SQL is a standard language for accessing and manipulating databases.
What is SQL?
- SQL stands for Structured Query Language
- SQL lets you access and manipulate databases
SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987
Every table is broken up into smaller entities called fields. The fields in the Customers table consist of CustomerID, CustomerName, ContactName, Address, City, PostalCode and Country. A field is a column in a table that is designed to maintain specific information about every record in the table.
A record, also called a row, is each individual entry that exists in a table. For example, there are 91 records in the above Customers table. A record is a horizontal entity in a table.
A column is a vertical entity in a table that contains all information associated with a specific field in a table.
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
"abc" #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
#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
#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 ('2022-05-10');
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 or 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 or 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("My","Database");
Lower/Lcase
It converts a string into lowercase
Select Lower("My Database");
Upper/Ucase
It converts a string into uppercase
Select Upper("mydatabase");
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 ' My Data');
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(69);
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;
Note:
In Windows
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