如何在SQLAlchemy和postgresql中联合两个子查询[英] How to union two subqueries in SQLAlchemy and postgresql

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问题描述

需要原始 SQL:

SELECT
    id
FROM
   (SELECT some_table.id FROM some_table WHERE some_table.some_field IS NULL) AS subq1
   UNION
   (SELECT some_table.id WHERE some_table.some_field IS NOT NULL)
LIMIT 10;

这是python代码:

import sqlalchemy

SOME_TABLE = sqlalchemy.Table(
 'some_table',
 sqlalchemy.MetaData(),
 sqlalchemy.Column('id', sqlalchemy.Integer, primary_key=True),
 sqlalchemy.Column('some_field', sqlalchemy.Text))

stmt_1 = sqlalchemy.sql.select(SOME_TABLE.columns).where(SOME_TABLE.columns.some_field != None)
stmt_2 = sqlalchemy.sql.select(SOME_TABLE.columns).where(SOME_TABLE.columns.some_field == None)

# This gets a programming error.
stmt_1.union(stmt_2).limit(10);

以下是出现此错误的输出 SQL(已交换参数):错误:"UNION"处或附近的语法错误:

SELECT some_table.id, some_table.some_field
FROM some_table
WHERE some_table.some_field IS NOT NULL
 LIMIT 10 UNION SELECT some_table.id, some_table.some_field
FROM some_table
WHERE some_table.some_field IS NULL
 LIMIT 10
 LIMIT 10

如何给子查询设置别名?

推荐答案

我使用了一点不同的方法:

# the first subquery, select all ids from SOME_TABLE where some_field is not NULL
s1 = select([SOME_TABLE.c.id]).where(SOME_TABLE.c.some_field != None)

# the second subquery, select all ids from SOME_TABLE where some_field is NULL
s2 = select([SOME_TABLE.c.id]).where(SOME_TABLE.c.some_field != None)

# union s1 and s2 subqueries together and alias the result as "alias_name"
q = s1.union(s2).alias('alias_name')

# run the query and limit the aliased result to 10
session.query(q).limit(10)

这里是生成的sql:

SELECT alias_name.id AS alias_name_id 
FROM (SELECT some_table.id AS id 
FROM some_table 
WHERE some_table.some_field IS NOT NULL UNION SELECT some_table.id AS id 
FROM some_table 
WHERE some_table.some_field IS NULL) AS alias_name 
LIMIT 10

我想这就是你想要的结果.

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