Question

我正在尝试编写一个for循环，以检查我的postgres表是否已经存在，以及是否不存在，请在继续执行脚本的其余部分之前创建它。如果确实存在，则不必费心创建它，而是继续执行脚本（将文件写入数据库）。

某处，我的for循环中的某些内容不正确。有人有什么想法吗？

"""A tool for saving files to and from a postgresql db.
"""
import os
import sys
import argparse
import psycopg2

db_conn_str = "postgresql://word:word@111.11.111.1:5432/DBNAME"
create_table_stm = """
CREATE TABLE files (
    id serial primary key,
    orig_filename text not null,
    file_data bytea not null
)
"""

check_for_table = """
IF EXISTS (SELECT * FROM files)
"""

def main(argv):
    parser = argparse.ArgumentParser()
    parser_action = parser.add_mutually_exclusive_group(required=True)
    parser_action.add_argument("--store", action='store_const', const=True, help="Load an image from the named file and save it in the DB")
    parser_action.add_argument("--fetch", type=int, help="Fetch an image from the DB and store it in the named file, overwriting it if it exists. Takes the database file identifier as an argument.", metavar='42')
    parser.add_argument("filename", help="Name of file to write to / fetch from")
    args = parser.parse_args(argv[1:])

    conn = psycopg2.connect(db_conn_str)
    curs = conn.cursor()

    # Check if the table already exists
    if curs.execute(check_for_table) is None:

    # If it DOES NOT exist, create it
        curs.execute(create_table_stm)

    # If it DOES exist, ??
    elif     
        if args.store:
            with open(args.filename,'rb') as f:
                filedata = psycopg2.Binary(f.read())
                curs.execute("INSERT INTO files(id, orig_filename, file_data) VALUES (DEFAULT,%s,%s) RETURNING id", (args.filename, filedata))
                print curs
                returned_id = curs.fetchone()[0]
            print("Stored {0} into DB record {1}".format(args.filename, returned_id))
            conn.commit()

        elif args.fetch is not None:
            with open(args.filename,'wb') as f:
                curs.execute("SELECT file_data, orig_filename FROM files WHERE id = %s", (int(args.fetch),))
                (file_data, orig_filename) = curs.fetchone()
                f.write(file_data)
            print("Fetched {0} into file {1}; original filename was {2}".format(args.fetch, args.filename, orig_filename))

        conn.close()

if __name__ == '__main__':
    main(sys.argv)

Answer 1

您可以使用create table if not exists并继续。

create_table_stm = """
CREATE TABLE IF NOT EXISTS files (
    id serial primary key,
    orig_filename text not null,
    file_data bytea not null
)
"""

有一个警告，一个具有相同名称但具有不同架构（即不同列和类型）的表可能已经存在。 Postgres将不会验证它是否与您的create语句匹配，而只会验证表名称是否匹配。

如果您需要创建的是具有特定架构的空白表，请先使用drop table if exists files，然后使用create table files。

用于检查表是否存在的for循环，然后在继续执行脚本之前创建该表

1 个答案: