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Lecture 10

Databases

A database is an organized collection of structured information. A database is usually controlled by a database management system (DBMS).

Database types

  • Hierarchical

  • Network

  • Relational

  • Object-oriented

  • Graph

  • Document

ACID

  • Atomicity
  • Consistency
  • Isolation
  • Durability

Atomicity means that you either:

  • Commit to the entirety of the transaction occurring
  • Have no transaction at all

Essentially, an atomic transaction ensures that any commit you make finishes the entire operation successfully. Or, in cases of a lost connection in the middle of an operation, the database is rolled back to its state prior to the commit being initiated.

Consistency refers to maintaining data integrity constraints.

A consistent transaction will not violate integrity constraints placed on the data by the database rules. Enforcing consistency ensures that if a database enters into an illegal state (if a violation of data integrity constraints occurs) the process will be aborted and changes rolled back to their previous, legal state. Another way of ensuring consistency within a database throughout each transaction is by also enforcing declarative constraints placed on the database.

Isolated transactions are considered to be “serializable”, meaning each transaction happens in a distinct order without any transactions occurring in tandem.

Any reads or writes performed on the database will not be impacted by other reads and writes of separate transactions occurring on the same database. A global order is created with each transaction queueing up in line to ensure that the transactions complete in their entirety before another one begins.

Importantly, this doesn’t mean two operations can’t happen at the same time. Multiple transactions can occur as long as those transactions have no possibility of impacting the other transactions occurring at the same time.

Durability ensures that changes made to the database (transactions) that are successfully committed will survive permanently, even in the case of system failures.

SQL

SQL is a programming language used by nearly all relational databases to query, manipulate, and define data, and to provide access control. SQL was first developed at IBM in the 1970s. In the late 1970s, Relational Software, Inc. (now Oracle Corporation) saw the potential of the SQL and developed their own SQL-based RDBMS.

Manipulation

Column Constraints

Column constraints are the rules applied to the values of individual columns:

  • PRIMARY KEY constraint can be used to uniquely identify the row.
  • UNIQUE columns have a different value for every row.
  • NOT NULL columns must have a value.
  • DEFAULT assigns a default value for the column when no value is specified.

There can be only one PRIMARY KEY column per table and multiple UNIQUE columns.

CREATE TABLE student
(
    id    INT PRIMARY KEY,
    name  INT UNIQUE,
    grade INT NOT NULL,
    age   INT DEFAULT 10
);

CREATE TABLE Statement

The CREATE TABLE statement creates a new table in a database. It allows one to specify the name of the table and the name of each column in the table.

CREATE TABLE table_name
(
    column1 datatype,
    column2 datatype,
    column3 datatype
);

INSERT Statement

The INSERT INTO statement is used to add a new record (row) to a table.

It has two forms as shown:

  • Insert into columns in order.
  • Insert into columns by name.
-- Insert into columns in order:
INSERT INTO my_table
VALUES (value1, value2);

-- Insert into columns by name:
INSERT INTO my_table (column1, column2)
VALUES (value1, value2);

ALTER TABLE Statement

The ALTER TABLE statement is used to modify the columns of an existing table. When combined with the ADD COLUMN clause, it is used to add a new column.

ALTER TABLE my_table
    ADD new_column datatype;

DELETE Statement

The DELETE statement is used to delete records (rows) in a table. The WHERE clause specifies which record or records that should be deleted. If the WHERE clause is omitted, all records will be deleted.

DELETE
FROM my_table
WHERE some_column = some_value;

UPDATE Statement

The UPDATE statement is used to edit records (rows) in a table. It includes a SET clause that indicates the column to edit and a WHERE clause for specifying the record(s).

UPDATE my_table
SET column1 = value1,
    column2 = value2
WHERE some_column = some_value;

Queries

AND Operator

The AND operator allows multiple conditions to be combined. Records must match both conditions that are joined by AND to be included in the result set. The given query will match any car that is blue and made after 2014.

SELECT model
FROM cars
WHERE color = 'blue'
  AND release_year > 2014;

AS Clause

Columns or tables can be aliased using the AS clause. This allows columns or tables to be specifically renamed in the returned result set. The given query will return a result set with the column for title renamed to movie_title.

