GROUP BY and HAVING

GROUP BY — Summarise per Group

GROUP BY splits rows into groups and applies an aggregate to each group.

SELECT class, COUNT(*) AS total
FROM students
GROUP BY class;

Average per Group

SELECT class, AVG(marks) AS avg_marks
FROM students
GROUP BY class;

HAVING — Filter Groups

HAVING filters groups after aggregation (WHERE filters rows before).

SELECT class, AVG(marks) AS avg_marks
FROM students
GROUP BY class
HAVING AVG(marks) >= 80;

WHERE vs HAVING

Summary

• GROUP BY makes groups; aggregates run per group.
• HAVING filters groups; WHERE filters rows before grouping.

GROUP BY — हर Group का Summary

GROUP BY rows को groups में बाँटकर हर group पर aggregate लगाता है।

SELECT class, COUNT(*) AS total
FROM students
GROUP BY class;

हर Group का Average

SELECT class, AVG(marks) AS avg_marks
FROM students
GROUP BY class;

HAVING — Groups Filter करें

HAVING aggregation के बाद groups filter करता है (WHERE पहले rows filter करता है)।

SELECT class, AVG(marks) AS avg_marks
FROM students
GROUP BY class
HAVING AVG(marks) >= 80;

WHERE बनाम HAVING

सारांश

• GROUP BY groups बनाता है; aggregates हर group पर चलते हैं।
• HAVING groups filter करता है; WHERE grouping से पहले rows।