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doc: Make UPDATE FROM examples consistent
The original first half of the example used an employees table and an accounts.sales_person foreign key column, while the second half (added in commit 8f889b1) used a salesmen table and accounts.sales_id for the foreign key. This makes everything use the original names. Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org> Discussion: https://postgr.es/m/87o81vqjw0.fsf@wibble.ilmari.org
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doc/src/sgml/ref/update.sgml

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@@ -387,23 +387,23 @@ UPDATE employees SET sales_count = sales_count + 1 WHERE id =
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<para>
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Update contact names in an accounts table to match the currently assigned
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salesmen:
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salespeople:
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<programlisting>
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UPDATE accounts SET (contact_first_name, contact_last_name) =
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(SELECT first_name, last_name FROM salesmen
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WHERE salesmen.id = accounts.sales_id);
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(SELECT first_name, last_name FROM employees
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WHERE employees.id = accounts.sales_person);
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</programlisting>
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A similar result could be accomplished with a join:
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<programlisting>
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UPDATE accounts SET contact_first_name = first_name,
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contact_last_name = last_name
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FROM salesmen WHERE salesmen.id = accounts.sales_id;
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FROM employees WHERE employees.id = accounts.sales_person;
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</programlisting>
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However, the second query may give unexpected results
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if <structname>salesmen</structname>.<structfield>id</structfield> is not a unique key, whereas
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if <structname>employees</structname>.<structfield>id</structfield> is not a unique key, whereas
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the first query is guaranteed to raise an error if there are multiple
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<structfield>id</structfield> matches. Also, if there is no match for a particular
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<structname>accounts</structname>.<structfield>sales_id</structfield> entry, the first query
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<structname>accounts</structname>.<structfield>sales_person</structfield> entry, the first query
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will set the corresponding name fields to NULL, whereas the second query
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will not update that row at all.
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</para>

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