April 2025
Beginner to intermediate
570 pages
12h 7m
Italian
Content preview from SQL Cookbook, 2a edizione
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Capitolo 3. Lavorare con tabelle multiple
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Questo capitolo introduce l'uso dei join e delle operazioni di set per combinare i dati di più tabelle. Le unioni sono alla base di SQL. Anche le operazioni di set sono importanti. Se vuoi padroneggiare le query complesse che troverai nei capitoli successivi di questo libro, devi iniziare da qui, dalle unioni e dalle operazioni di set.
3.1 Impilare una serie di righe su un'altra
Problema
Vuoi restituire i dati memorizzati in più di una tabella, sovrapponendo concettualmente un set di risultati all'altro. Le tabelle non hanno necessariamente una chiave comune, ma le loro colonne hanno gli stessi tipi di dati. Ad esempio, vuoi visualizzare il nome e il numero di reparto dei dipendenti del reparto 10 nella tabella EMP, oltre al nome e al numero di reparto di ciascun reparto nella tabella DEPT. Vuoi che l'insieme dei risultati assomigli al seguente:
ENAME_AND_DNAME DEPTNO --------------- ---------- CLARK 10 KING 10 MILLER 10 ---------- ACCOUNTING 10 RESEARCH 20 SALES 30 OPERATIONS 40
Soluzione
Usa l'operazione UNION ALL per combinare le righe di più tabelle:
1 select ename as ename_and_dname, deptno 2 from emp 3 where deptno = 10 4 union all 5 select '----------', null 6 from t1 7 union all 8 select dname, deptno 9 from dept
Discussione
UNION ALL combina le righe di più fonti di righe in un unico ...
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