August 2024
Intermediate to advanced
380 pages
10h 16m
Spanish
Content preview from Aprender SQL, 3ª Edición
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Capítulo 3. Manual de consulta
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Hasta ahora, has visto unos cuantos ejemplos de consultas a bases de datos (también conocidas como sentencias select ) salpicadas a lo largo de los dos primeros capítulos. Ahora es el momento de examinar más detenidamente las distintas partes de la sentencia select y cómo interactúan. Después de terminar este capítulo, deberías tener una comprensión básica de cómo se recuperan, unen, filtran, agrupan y ordenan los datos; estos temas se tratarán en detalle en los Capítulos 4 a 10.
Mecánica de consulta
Antes de analizar la sentencia select, puede ser interesante ver cómo ejecuta las consultas el servidor MySQL (o, para el caso, cualquier servidor de bases de datos). Si estás utilizando la herramienta de línea de comandos mysql (que supongo que es lo que estás haciendo), entonces ya has iniciado sesión en el servidor MySQL proporcionando tu nombre de usuario y contraseña (y posiblemente un nombre de host si el servidor MySQL se ejecuta en un ordenador diferente). Una vez que el servidor ha verificado que tu nombre de usuario y contraseña son correctos, se genera una conexión a la base de datos para que la utilices. Esta conexión la mantiene la aplicación que la solicitó (que, en este caso, es la herramienta mysql ) hasta que la aplicación libera la conexión (es decir, como resultado de teclear quit) o el servidor la cierra (es ...
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