SELECT title AS 'movie_title'
FROM movies;

OR Operator

The OR operator allows multiple conditions to be combined. Records matching either condition joined by the OR are included in the result set. The given query will match customers whose state is either 'CA' or 'NY'.

SELECT name
FROM customers
WHERE state = 'CA'
   OR state = 'NY';

SELECT Statement

The SELECT * statement returns all columns from the provided table in the result set. The given query will fetch all columns and records (rows) from the movies table.

SELECT *
FROM movies;

LIKE Operator

The LIKE operator can be used inside of a WHERE clause to match a specified pattern. The given query will match any movie that begins with Star in its title.

SELECT name
FROM movies
WHERE name LIKE 'Star%';

% Wildcard

The % wildcard can be used in a LIKE operator pattern to match zero or more unspecified character(s). The given query will match any movie that begins with The, followed by zero or more of any characters.

SELECT name
FROM movies
WHERE name LIKE 'The%';

_ Wildcard

The _ wildcard can be used in a LIKE operator pattern to match any single unspecified character. The given query will match any movie which begins with a single character, followed by ove.

SELECT name
FROM movies
WHERE name LIKE '_nception';

ORDER BY Clause

The ORDER BY clause can be used to sort the result set by a particular column either alphabetically or numerically. It can be ordered in two ways:

  • DESC is a keyword used to sort the results in descending order.
  • ASC is a keyword used to sort the results in ascending order (default).
SELECT *
FROM contacts
ORDER BY birth_date DESC;

DISTINCT Clause

Unique values of a column can be selected using a DISTINCT query. For a table contact_details having five rows in which the city column contains Chicago, Madison, Boston, Madison, and Denver, the given query would return Chicago, Madison, Boston, Denver

SELECT DISTINCT city
FROM contact_details;

BETWEEN Operator

The BETWEEN operator can be used to filter by a range of values. The range of values can be text, numbers, or date data. The given query will match any movie made between the years 1980 and 1990, inclusive.

SELECT *
FROM movies
WHERE year BETWEEN 1980 AND 1990;

LIMIT Clause

The LIMIT clause is used to narrow, or limit, a result set to the specified number of rows. The given query will limit the result set to 5 rows.

SELECT *
FROM movies LIMIT 5;

NULL Values

Column values can be NULL, or have no value. These records can be matched (or not matched) using the IS NULL and IS NOT NULL operators in combination with the WHERE clause. The given query will match all addresses where the address has a value or is not NULL.

SELECT address
FROM records
WHERE address IS NOT NULL;

WHERE Clause

The WHERE clause is used to filter records (rows) that match a certain condition. The given query will select all records where the pub_year equals 2017.

SELECT title
FROM library
WHERE pub_year = 2017;

Aggregate Functions

Aggregate functions perform a calculation on a set of values and return a single value:

  • COUNT()
  • SUM()
  • MAX()
  • MIN()
  • AVG()

Column References

The GROUP BY and ORDER BY clauses can reference the selected columns by number in which they appear in the SELECT statement. The example query will count the number of movies per rating, and will:

  • GROUP BY column 2 (rating)
  • ORDER BY column 1 (total_movies)
SELECT COUNT(*) AS 'total_movies', rating
FROM movies
GROUP BY 2
ORDER BY 1;

SUM() Aggregate Function

The SUM() aggregate function takes the name of a column as an argument and returns the sum of all the value in that column.

SELECT SUM(salary)
FROM salary_disbursement;

MAX() Aggregate Function

The MAX() aggregate function takes the name of a column as an argument and returns the largest value in a column. The given query will return the largest value from the amount column.

SELECT MAX(amount)
FROM transactions;

COUNT() Aggregate Function

The COUNT() aggregate function returns the total number of rows that match the specified criteria. For instance, to find the total number of employees who have less than 5 years of experience, the given query can be used.

Note: A column name of the table can also be used instead of _. Unlike COUNT(_), this variation COUNT(column)will not countNULL values in that column.

SELECT COUNT(*)
FROM employees
WHERE experience < 5;

GROUP BY Clause

The GROUP BY clause will group records in a result set by identical values in one or more columns. It is often used in combination with aggregate functions to query information of similar records. The GROUP BY clause can come after FROM or WHERE but must come before any ORDER BY or LIMIT clause.

The given query will count the number of movies per rating.

