SQL, Database, SQLAlchemy: Working Together

Concepts:

• SQL (Structured Query Language): A language for interacting with relational databases, used for creating, reading, updating, and deleting data.
• Database: A structured collection of data organized into tables, often managed by a dedicated software system.
• SQLAlchemy: A Python library for interacting with databases in a Pythonic way. It provides an object-relational mapper (ORM) that simplifies working with database tables as Python objects, but also allows executing raw SQL queries when needed.

Steps:

1. Import Libraries:

   import sqlalchemy as sa
   

2. engine = sa.create_engine('mysql://user:password@host/database')
   

3. Read SQL File: Open the SQL file using open and iterate through its lines.

   with open('my_script.sql', 'r') as f:
       sql_statements = f.readlines()
   

4. Process SQL Statements:

   • Ignore Comments: Skip lines starting with -- (common SQL comment syntax).
   • Combine Multi-Line Statements: Some SQL statements might span multiple lines (e.g., CREATE TABLE with columns). Concatenate consecutive non-empty lines until you encounter a semicolon (;), which marks the end of a statement.

   processed_statements = []
   current_statement = ''
   for line in sql_statements:
       line = line.strip()  # Remove leading/trailing whitespace
       if not line or line.startswith('--'):
           continue  # Skip comments and empty lines
       current_statement += line
       if line.endswith(';'):
           processed_statements.append(current_statement)
           current_statement = ''
   

5. Execute Statements: Use a loop to iterate through the processed SQL statements and execute them using the execute method of the engine object. Handle any potential errors appropriately.

   for statement in processed_statements:
       try:
           engine.execute(statement)
       except sa.exc.SQLAlchemyError as e:
           print(f"Error executing statement:\n{statement}\n{e}")
   

Complete Example:

import sqlalchemy as sa

engine = sa.create_engine('mysql://user:password@host/database')

with open('my_script.sql', 'r') as f:
    sql_statements = f.readlines()

processed_statements = []
current_statement = ''
for line in sql_statements:
    line = line.strip()
    if not line or line.startswith('--'):
        continue
    current_statement += line
    if line.endswith(';'):
        processed_statements.append(current_statement)
        current_statement = ''

for statement in processed_statements:
    try:
        engine.execute(statement)
    except sa.exc.SQLAlchemyError as e:
        print(f"Error executing statement:\n{statement}\n{e}")

print("SQL script executed successfully!")

import sqlalchemy as sa

# Replace with your actual database connection string
engine = sa.create_engine('MYSQL+pymysql://your_username:your_password@your_host/your_database')

with open('my_script.sql', 'r') as f:
    sql_statements = f.readlines()

processed_statements = []
current_statement = ''
for line in sql_statements:
    line = line.strip()
    if not line or line.startswith('--'):
        continue
    current_statement += line
    if line.endswith(';'):
        processed_statements.append(current_statement)
        current_statement = ''

for statement in processed_statements:
    try:
        engine.execute(statement)
    except sa.exc.SQLAlchemyError as e:
        print(f"Error executing statement:\n{statement}\n{e}")

print("SQL script executed successfully!")

Make sure to replace the following with your own information:

• 'MYSQL+pymysql://your_username:your_password@your_host/your_database' (replace with your specific database connection string, including database type, username, password, host, and database name)
• 'my_script.sql' (replace with the actual filename of your SQL script)

Using text construct:

This method allows you to define the SQL statements within your Python code as strings and then execute them directly using the text construct. It simplifies the file reading step but requires including the SQL within your script.

import sqlalchemy as sa

engine = sa.create_engine('mysql://user:password@host/database')

with open('my_script.sql', 'r') as f:
    sql_statements = f.read()

for statement in sql_statements.splitlines():
    if not statement.strip():
        continue
    try:
        engine.execute(sa.text(statement))
    except sa.exc.SQLAlchemyError as e:
        print(f"Error executing statement:\n{statement}\n{e}")

print("SQL script executed successfully!")

Leveraging with statement for connection management:

This approach utilizes a with statement to automatically connect to the database, execute the script, and then close the connection, ensuring proper resource management.

import sqlalchemy as sa

def execute_script(filename, engine):
    with engine.begin() as conn:
        with open(filename, 'r') as f:
            for line in f:
                if not line.strip() or line.startswith('--'):
                    continue
                conn.execute(line)

engine = sa.create_engine('mysql://user:password@host/database')
execute_script('my_script.sql', engine)
print("SQL script executed successfully!")

Calling executescript on the raw connection (for specific scenarios):

Important Note: This method should be used cautiously as it bypasses some of SQLAlchemy's security features and may not be suitable for all situations. It's generally recommended for internal scripts or specific use cases where you have complete control over the SQL content.

import sqlalchemy as sa

engine = sa.create_engine('sqlite:///my_database.db')
with engine.begin() as conn:
    dbapi_conn = conn.connection
    with open('my_script.sql', 'r') as f:
        sql_script = f.read()
    dbapi_conn.executescript(sql_script)  # Use with caution

print("SQL script executed successfully!")