MySQL-cheatsheet

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MySQL-cheatsheet

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Cheatsheet for MySQL

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aggregation.sql
constraints.sql
ddl-commands.sql
dml-commands.sql
dql-commands.sql
joins.sql
LICENSE
README.md
transaction.sql
views.sql
where.sql

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# MySQL cheatsheet

I have even uploaded the .sql file which you can download and directly run them in the sql prompt.

### General Commands
To run sql files
```sql
source <filename>.sql;
```

## Data Types

#### Integers

```sql
INT
```
```sql
TINYINT
```
```sql
SMALLINT
```
```sql
MEDIUMINT
```
```sql
BIGINT
```

#### Float

```sql
FLOAT(M,D)
```

#### Double

```sql
DOUBLE(M,D)
```

#### Decimal

```sql
DECIMAL(M,D)
```

#### Date

```sql
DATE -- Format - (YYYY-MM-DD)
```

#### Date Time

```sql
DATETIME -- Format - (YYYY-MM-DD HH:MM:SS)
```

#### Time

```sql
TIME -- Format - (HH:MM:SS)
```

#### String

```sql
CHAR(M)
```

```sql
VARCHAR(M)
```

```sql
BLOB or TEXT
```

## Comments

```sql
/* Multi
line
comment */
```

```sql
# Single Line Comment
```

```sql
-- Single Line Comment
```

## Data Definition Language (DDL)

#### Create Database
```sql
create database cheatsheet;
```

#### Use Database
```sql
use cheatsheet;
```

#### Show Databases
```sql
show databases;
```

#### Create Table
```sql
create table employee
(
    employee_id int primary key,              -- Setting primary key(1st method)
    first_name varchar(50),
    last_name varchar(50),
    dept_number int,
    age int,
    salary real
);

create table department
(
    dept_number int,
    dept_name varchar(50),
    dept_location varchar(50),
    emp_id int,
    primary key(dept_number)                -- Setting primary key(2nd method)
);
```

#### Show Tables
```sql
show tables;
```

#### Describe Table
```sql
describe employee;
desc employee;
show columns in employee;
```

#### Rename Table
```sql
rename table employee to employee_table;
alter table employee_table rename to employee;
```

#### Renaming Column
```sql
alter table employee change column employee_id emp_id int;
```

#### Add Constraint to Column
```sql
alter table employee change column first_name first_name varchar(50) not null;
```

#### Add Column
```sql
alter table employee add column salary real;
```

#### Drop Column
```sql
alter table employee drop column salary;
```

#### Modify the Datatype of column
```sql
alter table employee modify column salary int;
```

#### Truncate Table
```sql
truncate employee;
```

#### Drop Table
```sql
drop table department;
```

#### Drop Database
```sql
drop database cheatsheet;
```
## Data Manipulation Language (DML)

#### Insertion (Complete)
```sql
insert into employee (employee_id, first_name, last_name, dept_number, age, salary) values (1, "Anurag", "Peddi", 1, 20, 93425.63);

insert into employee values (2, "Anuhya", "Peddi", 2, 20, 83425.63);
```
#### Insertion (Partial)
```sql
insert into employee (employee_id, first_name) values (3, "Vageesh");
```

