August 2024
Intermediate to advanced
380 pages
10h 16m
Spanish
Content preview from Aprender SQL, 3ª Edición
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Capítulo 9. Subconsultas
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Las subconsultas son una potente herramienta que puedes utilizar en las cuatro sentencias de datos SQL. En este capítulo, te mostraré cómo se pueden utilizar las subconsultas para filtrar datos, generar valores y construir conjuntos de datos temporales. Después de experimentar un poco, creo que estarás de acuerdo en que las subconsultas son una de las características más potentes del lenguaje SQL.
¿Qué es una subconsulta?
Una subconsulta es una consulta contenida dentro de otra sentencia SQL (a la que me referiré como sentencia contenedora para el resto de esta discusión). Una subconsulta siempre va entre paréntesis y suele ejecutarse antes de la sentencia contenedora. Como cualquier consulta, una subconsulta devuelve un conjunto de resultados que puede estar formado por:
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Una sola fila con una sola columna
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Varias filas con una sola columna
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Varias filas con varias columnas
El tipo de conjunto de resultados devuelto por la subconsulta determina cómo puede utilizarse y qué operadores puede utilizar la sentencia contenedora para interactuar con los datos que devuelve la subconsulta. Cuando la sentencia contenedora ha terminado de ejecutarse, los datos devueltos por cualquier subconsulta se descartan, haciendo que una subconsulta actúe como una tabla temporal con ámbito de sentencia (lo que significa que el servidor libera cualquier ...
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