python - 比较 SQLite 查询中两个可能为 NULL 的值

Question

我有一个 SQLite 查询，如:

SELECT max(e), url, branch FROM (SELECT max(T1.entry) e, T1.url, T1.branch FROM repo_history T1
                                                        WHERE (SELECT active FROM repos T2 WHERE url = T1.url AND branch = T1.branch AND project = ?1)
                                                        GROUP BY T1.url, T1.branch
                                                        UNION
                                                        SELECT null, T3.url, T3.branch FROM repos T3 WHERE active AND project = ?1 )
                               GROUP BY url ORDER BY e

请注意，出现了两次 ?1范围。无论如何，在某些情况下，它可以为空(据我所知，python 中的 None 在 SQLite 中变为 NULL)。这是一个问题，因为我不了解 null 处理，但基本上我没有得到任何返回。

那么，我该如何处理 where "project" = ?1当?1是NULL吗？我想避免对它进行 2 个单独的查询。我环顾四周，但我只能找到有关 IS NULL 的信息。/IS NOT NULL ，这对我不起作用，因为我不是要检查列是否为空或不为空，而是要检查两个可为空的值是否匹配，是空值还是非空值。

Answer 1

在 SQLite 中，您可以使用 IS运算符而不是 = 用于 NULL tolarant 比较。也适用于 ?插入(不像 MikeT 的意思)。

python 示例:

>>> c.execute('SELECT * FROM mytable WHERE userid = ? AND recipe = ?', (3, None)).fetchall()
[]

>>> c.execute('SELECT * FROM mytable WHERE userid = ? AND recipe IS ?', (3, None)).fetchall()
[<Row object>, <Row object>]

>>> c.execute('SELECT * FROM mytable WHERE userid = ? AND recipe is ?', (3, 'TestRecipe')).fetchall()
[<Row object>]

The IS and IS NOT operators work like = and != except when one or both of the operands are NULL. In this case, if both operands are NULL, then the IS operator evaluates to 1 (true) and the IS NOT operator evaluates to 0 (false). If one operand is NULL and the other is not, then the IS operator evaluates to 0 (false) and the IS NOT operator is 1 (true). It is not possible for an IS or IS NOT expression to evaluate to NULL. Operators IS and IS NOT have the same precedence as =.

对于较旧的 MySQL/Mariadb 版本，至少 NULL tolarant 比较运算符是 <=>在 PostgreSQL 中它的 IS NOT DISTINCT FROM .

PostgreSQL 变体在 SQL:2003 标准中定义。
为了临时兼容性，可以从 Python dict 中插入合适的运算符...