# 175. Combine Two Tables

> Nguồn: https://tiennhm.io.vn/en/docs/leetcode/database/175-combine-two-tables
> Write a solution to report the first name, last name, city, and state of each person in the `Person` table. If the address of a `personId` is not present in the `Address` table, report null instead.

## Giới thiệu bài toán

Chi tiết: https://leetcode.com/problems/combine-two-tables

Table: `Person`

| Column Name | Type    |
| ----------- | ------- |
| personId    | int     |
| lastName    | varchar |
| firstName   | varchar |

- `personId` is the primary key (column with unique values) for this table.
- This table contains information about the ID of some persons and their first and last names.

Table: `Address`

| Column Name | Type    |
| ----------- | ------- |
| addressId   | int     |
| personId    | int     |
| city        | varchar |
| state       | varchar |

- `addressId` is the primary key (column with unique values) for this table.
- Each row of this table contains information about the city and state of one person with ID = PersonId.

> **Tip: Yêu cầu**
>
> Write a solution to report the first name, last name, city, and state of each person in the `Person` table. If the address of a `personId` is not present in the `Address` table, report null instead.
>
> Return the result table in any order. The result format is in the following example.
Example 1:

- Input:
`Person` table:

| personId | lastName | firstName |
| -------- | -------- | --------- |
| 1        | Wang     | Allen     |
| 2        | Alice    | Bob       |

`Address` table:

| addressId | personId | city          | state      |
| --------- | -------- | ------------- | ---------- |
| 1         | 2        | New York City | New York   |
| 2         | 3        | Leetcode      | California |

- Output:

| firstName | lastName | city          | state    |
| --------- | -------- | ------------- | -------- |
| Allen     | Wang     | Null          | Null     |
| Bob       | Alice    | New York City | New York |

- Explanation:
There is no address in the address table for the personId = 1 so we return null in their city and state.
addressId = 1 contains information about the address of personId = 2.

## Giải quyết bài toán

```sql
SELECT
    p.firstName,
    p.lastName,
    a.city,
    a.state
FROM
    Person p LEFT JOIN Address a
    ON p.personId = a.personId;
```

Tham khảo: https://leetcode.com/submissions/detail/1039116460/