number}
var climbStairs = function(n) {
    // if n = 1, then there is only 1 way to climb the stairs
    if (n === 1) {
        return 1;
    // if n = 2, then there are 2 ways to climb the stairs    
    } else if (n === 2) {
        return 2;
    // if n > 2, then the number of ways to climb the stairs is equal to the sum of the number of ways to climb the stairs when n = n - 1 and n = n - 2    
    } else {
        return climbStairs(n - 1) + climbStairs(n - 2);
1. First, we need to create a new table to store the combined data from the two tables.
2. Then, we need to insert the data from the first table into the new table.
3. Finally, we need to insert the data from the second table into the new table.
* Definition for a binary tree node.
* public class TreeNode {
*     public int val;
*     public TreeNode left;
*     public TreeNode right;
*     public TreeNode(int x) { val = x; }
* }
public class Solution {
    public TreeNode MergeTrees(TreeNode t1, TreeNode t2) {
        //if t1 and t2 are both null, return null
        if(t1 == null && t2 == null)
            return null;
        //if t1 is null, return t2
        else if(t1 == null)
            return t2;
        //if t2 is null, return t1
        else if(t2 == null)
            return t1;
        //if t1 and t2 are not null, create a new node with the value of t1.val + t2.val
        TreeNode node = new TreeNode(t1.val + t2.val);
        //set the left child of the new node to the result of calling MergeTrees on t1.left and t2.left
        node.left = MergeTrees(t1.left, t2.left);
        //set the right child of the new node to the result of calling MergeTrees on t1.right and t2.right
        node.right = MergeTrees(t1.right, t2.right);
        //return the new node
        return node;

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