In this article, we will discuss the overview of SQL and will mainly focus on Concepts and Queries and will understand each with the help of examples. Let's discuss it one by one.
Overview :
SQL is a computer language that is used for storing, manipulating, and retrieving data in a structured format. This language was invented by IBM. Here SQL stands for Structured Query Language. Interacting databases with SQL queries, we can handle a large amount of data. There are several SQL-supported database servers such as MySQL, PostgreSQL, sqlite3 and so on. Data can be stored in a secured and structured format through these database servers. SQL queries are often used for data manipulation and business insights better.
SQL Database :
Here, we will discuss the queries and will understand with the help of examples.
Query-1 :
Show existing databases -
Let's consider the existing database like information_schema, mysql, performance_schema, sakila, student, sys, and world. And if you want to show the exiting database then we will use the show database query as follows.
SHOW DATABASES;
Output :
Existing database Name |
---|
information_schema |
mysql |
performance_schema |
sakila |
student |
sys |
world |
Query-2 :
Drop a database -
Suppose we want to drop the database namely student.
DROP DATABASE student;
SHOW DATABASES;
Database Name |
---|
information_schema |
mysql |
performance_schema |
sakila |
sys |
world |
Query-3 :
Create a database -
Suppose we want to create a database namely a bank.
CREATE DATABASE bank;
SHOW DATABASES;
Database Name |
---|
bank |
information_schema |
mysql |
performance_schema |
sakila |
sys |
world |
Query-4 :
Using a database -
USE bank;
Query-5 :
Create a Table -
Here data type may be varchar, integer, date, etc.
CREATE TABLE table_name (
column1 datatype,
column2 datatype,
....
);
Example -
CREATE TABLE IF NOT EXISTS Employee (
EmployeeID int,
FirstName varchar(55),
LastName varchar(55),
Email varchar(150),
DOB date
);
Query-6 :
Show tables in the same database -
SHOW TABLES;
Query-7 :
Dropping a Table -
DROP TABLE table_name;
Query-8 :
Inserting values into an existing table -
INSERT INTO Employee
VALUES(1111,'Dipak','Bera','[email protected]','1994-11-22');
Query-9 :
Fetching values in a table -
SELECT * FROM Employee;
Query-10 :
Not Null -
We can specify which column does not accept the null value when we insert a value(row) in a table. It will be done at the time of table creation.
CREATE TABLE table_name (
column1 datatype NOT NULL,
column2 datatype,
....
);
Query-11 :
Unique -
We can also specify that entries in a particular column should be unique.
CREATE TABLE table_name (
column1 datatype UNIQUE,
column2 datatype,
....
);
Example -
CREATE TABLE demo_table
(
EmployeeID int NOT NULL UNIQUE,
FirstName varchar(55),
LastName varchar(55)
);
KEY CONCEPTS in SQL :
Here, we will discuss some important concepts like keys, join operations, having clauses, order by, etc. Let's discuss it one by one.
- PRIMARY KEY -
The constraint PRIMARY KEY suggests that entries should be neither null nor duplicate corresponding to the specified column.
CREATE TABLE IF NOT EXISTS Customer(
CustID int NOT NULL,
FName varchar(55),
LName varchar(55),
Email varchar(100),
DOB date,
CONSTRAINT customer_custid_pk PRIMARY KEY(CustID)
);
- FOREIGN KEY -
The FOREIGN KEY is used to build a connection between the current table and the previous table containing the primary key.
CREATE TABLE Account(
AccNo int NOT NULL,
AType varchar(20),
OBal int,
OD date,
CurBal int,
CONSTRAINT customer_AccNo_fk FOREIGN KEY(AccNo) REFERENCES Customer(CustID)
);
- Here, AccNo column in the Account table is referred to by CustID column in the Customer table. Here Account table is a child table and the Customer table is the parent table.
ORDER BY :
TheORDER BY keyword is used to show the result in ascending or descending order. By default, it is in ascending order.
Syntax -
SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;
- Scenario-1 :
Suppose we have the Account Table as follows.
