August 2024
Intermediate to advanced
380 pages
10h 16m
Spanish
Content preview from Aprender SQL, 3ª Edición
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Capítulo 12. Transacciones
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Hasta ahora, todos los ejemplos de este libro han consistido en sentencias SQL individuales e independientes. Aunque ésta puede ser la norma para los informes ad hoc o los scripts de mantenimiento de datos, la lógica de la aplicación incluirá con frecuencia varias sentencias SQL que deben ejecutarse juntas como una unidad lógica de trabajo. Este capítulo explora las transacciones, que son el mecanismo utilizado para agrupar un conjunto de sentencias SQL de forma que todas o ninguna de las sentencias tengan éxito.
Bases de datos multiusuario
Los sistemas de gestión de bases de datos permiten a un único usuario consultar y modificar datos, pero en el mundo actual puede haber miles de personas realizando cambios en una base de datos simultáneamente. Si cada usuario sólo ejecuta consultas, como podría ser el caso de un almacén de datos durante el horario laboral normal, el servidor de la base de datos tendrá que enfrentarse a muy pocos problemas. Sin embargo, si algunos de los usuarios añaden y/o modifican datos, el servidor debe ocuparse de bastantes más asuntos.
Supongamos, por ejemplo, que estás ejecutando un informe que resume la actividad de alquiler de películas de la semana en curso. Sin embargo, al mismo tiempo que ejecutas el informe, se están produciendo las siguientes actividades:
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Un cliente alquila una película.
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Un cliente ...
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