Week 7 - SQL

Welcome to CS50! This is my review for Week 7's content about SQL.

Lecture

In SQL, we have a CRUD paradigm, where
- C stands for CREATE, sometimes includes INSERT
- R stands for READ
- U stands for UPDATE
- D stands for DELETE, sometimes includes DROP
In SQL, we use the tradition that all the key words, like SELECT..., should be captalized. This is just to make our code easy to look at. Not captalizing the key words won't affect the execution of our SQL code.
Show the selected columns from the table in the terminal

SELECT columns FROM table;

To define a variable in SQL

SELECT COUNT(*) AS n FROM table;

Insert into table

INSERT INTO table (column, ...) VALUES(value, ...);

Delete the whole table

DELETE FROM table;

This command will delete the whole table, which sometimes may cause horror story to happen.

7. An Interesting SQL attack: SQL Injection Attack Hackers will use the property of the comment (--) in SQL to hack into the user's account without knowing the password.

rows = db.execute("SELECT * FROM users WHERE username = '{username}' AND password = '{password}'")

If the hacker input malan@harvard'-- as the username

    rows = db.execute("SELECT * FROM users WHERE username = `malan@harvard.edu'--' AND password = '{password}'")

In this case, the hacker doesn’t need to know the password but he can enter the account. To avoid this problem, we should use the code below:

    from cs50 import SQL
    rows = db.execute("SELECT * FROM users WHERE username = ? AND password = ?", username, password)

Section

Some basic jargons in Database:
- Database: A collection of data organized for creating, reading, updating, and deleting. (This is CRUD principle)
- Database Management System (DBMS): Software via which you can interact with a data base. And below are some examples:
  - MySQL
  - Oracle
  - PostgreSQL
  - SQLite ...
- SQL: A language via which you can create, read, update, and delete data in a database.
Design Principles for a database:
- Create one table for each entity in your dataset. For example, you should create two tables for the books and authors.
- All tables should have a primary key. This is a unique id that differentiate one row from every other row.
- The information in the table should depend on the primary key only.
Basic SQL Commands:

Create a table:

CREATE TABLE table_name (
    column0 INTEGER,
    column1 TEXT,
    column2 NUMERIC,
    column3 REAL,
    PRIMARY KEY(column0)
);

In this code, column0 and so on is just the name for one specific column. Remember that you should also define the PRIMARY KEY for this table, which abides by our designing principle stated above.

Create a joint/associative table:

CREATE TABLE table_name (
    column0 INTEGER,
    column1 INTEGER,
    PRIMARY KEY(column0, column1)
    FOREIGN KEY(column0) REFERENCES other_table_name(column0),
    FOREIGN KEY(column1) REFERENCES other_table_name(column1)
);

In this code, column0 and column1 are the foreign keys that reference to the primary keys in other tables.

Delete a table:

DROP TABLE table_name;

Insert:

INSERT INTO table (column0, column1)
VALUES(value0, value1);

When inserting, you don’t need to include the id/PRIMARY KEY column.

Delete:

DELETE FROM table WHERE condition;

Note that WHERE is usaully used to filter rows. It should appear right after you SELECT your columns.

Show basic information about a table (Usually the column information):

.schema table_name

Show all the tables in the database:

.tables

Order the result you see:

SELECT * FROM table ORDER BY column_name;

Note that ORDER BY is usually at the end of the statement. You can specify the ordering by add DESC (descending) and ASC (ascending) right after the column_name after ORDER BY.

Aggregate functions:
- COUNT(*): Count the number of rows in the table.
- SUM(column_name): Sum the values in the column.
- MAX(column_name): Find the maximum value in the column.
- MIN(column_name): Find the minimum value in the column.
- AVG(column_name): Find the average value in the column.
- Usage:
```
SELECT AVG(column_name) FROM table;
```

This will output a number, which is the average of a specifc column from the table.

Shorts

All our queries will refer to rows of the table since we specify all of the columns in that table.
In a joint table, it is okay to establish a joint primary key - a combination of two columns that is always guaranteed to be unique. Otherwise, there will be duplicates if we only specify one column as the PRIMARY KEY
In SQL, it is usally to define the PRIMARY KEY column as INTEGER. Otherwise, you need to configure that column to autoincrement.

Problem Set 7

Songs

Nothing more to note down.

Movies

In ORDER BY, we can specify more than one parameters and its order matters. For example, ORDER BY column1 DESC, column2 ASC will first sort the table by column1 in descending order and then by column2 in ascending order.
JOIN must appear before WHERE. For example,

SELECT title
FROM movies
JOIN ratings ON ratings.movie_id = id
WHERE id IN (
    ---
);

We can use INTERSECT to get the intersection of two columns. For example,

SELECT movie_id
FROM stars
WHERE person_id IN (
    SELECT people.id
    FROM people
    WHERE name = 'Bradley Cooper'
)
-- INTERSECT two columns
INTERSECT
SELECT movie_id
FROM stars
WHERE person_id IN (
    SELECT people.id
    FROM people
    WHERE name = 'Jennifer Lawrence'
)

In WHERE statement, you can specify more than one condition and use the AND or OR to connect them. For example,

SELECT title
FROM movies
WHERE year = 2014 AND title = 'Harry Potter%'

In SQL, we can use % to match any string. For example, 'Harry Potter%' will match any string that starts with 'Harry Potter'.

Fiftyville

Things to notice in the problem statement

In the phone_calls table, the unit of duration is seconds.

Divide and Conquer

Fiftyville Algorithm:
1. Get the description from the crime scene
2. Get the three interviews
3. Analyze these three interviews

Useful Snippts

Will update at the end of this course. My log.sql is quite long

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