AccNo | AType | OBal | OD | CurBal |
---|
1111 | savings | 1000 | 1990-11-09 | 6000 |
1114 | current | 2000 | 1992-10-07 | 1000 |
1113 | current | 7000 | 1992-11-03 | 4000 |
1112 | savings | 1000 | 2003-12-12 | 3000 |
- Now, we will use the Order By command as follows.
SELECT * FROM Account ORDER BY CurBal;
Output :
(By default it will be in increasing order)
AccNo | AType | OBal | OD | CurBal |
---|
1114 | current | 2000 | 1992-10-07 | 1000 |
1112 | savings | 1000 | 2003-12-12 | 3000 |
1113 | current | 7000 | 1992-11-03 | 4000 |
1111 | savings | 1000 | 1990-11-09 | 6000 |
- Scenario-2 :
For descending order :
SELECT * FROM Account ORDER BY CurBal DESC;
Output :
AccNo | AType | OBal | OD | CurBal |
---|
1111 | savings | 1000 | 1990-11-09 | 6000 |
1113 | current | 7000 | 1992-11-03 | 4000 |
1112 | savings | 1000 | 2003-12-12 | 3000 |
1114 | current | 2000 | 1992-10-07 | 1000 |
GROUP BY :
This keyword is used for grouping the results.
Example -
SELECT COUNT(AType) FROM Account GROUP BY AType;
Output :
AType | count(AType) |
---|
savings | 2 |
current | 2 |
JOIN CONCEPTS :
Here, we will discuss the join concept as follows.
- LEFT JOIN :
The LEFT JOIN keyword returns all records from the left table (table1) along with the matching records from the right table (table2).
Syntax -
SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;
LEFT JOIN- RIGHT JOIN :
The RIGHT JOIN keyword returns all records from the right table (table2) along with the matching records from the left table (table1). - Syntax -
SELECT column_name(s)
FROM table1
RIGHT JOIN table2
ON table1.column_name = table2.column_name;
RIGHT JOIN- INNER JOIN :
The INNER JOIN keyword returns all matching records from both the table. - Syntax -
SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name;
INNER JOIN- FULL JOIN :
The FULL JOIN or FULL OUTER JOIN keyword returns all records from both the table. - Syntax -
SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name = table2.column_name;
FULL JOIN- Note -
This keyword is not used in MySQL's latest version. Instead, the keyword UNION is used. Here the syntax is as follows.
SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;
- SELF JOIN :
This is a regular join between aliases of the same table. - Syntax -
SELECT column_name(s)
FROM table1 T1, table1 T2
WHERE condition;
WHERE CLAUSE :
This clause is used for filtering our data.
Syntax -
SELECT column1, column2, ...
FROM table_name
WHERE condition;
Example -
SELECT AccNo,CurBal FROM Account WHERE CurBal>=1000;
Output :
AccNo | CurBal |
---|
1111 | 6000 |
1113 | 4000 |
1114 | 1000 |
HAVING CLAUSE :
This is required as the WHERE clause does not support aggregate functions such as count, min, max, avg, sum, and so on.
SELECT column1, column2, ...
