August 2024
Intermediate to advanced
380 pages
10h 16m
Spanish
Content preview from Aprender SQL, 3ª Edición
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Capítulo 8. Agrupación y agregados
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Por lo general, los datos se almacenan al nivel más bajo de granularidad que necesite cualquiera de los usuarios de una base de datos; si Chuck, de contabilidad, necesita ver las transacciones individuales de un cliente, entonces tiene que haber una tabla en la base de datos que almacene las transacciones individuales. Eso no significa, sin embargo, que todos los usuarios deban tratar los datos tal y como están almacenados en la base de datos. Este capítulo se centra en cómo pueden agruparse y agregarse los datos para que los usuarios puedan interactuar con ellos a un nivel de granularidad superior al almacenado en la base de datos.
Conceptos de agrupación
A veces querrás encontrar tendencias en tus datos que requerirán que el servidor de la base de datos cocine un poco los datos antes de que puedas generar los resultados que buscas. Por ejemplo, supongamos que te encargas de enviar cupones para alquileres gratuitos a tus mejores clientes. Podrías hacer una consulta sencilla para ver los datos sin procesar:
mysql> SELECT customer_id FROM rental; +-------------+ | customer_id | +-------------+ | 1 | | 1 | | 1 | | 1 | | 1 | | 1 | | 1 | ... | 599 | | 599 | | 599 | | 599 | | 599 | | 599 | +-------------+ 16044 rows ...
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