Select Clause

The SELECT clause is one of the most fundamental and frequently used components of SQL. It allows you to retrieve data from one or more tables in a database.

Basic Syntax

The basic syntax of the SELECT statement is:

SELECT column1, column2, ...
FROM table_name;

Examples

1. Selecting All Columns

To select all columns from a table, use the asterisk (*):

SELECT *
FROM employees;

This query retrieves all columns and all rows from the employees table.

2. Selecting Specific Columns

To select only specific columns:

SELECT first_name, last_name, email
FROM employees;

This query retrieves only the first_name, last_name, and email columns from the employees table.

3. Using Aliases

You can use aliases to give columns or tables different names in the result set:

SELECT 
    first_name AS "First Name",
    last_name AS "Last Name",
    salary * 12 AS "Annual Salary"
FROM employees;

This query renames the columns in the output and calculates an annual salary.

4. Selecting Distinct Values

To select only unique values, use the DISTINCT keyword:

SELECT DISTINCT department
FROM employees;

This query returns a list of unique departments from the employees table.

5. Using Expressions

You can use expressions in your SELECT statement:

SELECT 
    product_name,
    unit_price,
    units_in_stock,
    unit_price * units_in_stock AS total_value
FROM products;

This query calculates the total value of each product.

6. Concatenating Columns

In many databases, you can concatenate columns:

SELECT 
    CONCAT(first_name, ' ', last_name) AS full_name,
    email
FROM employees;

This query combines first_name and last_name into a single full_name column.

7. Using Functions

SQL provides various built-in functions that you can use in SELECT statements:

SELECT 
    COUNT(*) AS total_employees,
    AVG(salary) AS average_salary,
    MAX(salary) AS highest_salary,
    MIN(salary) AS lowest_salary
FROM employees;

This query uses aggregate functions to calculate statistics about employee salaries.

Remember, the SELECT clause is often used in conjunction with other clauses like WHERE, GROUP BY, HAVING, and ORDER BY to create more complex and specific queries.