FROM table_name
HAVING condition;
Example -
SELECT AccNo,CurBal FROM Account HAVING CurBal=MAX(CurBal);
Output :
Similar Reads
SQL Tutorial Structured Query Language (SQL) is the standard language used to interact with relational databases. Mainly used to manage data. Whether you want to create, delete, update or read data, SQL provides the structure and commands to perform these operations. Widely supported across various database syst
8 min read
Basics
What is SQL?Structured Query Language (SQL) is the standard language used to interact with relational databases. Allows users to store, retrieve, update, and manage data efficiently through simple commands. Known for its user-friendly syntax and powerful capabilities, SQL is widely used across industries.How Do
6 min read
SQL Data TypesSQL data types define the kind of data a column can store, dictating how the database manages and interacts with the data. Each data type in SQL specifies a set of allowed values, as well as the operations that can be performed on the values.SQL data types are broadly categorized into several groups
4 min read
SQL OperatorsSQL operators are symbols or keywords used to perform operations on data in SQL queries. These operations can include mathematical calculations, data comparisons, logical manipulations, other data-processing tasks. Operators help in filtering, calculating, and updating data in databases, making them
5 min read
SQL Commands | DDL, DQL, DML, DCL and TCL CommandsSQL commands are the fundamental building blocks for communicating with a database management system (DBMS). It is used to interact with the database with some operations. It is also used to perform specific tasks, functions, and queries of data. SQL can perform various tasks like creating a table,
7 min read
SQL Database OperationsSQL databases or relational databases are widely used for storing, managing and organizing structured data in a tabular format. These databases store data in tables consisting of rows and columns. SQL is the standard programming language used to interact with these databases. It enables users to cre
3 min read
SQL CREATE TABLEIn SQL, creating a table is one of the most essential tasks for structuring your database. The CREATE TABLE statement defines the structure of the database table, specifying column names, data types, and constraints such as PRIMARY KEY, NOT NULL, and CHECK. Mastering this statement is fundamental to
5 min read
Queries & Operations
SQL SELECT QueryThe SQL SELECT query is one of the most frequently used commands to retrieve data from a database. It allows users to access and extract specific records based on defined conditions, making it an essential tool for data management and analysis. In this article, we will learn about SQL SELECT stateme
4 min read
SQL INSERT INTO StatementThe SQL INSERT INTO statement is one of the most essential commands for adding new data into a database table. Whether you are working with customer records, product details or user information, understanding and mastering this command is important for effective database management. How SQL INSERT I
6 min read
SQL UPDATE StatementIn SQL, the UPDATE statement is used to modify existing records in a table. Whether you are updating a single record or multiple records at once, SQL provides the necessary functionality to make these changes. Whether you are working with a small dataset or handling large-scale databases, the UPDATE
6 min read
SQL DELETE StatementThe SQL DELETE statement is an essential command in SQL used to remove one or more rows from a database table. Unlike the DROP statement, which removes the entire table, the DELETE statement removes data (rows) from the table retaining only the table structure, constraints, and schema. Whether you n
4 min read
SQL | WHERE ClauseThe SQL WHERE clause allows filtering of records in queries. Whether you are retrieving data, updating records, or deleting entries from a database, the WHERE clause plays an important role in defining which rows will be affected by the query. Without WHERE clause, SQL queries would return all rows
4 min read
SQL | AliasesIn SQL, aliases are temporary names assigned to columns or tables for the duration of a query. They make the query more readable, especially when dealing with complex queries or large datasets. Aliases help simplify long column names, improve query clarity, and are particularly useful in queries inv
4 min read
SQL Joins & Functions
SQL Joins (Inner, Left, Right and Full Join)SQL joins are fundamental tools for combining data from multiple tables in relational databases. Joins allow efficient data retrieval, which is essential for generating meaningful observations and solving complex business queries. Understanding SQL join types, such as INNER JOIN, LEFT JOIN, RIGHT JO
4 min read
SQL CROSS JOINIn SQL, the CROSS JOIN is a unique join operation that returns the Cartesian product of two or more tables. This means it matches each row from the left table with every row from the right table, resulting in a combination of all possible pairs of records. In this article, we will learn the CROSS JO
3 min read
SQL | Date Functions (Set-1)SQL Date Functions are essential for managing and manipulating date and time values in SQL databases. They provide tools to perform operations such as calculating date differences, retrieving current dates and times and formatting dates. From tracking sales trends to calculating project deadlines, w
5 min read
SQL | String functionsSQL String Functions are powerful tools that allow us to manipulate, format, and extract specific parts of text data in our database. These functions are essential for tasks like cleaning up data, comparing strings, and combining text fields. Whether we're working with names, addresses, or any form
7 min read
Data Constraints & Aggregate Functions
SQL NOT NULL ConstraintIn SQL, constraints are used to enforce rules on data, ensuring the accuracy, consistency, and integrity of the data stored in a database. One of the most commonly used constraints is the NOT NULL constraint, which ensures that a column cannot have NULL values. This is important for maintaining data
3 min read
SQL PRIMARY KEY ConstraintThe PRIMARY KEY constraint in SQL is one of the most important constraints used to ensure data integrity in a database table. A primary key uniquely identifies each record in a table, preventing duplicate or NULL values in the specified column(s). Understanding how to properly implement and use the
5 min read
SQL Count() FunctionIn the world of SQL, data analysis often requires us to get counts of rows or unique values. The COUNT() function is a powerful tool that helps us perform this task. Whether we are counting all rows in a table, counting rows based on a specific condition, or even counting unique values, the COUNT()
7 min read
SQL SUM() FunctionThe SUM() function in SQL is one of the most commonly used aggregate functions. It allows us to calculate the total sum of a numeric column, making it essential for reporting and data analysis tasks. Whether we're working with sales data, financial figures, or any other numeric information, the SUM(
5 min read
SQL MAX() FunctionThe MAX() function in SQL is a powerful aggregate function used to retrieve the maximum (highest) value from a specified column in a table. It is commonly employed for analyzing data to identify the largest numeric value, the latest date, or other maximum values in various datasets. The MAX() functi
4 min read
AVG() Function in SQLSQL is an RDBMS system in which SQL functions become very essential to provide us with primary data insights. One of the most important functions is called AVG() and is particularly useful for the calculation of averages within datasets. In this, we will learn about the AVG() function, and its synta
4 min read
Advanced SQL Topics
SQL | SubqueryIn SQL, a subquery can be defined as a query embedded within another query. It is often used in the WHERE, HAVING, or FROM clauses of a statement. Subqueries are commonly used with SELECT, UPDATE, INSERT, and DELETE statements to achieve complex filtering and data manipulation. They are an essential
5 min read
Window Functions in SQLSQL window functions are essential for advanced data analysis and database management. It is a type of function that allows us to perform calculations across a specific set of rows related to the current row. These calculations happen within a defined window of data and they are particularly useful
6 min read
SQL Stored ProceduresStored procedures are precompiled SQL statements that are stored in the database and can be executed as a single unit. SQL Stored Procedures are a powerful feature in database management systems (DBMS) that allow developers to encapsulate SQL code and business logic. When executed, they can accept i
7 min read
SQL TriggersA trigger is a stored procedure in adatabase that automatically invokes whenever a special event in the database occurs. By using SQL triggers, developers can automate tasks, ensure data consistency, and keep accurate records of database activities. For example, a trigger can be invoked when a row i
7 min read
SQL Performance TuningSQL performance tuning is an essential aspect of database management that helps improve the efficiency of SQL queries and ensures that database systems run smoothly. Properly tuned queries execute faster, reducing response times and minimizing the load on the serverIn this article, we'll discuss var
8 min read
SQL TRANSACTIONSSQL transactions are essential for ensuring data integrity and consistency in relational databases. Transactions allow for a group of SQL operations to be executed as a single unit, ensuring that either all the operations succeed or none of them do. Transactions allow us to group SQL operations into
8 min read
Database Design & Security
Introduction of ER ModelThe Entity-Relationship Model (ER Model) is a conceptual model for designing a databases. This model represents the logical structure of a database, including entities, their attributes and relationships between them. Entity: An objects that is stored as data such as Student, Course or Company.Attri
10 min read
Introduction to Database NormalizationNormalization is an important process in database design that helps improve the database's efficiency, consistency, and accuracy. It makes it easier to manage and maintain the data and ensures that the database is adaptable to changing business needs.Database normalization is the process of organizi
6 min read
SQL InjectionSQL Injection is a security flaw in web applications where attackers insert harmful SQL code through user inputs. This can allow them to access sensitive data, change database contents or even take control of the system. It's important to know about SQL Injection to keep web applications secure.In t
7 min read
SQL Data EncryptionIn todayâs digital era, data security is more critical than ever, especially for organizations storing the personal details of their customers in their database. SQL Data Encryption aims to safeguard unauthorized access to data, ensuring that even if a breach occurs, the information remains unreadab
5 min read
SQL BackupIn SQL Server, a backup, or data backup is a copy of computer data that is created and stored in a different location so that it can be used to recover the original in the event of a data loss. To create a full database backup, the below methods could be used : 1. Using the SQL Server Management Stu
4 min read
What is Object-Relational Mapping (ORM) in DBMS?Object-relational mapping (ORM) is a key concept in the field of Database Management Systems (DBMS), addressing the bridge between the object-oriented programming approach and relational databases. ORM is critical in data interaction simplification, code optimization, and smooth blending of applicat
7 min read