SELECT Statement

Introduction

SQL (Structured Query Language) is used to communicate with relational databases. SQL statements are composed of clauses, expressions, and predicates that define what actions to perform on the database. The SELECT statement is the cornerstone of any SQL query. It is used to retrieve data from one or more tables within a database. In MS SQL, the SELECT statement allows you to query specific columns, filter data, perform aggregations, and even combine data from multiple tables.

Here’s a basic structure of an SQL statement:

• SELECT: Specifies the columns to retrieve.
• FROM: Specifies the table to retrieve data from.

Syntax

The general syntax of a SELECT statement in MS SQL is as follows:

SELECT column1, column2, ...
FROM table_name;

You specify the columns you want to retrieve after the SELECT keyword, followed by the table name after FROM.

Example Let’s assume you have a table called Customers. To retrieve all employee names and their department IDs, you can run:

Table

Customers

SELECT customer_name, City
FROM Customers;

This query will return the values from the Customer_name and City columns.

Selecting All Columns with SELECT *

You can retrieve all columns from a table by using the wildcard character *:

SELECT *
FROM Customers;

This will return every column in the Customers table. While it is convenient, be cautious when using SELECT * in production environments, as it can negatively impact performance, especially if the table has many columns or records.

Conclusion

The SELECT statement is a powerful and versatile tool in MS SQL for querying and manipulating data. Whether you're fetching specific rows, aggregating data, or joining multiple tables, mastering the SELECT statement is essential for any SQL professional. By understanding its various clauses and functions, you can write more efficient, readable, and maintainable queries.

Asked in Interview

• What is the purpose of the SELECT statement in SQL?
• How would you retrieve unique values from a column using SELECT?
• What is the difference between SELECT *` and specifying individual columns?