#### Updating all rows
```sql
update employee set salary = 1.1 * salary;
```

#### Updating a specified row
```sql
update employee set salary = 1.2 * salary where employee_id = 1;
```

#### Delete a specified row
```sql
delete from employee where employee_id = 2;
```

#### Delete all rows
```sql
delete from employee;
```

#### Enabling foreign key checks
```sql
set foreign_key_checks = 1;
```

#### Disabling foreign key checks
```sql
set foreign_key_checks = 0;
```

## Data Query Language (DQL)

#### Display Table
```sql
select * from employee;
```

#### Select only specified columns
```sql
select employee_id, first_name from employee;
```

#### Select only few rows
```sql
select employee_id, first_name from employee where age > 25;
```

### Where Clause

#### Greater than(>)
```sql
select * from employee where salary > 3100;
```

#### Greater than equal to(>=)
```sql
select * from employee where salary >= 3100;
```

#### Less than(<)
```sql
select * from employee where salary < 4500;
```

#### Less than equal to(<=)
```sql
select * from employee where salary <= 4350;
```

#### Range
```sql
select * from employee where salary > 3000 and salary < 4000;
```

#### BETWEEN and AND
```sql
select * from employee where salary between 3000 and 4000;
```

### OR
```sql
select * from employee where salary = 3000 or salary = 4000;
```

#### Null
```sql
select * from employee where salary is NULL;
```

#### Not null
```sql
select * from employee where salary is NOT NULL;
```

### ORDER BY Clause
```sql
select * from employee ORDER BY salary DESC;
```

#### Like Operator
```sql
select * from employee where name like '%Jo%';          -- Similar to *Jo* in regrex
```
```sql
select * from employee where name like 'Jo_';           -- Similar to Jo. in regrex
```

## Views

#### Create a view
```sql
create view personal_info as select first_name, last_name, age from employees;
```

#### Displaying view
```sql
select * from personal_info;
```

#### Updating in view
```sql
update personal_info set salary = 1.1 * salary;
```

#### Deleting record from view
```sql
delete from personal_info where age < 40;
```

#### Droping a view
```sql
drop view personal_info;
```

## Joins
#### Inner join
```sql
select e.fname, p.pname from employees as e inner join project as p on e.eid = p.eid;

-- or

select e.fname, p.pname from employees as e join project as p on e.eid = p.eid;
```

#### Full outer join
```sql
select e.fname, p.pname from employees as e left outer join project as p on e.eid = p.eid
union
select e.fname, p.pname from employees as e right outer join project as p on e.eid = p.eid;
```

#### Left outer join
```sql
select e.fname, p.pname from employees as e left outer join project as p on e.eid = p.eid;
```

#### Right outer join
```sql
select e.fname, p.pname from employees as e right outer join project as p on e.eid = p.eid;
```

#### Left outer join - inner join
```sql
select e.fname, p.pname from employees as e left outer join project as p on e.eid = p.eid where p.pname is null;
```

#### Right outer join - inner join
```sql
select e.fname, p.pname from employees as e right outer join project as p on e.eid = p.eid where e.fname is null;
```

## Aggregation

#### Sum function
```sql
select sum(population) from city group by population;
```

#### Average function
```sql
select avg(population) from city group by population;
```

#### Count function
```sql
select district, count(district) from city group by district;
```

#### Maximum function
```sql
select max(population) from city group by population;
```

#### Minimum function
```sql
select min(population) from city group by population;
```

#### Standard deviation function
```sql
select stddev(population) from city group by population;
```

#### Group concat function
```sql
select group_concat(population) from city group by population;
```

> Only COUNT function considers NULL values

## Procedure

#### Creating procedure
```sql
create procedure display_dbs()
show databases;
```

#### Calling procedure
```sql
call display_dbs();
```

#### Drop procedure
```sql
drop procedure display_dbs;
```

## Transaction

#### Begin transaction
```sql
start transaction;
```

#### Create savepoint
```sql
savepoint sv_pt;
```

```sql
delete from city;       -- changing data in table
```

#### Rollback
```sql
rollback to sv_pt;
```

#### Releasing savepoint
```sql
release savepoint sv_pt;
```

#### Commiting changes
```sql
commit;
```

## Constraints

#### Not Null
```sql
alter table Employee
change
    Age
    Age int NOT NULL;
```

#### Unique
```sql
alter table Employee
add constraint u_q unique(ID);
```
```sql
alter table Employee -- drop the constraint
drop constraint u_q;
```

#### Primary Key
```sql
alter table Employee
add constraint p_k primary key(ID);
```
```sql
alter table Employee -- drop the constraint
drop constraint p_k;
```

