Escribir consultas

Introducción a SQL

Jasmin Ludolf

Senior Data Science Content Developer, DataCamp

Alias

Usa alias para renombrar columnas

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       |

Seleccionar registros distintos

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       |

DISTINCT con varios campos

la tabla de empleados,

SELECT dept_id, year_hired
FROM employees;

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

DISTINCT con varios campos

SELECT DISTINCT dept_id, year_hired
FROM employees;

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

Las vistas

Una vista es una tabla virtual que es el resultado de una instrucción SQL guardada SELECT.
Cuando se accede a ellas, las vistas se actualizan automáticamente en respuesta a las actualizaciones de los datos subyacentes.

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

Uso de vistas

SELECT id, name
FROM employee_hire_years;

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

¡Vamos a practicar!

Introducción a SQL