SQL Statement Management
It’s very difficult explaining to students new to relational databases how SQL works. There are many parts that seem intuitive and then there are others that confuse and confound.
For beginners, the idea that a SQL statement is simply a text string that you must dispatch to a SQL statement processing engine is new. That’s because they use an Integrated Development Environment (IDE) that hides, or abstracts the complexity, of how SQL executes.
I start my core SQL class by demonstrating how to run a text literal query without a FROM clause in MySQL Workbench, like this:
SELECT 'Hello World!' AS "Output"; |
After writing the query, I highlight everything except the semicolon and click the lightening bolt that dispatches the static string to the SQL statement engine. They see this result:
Then, I launch a mysql Monitor session and write the query with a semicolon to dispatch the SQL static string to the SQL statement engine:
SELECT 'Hello World!' AS "Output"; |
and, with a \g to dispatch the SQL static string to the SQL statement engine:
SELECT 'Hello World!' AS "Output"\g |
Both queries return the same output, as shown below:
+--------------+ | output | +--------------+ | Hello World! | +--------------+ 1 row in set (0.00 sec)
Rewriting the query with a \G to dispatch the SQL static string to the SQL statement engine:
SELECT 'Hello World!' AS "Output"\G |
Both queries return the following output:
*************************** 1. row *************************** output: Hello World! 1 row in set (0.00 sec)
The next step requires removing the MySQL Workbench and MySQL Monitor from the demonstration. Without either of those tools, a Python program can demonstrate how to run a static SQL string.
The query is now a string literal into a query.sql file. The Python program reads the query.sql file, dispatches the embedded query, and displays the query results.
This is the query.sql file is:
SELECT 'Hello World!' AS "output"; |
This is the query.py file is:
#!/usr/bin/python # Import libraries. import sys import mysql.connector from mysql.connector import errorcode # ============================================================ # Use a try-catch block to read and parse a query from a # a file found in the same local directory as the Python # program. # ============================================================ try: file = open('query.sql','r') query = file.read().replace('\n',' ').replace(';','') file.close() except IOError: print("Could not read file:", fileName) # ============================================================ # Attempt connection in a try-catch block. # ============================================================ # -------------------------------------------------------- # Open connection, bind variable in query and format # query output before closing the cursor. # -------------------------------------------------------- try: # Open connection. cnx = mysql.connector.connect(user='student', password='student', host='127.0.0.1', database='studentdb') # Create cursor. cursor = cnx.cursor() # Execute cursor, and coerce string to tuple. cursor.execute(query) # Display the rows returned by the query. for row in cursor: print(row[0]) # Close cursor. cursor.close() # -------------------------------------------------------- # Handle MySQL exception # -------------------------------------------------------- except mysql.connector.Error as e: if e.errno == errorcode.ER_ACCESS_DENIED_ERROR: print("Something is wrong with your user name or password") elif e.errno == errorcode.ER_BAD_DB_ERROR: print("Database does not exist") else: print("Error code:", e.errno) # error number print("SQLSTATE value:", e.sqlstate) # SQLSTATE value print("Error message:", e.msg) # error message # -------------------------------------------------------- # Close connection after try-catch completes. # -------------------------------------------------------- # Close the connection when the try block completes. else: cnx.close() |
In Linux or Unix from the relative directory where both the query.sql and query.py files are located:
./query.py |
It returns:
Hello World!
These examples demonstrate that a query without variable substitution is only a static string. In all the cases, the static SQL strings are dispatched to the SQL engine by a terminator like a semicolon or through an ODBC library call that executes the static SQL string.
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