[英]How to union two subqueries in SQLAlchemy and postgresql
原始SQL需要:
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;
這是python代碼:
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);
以下是輸出的SQL(帶有參數交換),它會收到此錯誤: 錯誤:“UNION”處或附近的語法錯誤:
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
我如何為子查詢添加別名?
我用了一點點不同的方法:
# 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)
這是生成的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
我想這就是你想要的結果。
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