MySQL Cheatsheet

Database

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

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
"Harry" # 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

# Date 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 comment
 
-- 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 the 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.

Insert 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 <col1>, <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("Harry","Bhai");

Lower/Lcase

It converts a string into lowercase.

Select Lower("Harry Bhai");

Upper/Ucase

It converts a string into uppercase.

Select Upper("CodeWithHarry");

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 ' Harry Bhai');

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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