编写查询

SQL 入门

Jasmin Ludolf

Senior Data Science Content Developer, DataCamp

别名

使用别名重命名列

SELECT name AS first_name, year_hired
FROM employees;

| first_name | year_hired |
|------------|------------|
| Darius     | 2020       |
| Raven      | 2017       |
| Eduardo    | 2022       |
| Maggie     | 2021       |
| Amy        | 2020       |
| Meehir     | 2021       |

选择去重记录

SELECT year_hired
FROM employees;

| year_hired |
|------------|
| 2020       |
| 2017       |
| 2022       |
| 2021       |
| 2020       |
| 2021       |

SELECT DISTINCT year_hired
FROM employees;

| year_hired |
|------------|
| 2020       |
| 2017       |
| 2022       |
| 2021       |

多列使用 DISTINCT

employees 表

SELECT dept_id, year_hired
FROM employees;

| dept_id | year_hired |
|---------|------------|
| 1       | 2020       |
| 2       | 2017       |
| 2       | 2022       |
| 3       | 2021       |
| 2       | 2020       |
| 3       | 2021       |

多列使用 DISTINCT

SELECT DISTINCT dept_id, year_hired
FROM employees;

| dept_id | year_hired |
|---------|------------|
| 1       | 2020       |
| 2       | 2017       |
| 2       | 2022       |
| 3       | 2021       |
| 2       | 2020       |

视图

视图是由已保存的 SQL SELECT 语句生成的虚拟表
访问时，视图会随底层数据的更新而自动更新

CREATE VIEW employee_hire_years AS
SELECT id, name, year_hired
FROM employees;

使用视图

SELECT id, name
FROM employee_hire_years;

| id    | name    |
|-------|---------|
| 54378 | Darius  |
| 94722 | Raven   |
| 45783 | Eduardo |
| 90123 | Maggie  |
| 67284 | Amy     |
| 26148 | Meehir  |

Let's practice!

SQL 入门