MySQL Cheatsheet

Database

It is defined as a collection of interrelated data stored together to serve multiple applications.

What is MySQL?

MySQL is an open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter, and "SQL", the abbreviation for Structured Query Language. A relational database organizes data into one or more data tables in which data types may be related to each other; these relations help structure the data. SQL is a language programmers use to create, modify and extract data from the relational database, as well as control user access to the database. In addition to relational databases and SQL, an RDBMS like MySQL works with an operating system to implement a relational database in a computer's storage system, manages users, allows for network access and facilitates testing database integrity and creation of backups.

MySQL is free and open-source software under the terms of the GNU General Public License, and is also available under a variety of proprietary licenses. MySQL was owned and sponsored by the Swedish company MySQL AB, which was bought by Sun Microsystems (now Oracle Corporation). In 2010, when Oracle acquired Sun, Widenius forked the open-source MySQL project to create MariaDB. 

MySQL Elements

MySQL has certain elements that play an important role in querying a database.

Literals

Literals refer to a fixed data value

17 #It is a numeric literal
    "Example" #It is a text literal
    12.5 #It is a real literal

Data Types

Data types are means to identify the type of data.

#Numeric
    
    INT -- Integer data type
    TINYINT
    SMALLINT
    MEDIUMINT
    BIGINT
    
    FLOAT(M,D) -- Floating point data type
    DOUBLE(M,D) -- Double data type also stores decimal values
    DECIMAL(M,D) -- Decimal data type

#Data and Time 
    
    DATE -- Date data type (YYYY-MM-DD)
    DATETIME -- It's a date and time combination (YYYY-MM-DD HH:MM:SS)
    TIME -- It stores time (HH:MM:SS)

#String/Text 
    
    CHAR(M) -- Character data type
    VARCHAR(M) -- Variable character data type
    BLOB or TEXT

NULL Values

If a column has no value, then it is said to be NULL

Comments

A comment is a text that is not executed.

/* This is a multi-line
    comment in MySQL */
    
    # It is a single-line commend
    
    -- It is also a single-line comment

MySQL Simple Calculations

You can perform simple calculations in MySQL, just by using the Select command, there's no need to select any particular database to perform these commands.

Addition

It will add two numbers

Select 5+8;

Subtraction

It will subtract the second number from first

Select 15-5;

Multiplication

It will give the product of supplied numbers

Select 5*5;

Division

It will divide the number

Select 24/4;

-- SQL is not a case-sensitive language

Accessing Database

These commands allow one to check all the databases and tables

Show command

It will show all the databases in the system

Show databases;

It will show all the tables in a selected database

show tables;

Use command

It will start using the specified database i.e. now you can create tables in the selected database

use database_name;

Creating tables

These commands allow you to create the table in MySQL

Create table command

This query is used to create a table in the selected database

Create table <table-name>
    (<column_name> <data_type>,
    <column_name> <data_type>,
    <column_name> <data_type>);

Insert command

It will add data into the selected table

Insert into <table_name> [<column-list>]
    Values (<value1>,<value2>...);

Inserting NULL values

This query will add NULL value in the col3 of the selected table

Inset into <table-name> (col1, col2,col3) 
    Values (val1,val2,NULL);

Inserting Dates

It will add the following data into the selected column of the table

Insert into <table_name> (<col_name>) 
    Values ('2021-12-10');

Select Command

A select query is used to fetch the data from the database

Selecting All Data

It will retrieve all the data of the selected table

Select * From <table_name>;

Selecting Particular Rows

It will retrieve all the data of the row that will satisfy the condition

Select * from <table_name>
    Where <condition_to_satisfy>;

Selecting Particular Columns

It will retrieve data of selected columns that will satisfy the condition

Select column1, column2 from <table_name>
    Where <condition_to_satisfy>;

DISTINCT Keyword

It will retrieve only distinct data i.e. duplicate data rows will get eliminated

Select DISTINCT <column_name> from <table_name>;

ALL Keyword

It will retrieve all the data of the selected column

Select ALL <column_name> from <table_name>;

Column Aliases

It is used to give a temporary name to a table or a column in a table for the purpose of a particular query

Select <column1>,<column2> AS <new_name>
    From <table_name>;

Condition Based on a Range

It will only retrieve data of those columns whose values will fall between value1 and value2 (both inclusive)

Select <co11>, <col2> 
    From <table_name>
    Where <value1> Between <value2>;

Condition Based on a List

Select * from <table_name> 
    Where <column_name> IN (<val1>,<val2>,<val3>);

"Select * from <table_name> 
    Where <column_name> NOT IN (<val1>,<val2>,<val3>);"

Condition Based on Pattern Match

Select <col1>,<col2> 
    From <table_name>
    Where <column> LIKE 'Ha%';

Select <col1>,<col2> 
    From <table_name>
    Where <column> LIKE 'Ha__y%';

Searching NULL

It returns data that contains a NULL value in them

Select <column1>, <column2>
    From <table_name> Where <Val> IS NULL;

 

SQL Constraints

SQL constraints are the rules or checks enforced on the data columns of a table

NOT NULL

It will create a table with NOT NULL constraint to its first column

Create table <table_name>
    ( <col1> <data_type> NOT NULL,
    <col2> <data_type>,
    <col3> <data_type>);

DEFAULT

DEFAULT constraint provides a default value to a column

Create table <table_name>
    ( <col1> <data_type> DEFAULT 50,
    <col2> <data_type>,
    <col3> <data_type>);

UNIQUE

UNIQUE constraint ensures that all values in the column are different

Create table <table_name>
    ( <col1> <data_type> UNIQUE,
    <col2> <data_type>,
    <col3> <data_type>);

