SQL Default Attribute Values

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Questions and Answers

Which data types are mentioned as not supporting default values?

VARCHAR
BLOB (correct)
TEXT (correct)
INTEGER

What is the correct SQL syntax to change the default value of an attribute?

CHANGE TABLE relation DEFAULT value
ALTER TABLE relation ALTER attribut SET DEFAULT value (correct)
ALTER relation DROP DEFAULT
ALTER relation SET DEFAULT value

What happens to existing records when the default value for an attribute is changed?

The database will throw an error.
Existing records retain the old default value. (correct)
All existing records will update to the new default value.
Existing records are deleted.

What SQL command is used to remove a default value from an attribute?

<p>ALTER TABLE relation ALTER attribut DROP DEFAULT (D)</p>

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Which of the following statements is true regarding changing default values?

<p>New records will use the updated default value after changes. (C)</p>

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Flashcards

Modifying a Default Value

Changing the default value of an attribute in a database, setting it to a new value. This affects only new records, not existing ones.

Removing a Default Value

Removing a default value for an attribute. Subsequent records will no longer have a default value for that attribute.

ALTER TABLE ... ALTER ... SET DEFAULT

An SQL command used to alter the default value of an attribute in a table.

ALTER TABLE ... ALTER ... DROP DEFAULT

An SQL command used to remove the default value of an attribute from a table.

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Impact of Default Value Changes

Changing a default value in a database does not retroactively change values in existing rows. Only new rows added after the change will use the new (or removed) default value.

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Study Notes

Changing Default Attribute Values

Changing or removing default attribute values is possible.
Be mindful of data types like BLOB and TEXT, which may not accept default values.
Syntax for changing default values:
- ALTER TABLE relation ALTER attribut { SET DEFAULT value | DROP DEFAULT }
Example for changing a default value:
- ALTER TABLE Personnes ALTER 'telephone' SET DEFAULT '9999999999'
Example for removing a default value:
- ALTER TABLE Personnes ALTER 'telephone' DROP DEFAULT
Changes to default values do not affect existing entries that used the previous default value.