sqlite3如何同时搜索多个表？

Question

I'm tying to design a Sign In page. I have three tables students , professors , dean . I want to fetch the password for the given username and check if it's correct. I can do this for one table with:

def get_password_by_username(username):
    c.execute("SELECT password FROM students WHERE username = :username", {"username": username})
    return c.fetchone()

How can I search for the username in all three tables at the same time? thanks.

Answer 1

This query:

SELECT username, password FROM students 
UNION ALL
SELECT username, password FROM professors
UNION ALL
SELECT username, password FROM dean

returns all the usernames and passwords from the 3 tables and you can filter to get the password like this:

SELECT password
FROM (
  SELECT username, password FROM students 
  UNION ALL
  SELECT username, password FROM professors
  UNION ALL
  SELECT username, password FROM dean
)
WHERE username = :username

Answer 2

If you want to get all the data associated with some student based on student's username , you should use JOIN in your SQL queries to achieve that. You will maybe have to redesign your database a little bit to be able to get all the data you need.

There are plenty of nice guides around about how to use JOIN ( this one or this one ), read them to understand better how to join your tables in a desired way.

Very simple example:

SELECT 
    *
FROM 
    students s
LEFT JOIN professors p 
    ON s.professorID = p.professorID
WHERE
    s.username = 'some_username';

Answer 3

You can separate them with a comma

def get_password_by_username(username):
    c.execute("SELECT password FROM students, professors, dean WHERE username=?", (username,)
    return c.fetchone()