How SQL DISTINCT removes duplicate rows (and why it applies to every selected column), how it treats NULL, COUNT(DISTINCT), and how to use column and table aliases, including why an alias works in ORDER BY but not in WHERE.
How to calculate a running total and a running account balance in SQL Server with SUM() OVER, reset it per month or account, and avoid the two mistakes that give wrong or slow results.
Search text with LIKE, match a list with IN and filter ranges with BETWEEN, with simple examples, the NOT IN NULL trap and the date BETWEEN mistake that hides rows.
A clear, 9-stage plan to learn SQL from zero: what to learn first, in what order, how long each stage takes, and a free tutorial for every step.
The SQL string functions you’ll use most in SQL Server and MySQL: CONCAT, LEN, TRIM, SUBSTRING, CHARINDEX, REPLACE and STRING_AGG, plus the NULL trap that blanks whole results.
Understand what NULL means in SQL, why = NULL never works, and how to handle missing values with IS NULL, COALESCE and NULLIF.
Learn how to use the SQL CASE expression for if/then/else logic: labels, conditional counts, custom sorting and more, with simple examples.
A beginner-friendly guide to SQL window functions: OVER, PARTITION BY, ROW_NUMBER, RANK, running totals and LAG, with clear examples.
Learn what a SQL view is, how to create one, and how views simplify queries, improve security and keep reports consistent.
Learn how to add, change and remove data in SQL with INSERT, UPDATE and DELETE, plus the safety habits that stop costly mistakes.