SELECT rating,
       COUNT(*)
FROM movies
GROUP BY rating;

MIN() Aggregate Function

The MIN() aggregate function returns the smallest value in a column. For instance, to find the smallest value of the amount column from the table named transactions, the given query can be used.

SELECT MIN(amount)
FROM transactions;

AVG() Aggregate Function

The AVG() aggregate function returns the average value in a column. For instance, to find the average salary for the employees who have less than 5 years of experience, the given query can be used.

SELECT AVG(salary)
FROM employees
WHERE experience < 5;

HAVING Clause

The HAVING clause is used to further filter the result set groups provided by the GROUP BY clause. HAVING is often used with aggregate functions to filter the result set groups based on an aggregate property. The given query will select only the records (rows) from only years where more than 5 movies were released per year.

SELECT year,
    COUNT (*)
FROM movies
GROUP BY year
HAVING COUNT (*) > 5;

ROUND() Function

The ROUND() function will round a number value to a specified number of places. It takes two arguments: a number, and a number of decimal places. It can be combined with other aggregate functions, as shown in the given query. This query will calculate the average rating of movies from 2015, rounding to 2 decimal places.

SELECT year,
    ROUND(AVG (rating), 2)
FROM movies
WHERE year = 2015;

Operators

Operators

Logical:

  • AND
  • OR
  • NOT

Comparison:

  • <
  • >
  • <=
  • >=
  • =
  • <> or !=

Math:

  • /
  • %
  • ^
  • |/
  • @

Multiple Tables

Inner Join

The JOIN clause allows for the return of results from more than one table by joining them together with other results based on common column values specified using an ON clause. INNER JOIN is the default JOIN and it will only return results matching the condition specified by ON.

SELECT *
FROM books
JOIN authors
ON books.author_id = authors.id;

Outer Join

An outer join will combine rows from different tables even if the join condition is not met. In a LEFT JOIN, every row in the left table is returned in the result set, and if the join condition is not met, then NULL values are used to fill in the columns from the right table.

SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;

CROSS JOIN Clause

The CROSS JOIN clause is used to combine each row from one table with each row from another in the result set. This JOIN is helpful for creating all possible combinations for the records (rows) in two tables.

The given query will select the shirt_color and pants_color columns from the result set, which will contain all combinations of combining the rows in the shirts and pants tables. If there are 3 different shirt colors in the shirts table and 5 different pants colors in the pants table then the result set will contain 3 x 5 = 15 rows.

SELECT shirts.shirt_color,
       pants.pants_color
FROM shirts
CROSS JOIN pants;

WITH Clause

The WITH clause stores the result of a query in a temporary table (temporary_movies) using an alias.

Multiple temporary tables can be defined with one instance of the WITH keyword.

WITH temporary_movies AS (
    SELECT *
    FROM movies
)
SELECT *
FROM temporary_movies
WHERE year BETWEEN 2000 AND 2020;

UNION Clause

The UNION clause is used to combine results that appear from multiple SELECT statements and filter duplicates.

For example, given a first_names table with a column name containing rows of data “James” and “Hermione”, and a last_names table with a column name containing rows of data “James”, “Hermione” and “Cassidy”, the result of this query would contain three names: “Cassidy”, “James”, and “Hermione”.

SELECT name
FROM first_names
UNION
SELECT name
FROM last_names

Foreign Key

A foreign key is a reference in one table’s records to the primary key of another table. To maintain multiple records for a specific row, the use of foreign key plays a vital role. For instance, to track all the orders of a specific customer, the table order (illustrated at the bottom of the image) can contain a foreign key.

Primary Key

A primary key column in a SQL table is used to uniquely identify each record in that table. A primary key cannot be NULL. In the example, customer_id is the primary key. The same value cannot re-occur in a primary key column. Primary keys are often used in JOIN operations.

psql

  • Create database
CREATE DATABASE WITH OWNER some_user;
  • Create user
CREATE ROLE some_username WITH PASSWORD 'somepassword';
  • Allow login
ALTER ROLE some_username WITH LOGIN;
  • Delete user
DROP ROLE some_username;
  • Get all tables
\dt
  • Get all users
\du
  • Describe table schema:
\d+ some_table_name
  • Run script from file:
psql -f myscript.sql