Working with Data Sources 4

Posted 2020-08-20 阿难的机器学习计划

Querying SQLite from Python

1. We use connect() in the library sqlite3 to connect the database we would like to query. Once it is connected, the target database is the only one database we are currently connecting.

　　import sqlite3

　　conn = sqlite3.connect("jobs.db") # connect() only have one parameter, which is the target database.

2.Tuples is a set of data like list. But it can not be modified after creating, and it is faster than list.

　　t = (‘Apple‘, ‘Banana‘) # t is a tuple
　　apple = t[0]
　　banana = t[1]

 

3. When we would like to query some result from the query, we have to 

　　a. Connect the database with the local file by using connect() ,and store as a local variable.

　　b. Because the query we get is a string, we have to change them into a cursor class by using conn.cursor. 

　　c. cursor.exercute() change the cursor class into a instance. 

　　d. We fatch all the data from the cursor class and store it as a tuples format.

　　import sqlite3

　　conn = sqlite3.connect("jobs.db") # connect the database to local python environment　　

　　cursor = conn.cursor()　#store the connected result into cursor class

　　query_1 = "select Major from recent_grads;" # get a query and store as a string

　　cursor.execute(query_1) # Execute the query, convert the results to tuples, and store as a local variable.

　　majors = cursor.fetchall() # fetch all the result from the cursor into a variable

　　print(majors[0:2])

 

4. Every tuples have to be executed before fetch. We can use fetchone() or fetchall() to fetch certain number of results.

　　query = "select Major,Major_category from recent_grads"

　　cursor.execute(query) 

　　five_results = cursor.fetchmany(5) 

 

5. close the connection by using close()