October 2024
Intermediate to advanced
974 pages
27h 14m
Spanish
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Capítulo 16. Uso de uniones y subconsultas
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16.0 Introducción
La mayoría de las consultas de los capítulos anteriores utilizaban una sola tabla, pero para cualquier aplicación dede complejidad incluso moderada, es probable que necesites utilizar varias tablas. Algunas preguntas simplemente no pueden responderse utilizando una sola tabla, y la verdadera potencia de una base de datos relacional entra en juego cuando combinas la información de múltiples fuentes:
Combina filas de tablas para obtener una información más completa que la que puede obtenerse de las tablas individuales por sí solas
Mantener resultados intermedios para una operación de varias etapas
Modificar filas de una tabla a partir de la información de otra
Este capítulo se centra en dos tipos de sentencias que utilizan varias tablas: las uniones entre tablas y las subconsultas que anidan una SELECT dentro de otra. Abarca los siguientes temas:
- Comparar tablas para encontrar coincidencias o discordancias
Para resolver estos problemas, debes saber qué tipos de uniones aplica . Las uniones internas muestran las filas de una tabla que coinciden con las de otra. Las uniones externas muestran las filas que coinciden, pero también encuentran filas de una tabla que no coinciden con filas de otra.
- Eliminar filas no coincidentes
Si dos conjuntos de datos están relacionados, pero de forma imperfecta, puedes determinar qué ...
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