CHECK

CHECK constraint ensures that all values in a column satisfy certain conditions

Create table <table_name>
    ( <col1> <data_type> CHECK (condition),
    <col2> <data_type>,
    <col3> <data_type>);

Primary Key

Primary key is used to uniquely identify each row in a table

Create table <table_name>
    ( <col1> <data_type> Primary Key,
    <col2> <data_type>,
    <col3> <data_type>);

Foreign Key

CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
    );

Viewing Table Structure

Desc or Describe command

It allows you to see the table structure

Desc <table_name>;

Modifying Data

Update Command

It will update the values of selected columns

Update <table_name>
    SET <col1> = <new_value>, <col2> = <new_value>
    Where <condition>;

Deleting Data

Delete Command

It will delete the entire row that will satisfy the condition

Delete From <table_name>
    Where <condition>;

Ordering Records

Order by clause is used to sort the data in ascending or descending order of specified column

order by clause

It will return records in the ascending order of the specified column name's data

Select * from <table_name> order by <column_name>;

It will return records in the descending order of the specified column name's data

Select * from <table_name> order by <column_name> DESC;

Ordering data on multiple columns

It will return records in the ascending order of column1 and descending order of column2

Select * From <table_name> order by <column1> ASC, <column2> DESC;

Grouping Result

It is used to arrange identical data into groups so that aggregate functions can work on them

Group by clause

It allows you to group two or more columns and then you can perform aggregate function on them

Select <column>, Count(*) from <table_name> group by <column>;

Having clause

Having clause is used to put conditions on groups

Select avg(<column>), sum(<column>) from <table_name> group by <column_name> having <condition_to_satisfy>;

Altering Table

These commands allow you to change the structure of the table

To Add New Column

It will add a new column in your table

Alter Table <table_name>
    Add <new_column>;

To Modify Old Column

It will update the data type or size of old column

Alter Table <table_name>
    Modify <old_column_name> [<new_data_type><size>];

To Change Name of Column

It will change the name of the old column in the table

Alter Table Change <old_column_name> <new_column_name><data_type>;

Dropping Table

DROP command

It will delete the complete table from the database

Drop table <table_name>;

 

 

MySQL Functions:

There are many functions in MySQL that perform some task or operation and return a single value

Text/String Functions

Text function work on strings

Char Function

It returns the character for each integer passed

Select Char(72,97,114,114,121);

Concat Function

It concatenates two strings

Select Concat("Hello","World");

Lower/Lcase

It converts a string into lowercase

Select Lower("Hello","World");

Upper/Ucase

It converts a string into uppercase

Select Upper("Database");

Substr

It extracts a substring from a given string

Select Substr(string,m,n);

Trim

It removes leading and trailing spaces from a given string

Select Trim(leading ' ' FROM ' Example ');

Instr

It searches for given second string into the given first string

Select Instr(String1,String2);

Length

It returns the length of given string in bytes

Select Length(String)

Numeric Functions

Numeric function works on numerical data and returns a single output

MOD

It returns modulus of two numbers

Select MOD(11,4);

Power

It returns the number m raised to the nth power

Select Power(m,n);

Round

It returns a number rounded off number

Select Round(15.193,1);

Sqrt

It returns the square root of a given number

Select Sqrt(144);

Truncate

It returns a number with some digits truncated

Select Truncate(15.75,1);

Date/Time Functions

These are used to fetch the current date and time and allow you to perform several operations on them

Curdate Function

It returns the current date

Select Curdate();

Date Function

It extracts the date part of the expression

Select Date('2021-12-10 12:00:00');

Month Function

It returns the month from the date passed

Select Month(date);

Day Function

It returns the day part of a date

Select Day(date);

Year Function

It returns the year part of a date

Select Year(date);

Now Function

It returns the current date and time

Select now();

Sysdate Function

It returns the time at which function executes

Select sysdate();

Aggregate Functions

Aggregate functions or multiple row functions work on multiple data and returns a single result

AVG Function

It calculates the average of given data

Select AVG(<column_name>) "Alias Name" from <table_name>;

COUNT Function

It counts the number of rows in a given column

Select Count(<column_name>) "Alias Name" from <table_name>;

MAX Function

It returns the maximum value from a given column

Select Max(<column_name>) "Alias Name" from <table_name>;

MIN Function

It returns the minimum value from a given column

Select Min(<column_name>) "Alias Name" from <table_name>;

SUM Function

It returns the sum of values in given column

Select Sum(<column_name>) "Alias Name" from <table_name>;

MySQL Joins

Join clause is used to combine or merge rows from two or more tables based on a related attribute

INNER JOIN

It returns all rows from multiple tables where the join condition is satisfied. It is the most common type of join.

SELECT columns FROM table1 INNER JOIN table2 ON table1.column = table2.column;

LEFT OUTER JOIN

It returns all rows from the left-hand table specified in the ON condition and only those rows from the other table where the join condition is fulfilled.

SELECT columns FROM table1 LEFT [OUTER] JOIN table2 ON table1.column = table2.column;

RIGHT OUTER JOIN

It returns all rows from the RIGHT-hand table specified in the ON condition and only those rows from the other table where the join condition is satisfied

SELECT columns FROM table1 RIGHT [OUTER] JOIN table2 ON table1.column = table2.column;

FULL JOIN

It combines the results of both left and right outer joins

SELECT column_name FROM table1 FULL OUTER JOIN table2 ON table1.column_name = table2.column_name WHERE condition;

SELF JOIN

In this join, table is joined with itself

SELECT column_name FROM table1 T1, table1 T2 WHERE condition;

 

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