Sqlite NULL 且唯一？

Question

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Answer 1

虽然以下内容解决了多个空值，但它不解决与此类设计相关的任何“问题”，除了可能的数据库/SQL 可移植性 - 因此，它可能不 被认为是一个答案，并留在这里仅供引用。

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这实际上包含在 SQLite FAQ 中。这是一种设计选择 - SQLite(与 SQL Server 不同)选择多个 NULL 值不计入索引中的唯一性。

The SQL standard requires that a UNIQUE constraint be enforced even if one or more of the columns in the constraint are NULL, but SQLite does not do this. Isn't that a bug?

Perhaps you are referring to the following statement from SQL92:

• A unique constraint is satisfied if and only if no two rows in a table have the same non-null values in the unique columns.

That statement is ambiguous, having at least two possible interpretations:

1. A unique constraint is satisfied if and only if no two rows in a table have the same values and have non-null values in the unique columns.

2. A unique constraint is satisfied if and only if no two rows in a table have the same values in the subset of unique columns that are not null.

SQLite follows interpretation (1), as does PostgreSQL, MySQL, Oracle, and Firebird. It is true that Informix and Microsoft SQL Server use interpretation (2), however we the SQLite developers hold that interpretation (1) is the most natural reading of the requirement and we also want to maximize compatibility with other SQL database engines, and most other database engines also go with (1), so that is what SQLite does.

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