Python 3 检查数据库是否存在并创建失败

Question

我是 python 初学者。 我想使用 Python 3 来检查 SQL 服务器上的test2数据库。 如果它不存在，我将创建它。 但是，我发现报错了，不知道怎么办？

python 代码：

import pymssql 
 
 
def conn():
    ret = pymssql.connect(host='DESKTOP-4CDQOMR', user = 'sa', password = '123456') 
    if ret:
        print("connect successfully!")
    else:
        print("connect failed!")

    return ret
        

 
 
if __name__ == '__main__':
    conn = conn()  
    if conn:
        cursor = conn.cursor()
        sql = "if not exist (select * from sys.databases where name = 'test2')"
        conn.autocommit(True)
        cursor.execute(sql)
        conn.autocommit(False) 
        conn.close()

错误信息：

---------------------------------------------------------------------------
MSSQLDatabaseException                    Traceback (most recent call last)
src\pymssql.pyx in pymssql.Cursor.execute()

src\_mssql.pyx in _mssql.MSSQLConnection.execute_query()

src\_mssql.pyx in _mssql.MSSQLConnection.execute_query()

src\_mssql.pyx in _mssql.MSSQLConnection.format_and_run_query()

src\_mssql.pyx in _mssql.check_cancel_and_raise()

src\_mssql.pyx in _mssql.maybe_raise_MSSQLDatabaseException()

MSSQLDatabaseException: (156, b"Incorrect syntax near the keyword 'select'.DB-Lib error message 20018, severity 15:\nGeneral SQL Server error: Check messages from the SQL Server\nDB-Lib error message 20018, severity 15:\nGeneral SQL Server error: Check messages from the SQL Server\n")

During handling of the above exception, another exception occurred:

OperationalError                          Traceback (most recent call last)
F:\jupyter\connect.py in <module>
     20         sql = "if not exist (select * from sys.databases where name = 'test2')"
     21         conn.autocommit(True)
---> 22         cursor.execute(sql)
     23         conn.autocommit(False)
     24         conn.close()

src\pymssql.pyx in pymssql.Cursor.execute()

OperationalError: (156, b"Incorrect syntax near the keyword 'select'.DB-Lib error message 20018, severity 15:\nGeneral SQL Server error: Check messages from the SQL Server\nDB-Lib error message 20018, severity 15:\nGeneral SQL Server error: Check messages from the SQL Server\n")

Answer 1

我通过修改以下代码解决了我的问题：

sql = "if not exist (select * from sys.databases where name = 'test2')"

至

sql = "if not exists (select * from sys.databases where name = 'test2') begin create database test2 end"

Answer 2

我没有安装 MS SQL 服务器来测试，但是

if not exist (select * from sys.databases where name = 'test2')

对我来说看起来不像有效的 SQL。

也许你想做这样的事情：

cur = conn.cursor()
# Check if the database already exists.
cur.execute("""SELECT COUNT(*) FROM sys.databases WHERE name = 'test2'""")
res = cur.fetchone()[0]
if res == 0:
    conn.autocommit(True)
    cur.execute("""CREATE DATABASE test2""")
    conn.autocommit(False)

另一种方法可能是尝试直接创建数据库，如果异常已经存在，则捕获它。 如果其他人有可能在您获取计数和发送 create 语句之间创建数据库，这是一种更好的方法。

import sys

cur = conn.cursor()
conn.autocommit(True)
try:
    cur.execute("""CREATE DATABASE test2""")
except Exception as ex:
    # Rollback is probably unnecessary
    conn.rollback()
    print('Failed to create the database because', ex, file=sys.stderr)
finally:
    conn.autocommit(False)
    conn.close()

Answer 3

嗨希望下面的代码可以帮助你！

import mysql.connector

DB_NAME  = "test2"

client = mysql.connector.connect(host = "127.0.0.1",
                                 user = "admin",
                                 passwd = "password")
mycursor = client.cursor()
mycursor.execute("SHOW DATABASES")

if DB_NAME in mycursor:    print("Database exists :)")
else :
    print(f"Database {DB_NAME} doesn't exist, creating now....")
    mycursor.execute("CREATE DATABASE firstdatabase")
    print(f"Database {DB_NAME} created!")