SQL ALIASES

---

SQL Tutorial • 09-Sep-2023

by Malik Umair Sajjad

---

SQL, or Structured Query Language, is the standard language for managing and manipulating databases. As you dive deeper into the world of SQL, you’ll discover various techniques and tools that can make your queries more efficient and readable. One such tool is the SQL Alias. In this article, we’ll explore what SQL Aliases are, why they’re beneficial, and how to use them effectively with examples.

Introduction to SQL Aliases

SQL Aliases are temporary names assigned to a table or a column for the purpose of a particular SQL query. They are useful for providing a shorthand method of referencing columns or tables, especially if their names are long or cumbersome.

Syntax:

For a column:

SELECT column_name AS alias_name
FROM table_name;

For a table:

SELECT column_name
FROM table_name AS alias_name;

Benefits of SQL Aliases

1. Readability: Makes SQL statements more readable especially when using joins and multiple tables.
2. Efficiency: Reduces the amount of text you need to write, especially with complex queries.
3. Avoiding Conflicts: Helpful when two tables have columns with the same names.

When to Use an SQL Alias

1. Complex Queries: When working with multi-table queries or complex SQL queries.
2. Aggregated Data: When using aggregate functions like SUM, COUNT, etc.
3. Subqueries: With subqueries in the FROM clause.
4. Self-Joins: When a table is joined to itself.

Column Alias

Column aliases are used to rename a column’s header in the result set.

Syntax:

SELECT column_name AS alias_name
FROM table_name;

Example:

Suppose we have a table named Students:

StudentID	FirstName	LastName
1	John	Doe
2	Jane	Smith

Query:

SELECT FirstName AS First, LastName AS Last
FROM Students;

Result:

First	Last
John	Doe
Jane	Smith

Explanation:

Here, “FirstName” is displayed as First and LastName as Last.

Table Alias

Table aliases are used to provide a table with a temporary name, which can make queries shorter.

Syntax:

SELECT column_name
FROM table_name AS alias_name;

Example:

Suppose we have a table named Orders:

OrderID	ProductName	Quantity
1	Apple	10
2	Banana	5

Query:

SELECT o.OrderID, o.ProductName
FROM Orders AS o;

Result:

OrderID	ProductName
1	Apple
2	Banana

Explanation:

Here, the table Orders is referenced as o in the query.

Aliases in Complex Queries

Aliases are particularly beneficial when dealing with complex queries involving multiple tables.

Example:

Given two tables Employees and Departments.

Employees:

EmpID	Name	DeptID
1	Alice	101
2	Bob	102

Departments:

DeptID	DeptName
101	HR
102	Finance

Query:

SELECT e.Name, d.DeptName
FROM Employees AS e, Departments AS d
WHERE e.DeptID = d.DeptID;

Result:

Name	DeptName
Alice	HR
Bob	Finance

Explanation:

Here, we join the two tables using their respective aliases.

Alias with Aggregate Functions

When using aggregate functions, aliases can provide a clearer column name for the result.

Example:

Given a table Sales:

SaleID	Amount
1	100
2	150

Query:

SELECT SUM(Amount) AS TotalSales
FROM Sales;

Result:

TotalSales
250

Explanation:

Here, the sum of all sales is displayed under the column TotalSales.

Alias with Subqueries

Aliases are crucial when using subqueries, especially in the FROM clause.

Example:

Suppose we have a table Products:

ProdID	Price
1	50
2	100

Query:

SELECT AVG(s.Price) AS AveragePrice
FROM (SELECT Price FROM Products WHERE Price > 50) AS s;

Result:

AveragePrice
100

Explanation:

The sub-query fetches products with a price greater than 50, and the outer query calculates the average of those prices.

Using an Alias with a Self-Join

Self-joins are joins in which a table is joined with itself. Aliases are essential here to distinguish between the two instances of the table.

Example:

Given a table Employees:

EmpID	Name	ManagerID
1	Alice	NULL
2	Bob	1

Query:

SELECT e1.Name AS Employee, e2.Name AS Manager
FROM Employees AS e1, Employees AS e2
WHERE e1.ManagerID = e2.EmpID;

Result:

Employee	Manager
Bob	Alice

Explanation:

Here, Bob’s manager is Alice.

Using SQL Alias with JOIN

Using aliases can make JOIN operations more readable.

Example:

Given two tables Users and Orders.

Users:

UserID	UserName
1	John
2	Jane

Orders:

OrderID	UserID	Product
1	1	Laptop
2	2	Book

Query:

SELECT u.UserName, o.Product
FROM Users AS u
JOIN Orders AS o ON u.UserID = o.UserID;

Result:

UserName	Product
John	Laptop
Jane	Book

Explanation:

Here, we fetched products ordered by each user.

Using an Alias with Non-Unique Column Names

When two tables have columns with the same name, aliases can help differentiate them.

Example:

Given two tables Teachers and Students, both having a column “Name”.

Using the alias:

SELECT t.Name AS TeacherName, s.Name AS StudentName
FROM Teachers AS t, Students AS s
WHERE t.ClassID = s.ClassID;

Explanation:

This way, we can clearly differentiate between teacher names and student names in the result set.

Common Mistakes

1. Forgetting the AS Keyword: While the AS keyword is optional in many databases, it’s a good practice to use it for clarity.
2. Not Using Aliases in Joins: This can lead to confusion when tables have columns with the same name.
3. Inconsistent Alias Usage: Once you set an alias, ensure you use it consistently throughout the query.

FAQs

Q1: Can I use SQL aliases outside the query they are defined in?

• Answer: No, aliases are temporary and can only be used within the query they are defined in.

Q2: Can I use numerical values as aliases?

• Answer: Yes, but it’s not recommended as it can lead to confusion. Always aim for meaningful alias names.

Q3: Can two tables in the same query have the same alias?

• Answer: No, aliases within a query must be unique.

Practice Questions

1. Given a table Products, write a query to fetch the total price of all products using an alias TotalPrice.
2. Write a query to fetch the first name and last name from a table Persons as a single column alias FullName.
3. Using a table Books, write a query to fetch the average price of all books with a price greater than 50 using a sub-query and an alias AvgPriceAbove50.
4. Given two tables Authors and Books, write a query using aliases to fetch the name of the author and the title of the book they wrote.

Happy querying!