- 01Getting Started
- 02SELECT Statement Basics
- 03Filtering Rows with WHERE
- 04Sorting Results with ORDER BY
- 05Combining Tables with JOIN
- 06Aggregating Data with GROUP BY
- 07Fuzzy Matching with LIKE
- 08Modifying Data with INSERT, UPDATE, and DELETE
- 09Introduction to Subqueries
- 10Creating a VIEW
- 11Ranking Rows with Window Functions
- 12Understanding Transactions
- 13Writing Comments in SQL
- 14Logical Operators: AND, OR, NOT
- 15Working with NULL Values
- 16INNER JOIN vs. LEFT JOIN
- 17Filtering Aggregates with HAVING
- 18Removing Duplicates with DISTINCT
- 19Combining Results with UNION
- 20Conditional Values with CASE
- 21Checking Existence with EXISTS
- 22Changing Table Structure with ALTER TABLE
- 23Self Joins
- 24Speeding Up Searches with INDEX
- 25GROUP BY with Multiple Columns
- 26Pagination with LIMIT and OFFSET
- 27Pattern Matching with GLOB
- 28Date Calculations with the date() Function
- 29Joining Three Tables
- 30Matching Multiple Values with IN
- 31Derived Tables: Subqueries in FROM
- 32The Basics of TRIGGER
- 33Readable Queries with WITH (CTEs)
- 34UPSERT: Insert or Update in One Statement
- 35Combining MIN, MAX, and AVG
- 36Restricting Input with CHECK Constraints
- 37Auto-Numbering with AUTOINCREMENT
- 38Inspecting Query Plans with EXPLAIN QUERY PLAN
- 39ORDER BY with Multiple Columns
- 40Conditional Aggregation with CASE
- 41Removing Objects with DROP TABLE and DROP VIEW
- 42Combining Multiple CTEs
- 43Date Range Search with BETWEEN
- 44Finding What's Missing with NOT EXISTS
- 45Substituting NULL with COALESCE
- 46String Functions: UPPER, LENGTH, and SUBSTR
- 47Converting Types with CAST
- 48Merging Grouped Values with GROUP_CONCAT
- 49Enforcing Integrity with FOREIGN KEY
- 50Capstone: Build a Mini Library Management System
Filtering Aggregates with HAVING
This lesson covers filtering aggregated results with HAVING, and clarifies how it differs from WHERE. It's for anyone searching "SQL HAVING clause" or "SQL WHERE vs HAVING."
WHERE filters rows before aggregation happens; HAVING filters the results after GROUP BY has already summarized them — useful for something like "only show groups with 3 or more members."
The example groups by class_id, then uses HAVING COUNT(*) >= 2 to keep only classes with two or more students. The key point: a condition on an aggregate result belongs in HAVING, not WHERE.
A common early mistake is putting an aggregate function like COUNT() inside WHERE, which throws an error — WHERE runs before GROUP BY, and HAVING runs after, and once that execution order clicks, it's obvious why WHERE can't reference a count that doesn't exist yet. Needing "only groups above a certain size" comes up constantly in sales analysis and traffic reports, so knowing when to reach for HAVING instead of WHERE is an essential aggregation skill.
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