SQL Server Day 7 | WHERE vs HAVING, GROUP BY, UNION Explained | SQL for Data Analyst

In Day 7 of the SQL Server Complete Course, we focus on some of the most confusing yet most important SQL concepts: WHERE vs HAVING, GROUP BY rules, UNION vs UNION ALL, and table-level operations like COPY, DELETE, TRUNCATE, and DROP.
This session clears the confusion around row-level filtering vs aggregated filtering, explains why WHERE cannot be used after GROUP BY, and shows how HAVING solves real-world aggregation problems. You will also learn how to merge result sets, remove or keep duplicates, and safely manage tables and data in SQL Server and MySQL.
This class is extremely useful for:
SQL beginners moving to intermediate level
Data Analysts & BI professionals
Backend / Database developers
SQL interview preparation
⏱️ Timeline / Chapters (Day 7)
00:00 – 03:07 – GROUP BY & WHERE clause execution order
03:08 – 08:22 – HAVING clause explained & why it is required
08:23 – 16:52 – WHERE vs HAVING with aggregation, casting & rounding
16:53 – 29:34 – HAVING with ORDER BY, multiple conditions & aliasing
29:35 – 50:26 – GROUP BY rules with multiple columns
50:27 – 01:04:40 – UNION vs UNION ALL (duplicate handling)
01:04:41 – 01:18:51 – Table copy methods (SELECT INTO, CREATE TABLE AS)
01:18:52 – 01:22:42 – DROP vs DELETE vs TRUNCATE explained
🎯 Key Takeaways
✅ WHERE clause runs before GROUP BY and filters rows
✅ HAVING clause runs after GROUP BY and filters aggregated data
✅ WHERE cannot be used on aggregate functions directly
✅ All non-aggregated columns in SELECT must appear in GROUP BY
✅ UNION removes duplicates, UNION ALL keeps duplicates
✅ Tables can be copied using SELECT INTO or CREATE TABLE AS
✅ DROP removes structure + data
✅ DELETE removes data row-by-row
✅ TRUNCATE clears data fast but keeps structure
✅ Applying WHERE early improves query performance
❓ Frequently Asked Questions (FAQs)
1. Can WHERE be used after GROUP BY?
No. WHERE works before aggregation. Use HAVING after GROUP BY.
2. What is the main difference between WHERE and HAVING?
WHERE filters rows, HAVING filters aggregated results.
3. Difference between UNION and UNION ALL?
UNION removes duplicates, UNION ALL keeps duplicates.
4. How to copy a table without CREATE TABLE?
Use SELECT INTO, but syntax depends on the database system.
5. Difference between DROP, DELETE, and TRUNCATE?
DROP removes table completely, DELETE removes rows, TRUNCATE clears table instantly.
📌 Conclusion
Day 7 strengthens your SQL fundamentals by addressing common logical mistakes developers make with GROUP BY, WHERE, and HAVING. By mastering UNION operations and table-level commands, learners gain confidence in data manipulation, cleanup, and performance-aware querying. These concepts are frequently tested in SQL interviews and used daily in real projects.
🚀 Action Steps
Practice WHERE vs HAVING with aggregation
Rewrite queries to optimize filtering order
Test UNION vs UNION ALL with duplicates
Practice safe DELETE and TRUNCATE usage
Revise GROUP BY rules carefully
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