MySQL notes - Basic Commands and Definitions

MySQL Notes
Structured Query Language

What is Database?
1. Collection of data
2. A method for accessing and manipulating data
3. A structured set of computerized data with an
accessible interface

Data Deﬁnition Language (DDL)
A set of statements that allow the user to deﬁne or modify data structures and objects,
such as tables
- CREATE
- ALTER
- DROP
- RENAME
- TRUNCATE

Data Manipulation Language (DML)
Its statements allow us to manipulate the data in the tables of a database
- SELECT… FROM…
- INSERT INTO… VALUES…
- UPDATE… SET… WHERE…
- DELETE FROM… WHERE…

Data Control Language (DCL)
the GRANT and REVOKE statements
allow us to manage the rights users have in a database
GRANT type_of_permission ON database_name.table_name TO ‘username’@’localhost’
REVOKE type_of_permission ON database_name.table_name FROM ‘username’@’localhost’

Transaction Control Language (DCL)
not every change you make to a database is saved automatically
the COMMIT statement
- will save the changes you’ve made
- will let other users have access to the modiﬁed version of the database
- related to INSERT, DELETE, UPDATE
the ROLLBACK clause
the clause that will let you make a step back
- allows you to undo any changes you have made but don’t want to be saved permanently

SQL Syntax
DDL – Data Deﬁnition Language
SQL Syntax creation of data
DML – Data Manipulation Language
manipulation of data
DCL – Data Control Language
assignment and removal of permissions to use this data
TCL – Transaction Control Language
saving and restoring changes to a database

Primary Key
A column (or a set of columns) whose value exists and is unique for every record in a table
is called a primary key
- each table can have one and only one primary key
- in one table, you cannot have 3 or 4 primary keys
- primary keys are the unique identiﬁers of a table
- cannot contain null values!
- not all tables you work with will have a primary key

Foreign Key
Identiﬁes the relationships between tables, not the tables themselves
Unique Key
used whenever you would like to specify that you don’t want to see duplicate data
in a given ﬁeld

Relationships
relationships tell you how much of the data from a foreign key ﬁeld can be seen in the primary
key column of the table the data is related to and vice versa
types of relationships
- one-to-many (many-to-one)
- one-to-one
- many-to-many

Creating Database
show databases;
CREATE DATABASE
<name>;
CREATE DATABASE
soap_store;
CREATE DATABASE
DogApp;
CREATE DATABASE
My App;
USE <database name>;
SELECT database();

Diﬀerent Data Types in MySQL
Numeric Types
a. INT
b. SMALLINT
c. TINYINT
d. MEDIUMINT
e. BIGINT
f. DECIMAL
g. NUMERIC
h. FLOAT
i. DOUBLE
j. BIT
Strings Types
a. CHAR
b. VARCHAR
c. BINARY
d. VARBINARY
e. BLOB
f. TINYBLOB
g. MEDIUMBLOB
h. LONGBLOB
i. TEXT
j. TINYTEXT
k. MEDIUMTEXT
l. LONGTEXT
m. ENUM
Date Types
a. DATE
b. DATETIME
c. TIMESTAMP
d. TIME
e. YEAR

Creating Table
CREATE TABLE tablename
(
column_name data_type,
column_name data_type
);
CREATE TABLE cats
(
name VARCHAR(100),
age INT
);
SHOW TABLES;
SHOW COLUMNS FROM <tablename>;
DESC <tablename>;

Deleting Table
DROP TABLE <tablename>;
Insert into Table
INSERT INTO cats(name, age) VALUES ("Jetson", 7);
INSERT INTO cats(name, age) VALUES ('Charlie', 10) ,('Sadie', 3) ,('Lazy Bear', 1);

Using Database and Tables
USE sales;
SELECT * FROM customers;
SELECT * FROM sales.customers;
DROP TABLE sales;

Constraints
Speciﬁc rules, or limits, that we deﬁne in our tables
- the role of constraints is to outline the existing relationships between different tables in
our database
Example :- NOT NULL

Foreign Key
ON DELETE CASCADE
if a speciﬁc value from the parent table’s primary key has been deleted, all the records from the
child table referring to this value will be removed as well

DISTINCT / AGGREGATE FUNCTION
ORDER BY

IFNULL() / COALESCE()
For One Column
For Multiple Columns

CROSS JOIN / AGGREGATE FUNCTION WITH JOIN

VIEW
a virtual table whose contents are obtained from an existing table or tables, called base tables
- The view itself does not contain any real data
- the data is physically stored in the base table
- the view simply shows the data contained in the base table

MySQL notes - Basic Commands and Definitions

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