The SELECT statement is the foundation of SQL queries. Here’s how to use it effectively.

Basic SELECT

Retrieve all columns from a table:

SELECT * FROM employees;

Select specific columns:

SELECT name, email, salary FROM employees;

Column Aliases

Rename columns in output:

SELECT 
    name AS employee_name,
    salary AS annual_salary
FROM employees;

WHERE Clause - Filtering

Filter rows based on conditions:

-- Equals
SELECT * FROM employees WHERE department = 'Engineering';

-- Comparison operators
SELECT * FROM employees WHERE salary > 50000;
SELECT * FROM employees WHERE age >= 25;

-- Multiple conditions (AND/OR)
SELECT * FROM employees 
WHERE department = 'Engineering' AND salary > 60000;

SELECT * FROM employees 
WHERE department = 'Engineering' OR department = 'Marketing';

LIKE - Pattern Matching

Search for patterns in strings:

-- Names starting with 'A'
SELECT * FROM employees WHERE name LIKE 'A%';

-- Names ending with 'son'
SELECT * FROM employees WHERE name LIKE '%son';

-- Names containing 'an'
SELECT * FROM employees WHERE name LIKE '%an%';

IN - Multiple Values

Match against a list:

SELECT * FROM employees 
WHERE department IN ('Engineering', 'Marketing', 'Sales');

ORDER BY - Sorting

Sort results:

-- Ascending (default)
SELECT * FROM employees ORDER BY name;

-- Descending
SELECT * FROM employees ORDER BY salary DESC;

-- Multiple columns
SELECT * FROM employees ORDER BY department, salary DESC;

LIMIT - Restrict Results

Limit number of rows returned:

-- First 10 rows
SELECT * FROM employees LIMIT 10;

-- Skip first 5, get next 10
SELECT * FROM employees LIMIT 10 OFFSET 5;

DISTINCT - Unique Values

Get unique values:

SELECT DISTINCT department FROM employees;

NULL Handling

Check for NULL values:

-- Find rows with NULL
SELECT * FROM employees WHERE manager_id IS NULL;

-- Find rows without NULL
SELECT * FROM employees WHERE manager_id IS NOT NULL;

Complete Example

SELECT 
    name,
    department,
    salary
FROM employees
WHERE department IN ('Engineering', 'Marketing')
    AND salary > 50000
ORDER BY salary DESC
LIMIT 10;

Summary

Use SELECT to choose columns
Use WHERE to filter rows
Use ORDER BY to sort results
Use LIMIT to restrict row count
Use DISTINCT for unique values

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