Microsoft SQL Server

In SQL, the SELECT statement is part of the Data Query Language (DQL) and is used to retrieve data from a database. The SELECT * command is one of the simplest ways to retrieve all columns from a table. This is especially useful for beginners as it helps fetch complete records without specifying column names. However, understanding when and how to use SELECT * effectively is important as it can have performance implications, especially in larger databases.

Here’s a breakdown of how to use SELECT * in Microsoft SQL Server and additional details that will help you better understand this command.

1. Basic Syntax of SELECT *

The SELECT * query retrieves all columns from a specific table. Here’s a simple syntax:

SELECT * 
FROM table_name;

• Replace table_name with the actual table name you want to query.
• It will fetch all rows and columns from the specified table.

Example:

SELECT * 
FROM Products;

This query will return all columns (e.g., ProductID, ProductName, Price, etc.) from the Products table.

2. Using SELECT * with a WHERE Clause

If you want to filter the results to return only specific records, you can use the WHERE clause:

SELECT * 
FROM Sales 
WHERE SaleDate = '2023-09-01';

This query retrieves all columns from the Sales table where the SaleDate is equal to 2023-09-01.

3. Limiting Results Using TOP

To limit the number of rows returned by your query, you can use the TOP clause:

SELECT TOP 10 * 
FROM Customers;

This query will return only the first 10 rows from the Customers table.

4. Join Tables with SELECT *

You can also use SELECT * in a join to retrieve data from multiple tables at once:

SELECT * 
FROM Sales s
JOIN Customers c 
ON s.CustomerID = c.CustomerID;

This query returns all columns from both the Sales and Customers tables, where there is a match on the CustomerID.

5. Best Practices for Using SELECT *

Using SELECT * is convenient but can lead to inefficiencies, especially as the database grows. Consider these best practices:

1. Avoid SELECT * in production code – Instead, specify only the columns you need, which can reduce the amount of data retrieved and improve query performance.

   SELECT ProductName, Price 
   FROM Products;

2. Use Aliases for Better Readability – When using joins or multiple tables, it's a good practice to use table aliases to make your query more readable.

   SELECT c.CustomerName, s.SaleAmount 
   FROM Sales s
   JOIN Customers c 
   ON s.CustomerID = c.CustomerID;

3. Limit Data with WHERE and TOP – Always try to limit the amount of data returned by using WHERE conditions or the TOP clause.

4. Consider Indexes – Retrieving data with SELECT * from large tables without proper indexing can be slow. Ensure that your table has proper indexes to support frequent queries.

By following these steps, you’ll improve both the performance and maintainability of your SQL queries, even as your database grows in complexity.

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SELECT * FROM org_employee;

SELECT * FROM org_client;

SELECT * FROM act_payment;

SELECT client_id, first_name, last_name, gender, city
FROM org_client;

SELECT city, gender, first_name, last_name, email, client_id
FROM org_client;

SELECT city, gender, first_name, last_name, email FROM org_client;

SELECT city
,gender
,first_name
,last_name
,email
FROM org_client;

SELECT city, gender, first_name, last_name, email
FROM org_client
ORDER BY first_name;

SELECT email
FROM org_client;

Example 1:

Let's understand the process of extracting data from a specified table. In this case, we aim to retrieve all rows and columns from the employee table.

Example 1 - Raw data from employee table

Example 1 - Query

SELECT * FROM org_employee;

Example 1 - Query data mapping

In above image, the color green signifies that the output for our query will include all rows and columns being selected.

Example 1 - Query Output

Example 2:

Let's understand the process of extracting desired columns from a specified table. In this case, we aim to retrieve 4 columns from the employee table.

Example 2 - Raw data from employee table

Example 2 - Query

SELECT employee_id, first_name, last_name, email FROM org_employee;

Example 2 - Query data mapping

In above image, the color green signifies that the output for our query will include data from desired columns(employee_id, first_name, last_name and email).

Example 2 - Query Output