Question

我想从带有复合键的表中删除行。

我需要构建一个表单查询：

DELETE FROM t WHERE EXISTS (c1, c2, c3) IN (subquery)

我如何在SQLAlchemy中执行此操作？

这是一个示例，其中有一个表格，记录每个用户每场比赛的多个分数。我想删除用户参与的每个游戏中每个用户的最低分数。

from sqlalchemy import Table, Column, MetaData, String, Integer


metadata = MetaData()
t = Table('scores', metadata, 
           Column('game',String),
           Column('user',String),
           Column('score',Integer))

数据可能如下所示：

game     user    score  
g1       u1      44
g1       u1      33
g1       u1      2     (delete this)
g2       u1      55
g2       u1      1     (and this)

我想删除(g1,u1,2)和(g2, u1,1)。

到目前为止，我的尝试是使用SQLAlchemy：

from sqlalchemy import delete, select, func, exists, tuple_

selector_tuple = tuple_(t.c.game, t.c.user, t.c.score)
low_score_subquery = select([t.c.game, t.c.user, func.min(t.c.score)])\
                        .group_by(t.c.game, t.c.user)
in_clause = selector_tuple.in_(low_score_subquery)
print "lowscores = ", low_score_subquery # prints expected SQL
print "****"
print "in_clause = ", in_clause # prints expected SQL

虽然我得到in_clause和low_score_subquery的预期SQL，但删除查询（如下）不正确。我尝试过以下各种变体，但结果都不好：

>>> delete_query = delete(t, exists([t.c.game, t.c.user, t.c.score], 
...                                 low_score_subquery))
>>> print delete_query # PRODUCES INVALID SQL
DELETE FROM scores WHERE EXISTS (SELECT scores."game", scores."user", scores.score 
FROM (SELECT scores."game" AS "game", scores."user" AS "user", min(scores.score) AS min_1 
FROM scores GROUP BY scores."game", scores."user") 
WHERE (SELECT scores."game", scores."user", min(scores.score) AS min_1 
FROM scores GROUP BY scores."game", scores."user"))

我尝试过exists(in_clause)，exists([], in_clause)和in_clause.exists()，但这些都会导致异常。

Answer 1

你真的需要EXISTS吗？这不符合你的要求吗？

>>> delete_query = delete(t, in_clause)
>>> print(delete_query)
DELETE FROM scores WHERE (scores.game, scores."user", scores.score) IN (SELECT scores.game, scores."user", min(scores.score) AS min_1
FROM scores GROUP BY scores.game, scores."user")

如何使用SQLAlchemy构造“DELETE WHERE EXISTS IN”？

1 个答案: