Meritshot Tutorials
SQL Tutorial
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SQL SyntaxSQL Syntax
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SQL ORDER BY ClauseSQL ORDER BY Clause
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Introduction to SQLIntroduction to SQL
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SQL SELECT DISTINCTSQL SELECT DISTINCT
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SQL Logical OperatorsSQL Logical Operators
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SQL WHERE ClauseSQL WHERE Clause
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SQL SELECT StatementSQL SELECT Statement
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SQL DELETE StatementSQL DELETE Statement
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SQL INSERT INTOSQL INSERT INTO
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SQL Null ValuesSQL Null Values
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SQL Update StatementSQL Update Statement
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SQL Select TopSQL Select Top
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SQL Aggregate FunctionsSQL Aggregate Functions
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SQL LIKE and WildcardsSQL LIKE and Wildcards
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SQL IN and SQL BETWEENSQL IN and SQL BETWEEN
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SQL JOINSSQL JOINS
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SQL Group BySQL Group By
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SQL HavingSQL Having
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SQL EXISTSSQL EXISTS
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SQL SELECT INTOSQL SELECT INTO
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SQL INSERT INTO SELECTSQL INSERT INTO SELECT
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SQL CASE STATEMENTSQL CASE STATEMENT
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SQL NULL FunctionsSQL NULL Functions
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SQL Stored ProceduresSQL Stored Procedures
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SQL User-Defined FunctionsSQL User-Defined Functions
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SQL CommentsSQL Comments
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SQL OperatorsSQL Operators
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SQL Database Creation and ManagementSQL Database Creation and Management
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SQL CREATE DATABASE StatementSQL CREATE DATABASE Statement
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SQL CREATE TABLE StatementSQL CREATE TABLE Statement
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SQL DROP DATABASE StatementSQL DROP DATABASE Statement
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SQL DROP TABLE StatementSQL DROP TABLE Statement
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SQL ALTER TABLE StatementSQL ALTER TABLE Statement
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SQL NOT NULL ConstraintSQL NOT NULL Constraint
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SQL UNIQUE ConstraintSQL UNIQUE Constraint
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SQL PRIMARY KEY ConstraintSQL PRIMARY KEY Constraint
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SQL FOREIGN KEY ConstraintSQL FOREIGN KEY Constraint
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SQL CHECK ConstraintSQL CHECK Constraint
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SQL DEFAULT ConstraintSQL DEFAULT Constraint
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SQL IndexesSQL Indexes
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SQL Date FunctionsSQL Date Functions
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SQL ViewsSQL Views
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SQL InjectionSQL Injection
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SQL Data Types OverviewSQL Data Types Overview
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SQL AUTO_INCREMENTSQL AUTO_INCREMENT
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SQL Keywords ReferenceSQL Keywords Reference
SQL Views
A VIEW in SQL is a virtual table that provides a way to present data from one or more tables in a specific format or structure. Views do not store data themselves but are defined by a query that retrieves data from underlying tables. They can be used to simplify complex queries, encapsulate data access, and provide a consistent interface to data.
Syntax
— Creating a view
CREATE VIEW view_name AS
SELECT column1, column2, …
FROM table_name
WHERE condition;
— Updating a view
CREATE OR REPLACE VIEW view_name AS
SELECT column1, column2, …
FROM table_name
WHERE condition;
— Dropping a view
DROP VIEW view_name;
- view_name: The name of the view to be created, updated, or dropped.
- column1, column2, …: The columns to be included in the view.
- table_name: The underlying table(s) from which the view retrieves data.
- condition: The criteria for filtering data in the view.
Example:
1 Creating a Basic View
Suppose you have a table Employees and you want to create a view that shows only employees from the Sales department:
CREATE VIEW SalesEmployees AS
SELECT EmployeeID, FullName, Position
FROM Employees
WHERE Department = ‘Sales’;
This view, SalesEmployees, will display only employees who work in the Sales department.
2 Updating an Existing View
If you want to update the SalesEmployees view to include the HireDate column:
CREATE OR REPLACE VIEW SalesEmployees AS
SELECT EmployeeID, FullName, Position, HireDate
FROM Employees
WHERE Department = ‘Sales’;
3 Updating an Existing View
If you want to update the SalesEmployees view to include the HireDate column:
CREATE OR REPLACE VIEW SalesEmployees AS
SELECT EmployeeID, FullName, Position, HireDate
FROM Employees
WHERE Department = ‘Sales’;
4 Creating a View with Joins
Suppose you have two tables, Orders and Customers, and you want to create a view that combines order details with customer information:
CREATE VIEW CustomerOrders AS
SELECT o.OrderID, o.OrderDate, c.CustomerName, c.ContactNumber
FROM Orders o
JOIN Customers c ON o.CustomerID = c.CustomerID;
This view, CustomerOrders, will show order details along with the customer names and contact numbers.
Tips to Remember
- Views Are Read-Only: In many database systems, views are read-only. To update data, you may need to perform updates on the underlying tables directly.
- Use for Security: Views can help in restricting access to sensitive data by exposing only specific columns or rows.
- Performance Considerations: While views simplify query writing, be aware that complex views or views based on other views can affect query performance.
Frequently Asked Questions
Q1: Can a view be based on another view?
A1: Yes, views can be nested. A view can be created based on another view, but be cautious of performance impacts and complexity.
Q2: Can I use views to enforce data integrity?
A2: Views themselves do not enforce data integrity, but they can provide a simplified and controlled way to access data, reducing the risk of erroneous data manipulation.
Q3: How do I refresh a view if the underlying data changes?
A3: Views are updated automatically with the latest data from the underlying tables. No action is needed to refresh the view.
Q4: Can I include aggregated data in a view?
A4: Yes, you can include aggregated data using functions like SUM(), COUNT(), AVG(), etc., in a view.
Q5: Are there any limitations to using views?
A5: Limitations may vary by database system. Common limitations include restrictions on updating views directly and potential performance impacts for complex views.