#### Check
```sql
alter table Employee
add constraint Age check (age>=30);
```
```sql
alter table Employee -- drop the constraint
drop check Age;
```

#### Default
```sql
alter table Employee
alter Age set default 10;
```
```sql
alter table Employee -- drop the constraint
alter Age drop default;
```

## Cloning

#### Duplicate a Table Schema
```sql
create table emp_dup like employee;
```

#### Duplicate a Table
```sql
create table emp_dup select * from employee;
```

## Access Controls

#### Creating New User
```sql
CREATE USER 'username'@'localhost' IDENTIFIED BY 'password';
```
the hostname part is set to `localhost`, so the user will be able to connect to the MySQL server only from the localhost.  
To grant access from another host, change the hostname part with the remote machine IP.  
```sql
CREATE USER 'username'@'172.8.10.5' IDENTIFIED BY 'user_password';
```
To create a user that can connect from any host, '%' is used in the hostname part:
```sql
CREATE USER 'username'@'%' IDENTIFIED BY 'user_password';
```

#### Grant All Permissions
```sql
GRANT ALL PRIVILEGES ON * . * TO 'username'@'localhost';
```
Asterisks(\*) refers to the database and table names respectively.  
By using asterisks we can give access of all the databases **or** tables to the user.

#### Flush Privileges
```sql
FLUSH PRIVILEGES
```
All the changes won't be in effect unless this query is fired.

#### Specific User Permissions
```sql
GRANT type_of_permission ON database_name.table_name TO 'username'@'localhost';
```
`type_of_permission` may have one of these value: 
* **ALL PRIVILEGES** - Allows user full access to a designated database (or if no database is selected, global access across the system).
* **CREATE** - allows them to create new tables or databases.
* **DROP** - allows them to them to delete tables or databases.
* **DELETE** - allows them to delete rows from tables.
* **INSERT** - allows them to insert rows into tables.
* **SELECT** - allows them to use the `SELECT` command to read through databases.
* **UPDATE** - allow them to update table rows.
* **GRANT OPTION** - allows them to grant or remove other users’ privileges.  
Multiple permissions are given with commas.

#### Revoking permissions
```sql
REVOKE type_of_permission ON database_name.table_name FROM 'username'@'localhost';
```

#### Show User's Current Permissions
```sql
SHOW GRANTS FOR 'username'@'localhost';
```

#### Delete a User
```sql
DROP USER 'username'@'localhost';
```

#### Set new password to a user
```sql
use mysql;
update user set authentication_string=PASSWORD("<new2-password>") where User='<user>';
flush privileges;
```

## Reset Root Password
Stop MySQL service 
```
sudo systemctl stop mysql
```
Restart MySQL service without loading grant tables
```bash
sudo mysqld_safe --skip-grant-tables &
```
The apersand (&) will cause the program to run in the background and `--skip-grant-tables` enables everyone to to connect to the database server without a password and with all privileges granted.
Login to shell
```
mysql -u root
```
Set new password for root
```sql
ALTER USER 'root'@'localhost' IDENTIFIED BY 'MY_NEW_PASSWORD';
FLUSH PRIVILEGES;
```
Stop and start the server once again
```
mysqladmin -u root -p shutdown
sudo systemctl start mysql
```
## Programming

#### Declare variables
```sql
set @num = 10;
set @name = 'Anurag';
```

#### Print them
```sql
select @name;
```

#### For loop
```sql
set @n = 21;
select repeat("* ", @n := @n - 1) from information_schema.tables where @n > 0;
```

## Miscellaneous

#### Round
```sql
select round(3.141596, 3);
```

#### Repeated concatenation
```sql
select repeat("* ", 20);
```

#### Random float
```sql
select rand();
```

#### Typecast to Int
```sql
select cast(23.01245 as signed);
```

#### Concatenation
```sql
select concat("Mahesh", " ", "Chandra", " ", "Duddu", "!");
```

#### Extract Month
```sql
select month("1998-12-30");
```

#### Extract Year
```sql
select year("1998-12-30");
```

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