SQL Clause Writing Order

This is the query writing order:

SELECT
FROM
JOIN
ON
WHERE
GROUP BY
HAVING
WINDOW
ORDER BY
LIMIT

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SQL Logical Execution Order

This is how SQL actually executes internally:

FROM
JOIN
ON
WHERE
GROUP BY
HAVING
SELECT
DISTINCT
ORDER BY
LIMIT

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SQL Formatting Style

Preferred Structure

SELECT
    column_name
FROM
    table_name AS t
WHERE
    condition = 'value'
ORDER BY
    column_name;

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SQL Style Guide

1. Uppercase SQL Keywords

SELECT
FROM
WHERE
GROUP BY
ORDER BY
JOIN
HAVING
LIMIT

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2. One Column Per Line

GOOD:

SELECT
    employee_id,
    employee_name,
    salary

BAD:

SELECT employee_id, employee_name, salary

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3. Use Meaningful Aliases

GOOD:

Employees AS e
Departments AS d
Orders AS o

BAD:

Employees a
Departments b

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4. Avoid SELECT *

GOOD:

SELECT
    employee_id,
    employee_name

BAD:

SELECT *

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5. Align Conditions

WHERE
    salary > 50000
    AND department_id = 1
    AND status = 'ACTIVE'

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Example Solution Template

Problem: Recyclable and Low Fat Products

Approach

• Filter products that are:
  • low fat
  • recyclable

Solution

-- Problem: Recyclable and Low Fat Products

SELECT
    p.product_id
FROM
    Products AS p
WHERE
    p.low_fats = 'Y'
    AND p.recyclable = 'Y';

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Common SQL Patterns

INNER JOIN

SELECT
    e.name,
    d.department_name
FROM
    Employees AS e
INNER JOIN Departments AS d
    ON e.department_id = d.department_id;

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GROUP BY

SELECT
    department_id,
    COUNT(*) AS total_employees
FROM
    Employees
GROUP BY
    department_id;

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HAVING

SELECT
    department_id,
    COUNT(*) AS total_employees
FROM
    Employees
GROUP BY
    department_id
HAVING
    COUNT(*) > 5;

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WINDOW FUNCTION

SELECT
    employee_id,
    salary,
    RANK() OVER (
        PARTITION BY department_id
        ORDER BY salary DESC
    ) AS salary_rank
FROM
    Employees;

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Recommended SQL Learning Order

1. SELECT / WHERE
2. ORDER BY
3. GROUP BY
4. HAVING
5. JOINs
6. Subqueries
7. CTEs
8. Window Functions
9. Advanced Aggregations
10. Optimization

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SQL Best Practices

• Keep queries readable
• Use consistent formatting
• Prefer explicit JOINs
• Use CTEs for complex logic
• Avoid unnecessary nesting
• Write maintainable SQL
• Think about scalability

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Author

Shohag Faraji

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License

MIT License