In SQL, what is one key similarity and one key difference between the WHERE clause and the HAVING clause?

Introduction / Context:
This question examines your understanding of the WHERE and HAVING clauses in SQL, especially in the context of grouped queries. Both clauses are used to restrict data, but they operate at different stages of query processing. Knowing when to use WHERE and when to use HAVING is fundamental for writing correct aggregate queries and is a frequent topic in database interviews.

Given Data / Assumptions:
• The environment is a relational database that supports SQL with GROUP BY, WHERE, and HAVING. • The question asks for one similarity and one difference between WHERE and HAVING. • We assume basic familiarity with aggregate functions such as SUM, COUNT, and AVG. • No numeric calculation is required; only conceptual understanding of query processing order is needed.

Concept / Approach:
Both WHERE and HAVING act as filters in a SELECT statement, but they apply at different stages. WHERE filters individual rows from the base tables before grouping and aggregation occur. It cannot directly reference aggregate functions like SUM or COUNT. HAVING, by contrast, filters the grouped results after GROUP BY and aggregation have been applied, and it is specifically designed to work with aggregate expressions. The similarity is that both clauses restrict the final output. The difference lies in whether the filtering happens before or after grouping and whether aggregates can be used in the condition.

Step-by-Step Solution:
Step 1: Recall that WHERE appears before GROUP BY in a SELECT statement and restricts rows from the underlying tables. Step 2: Remember that HAVING appears after GROUP BY and applies conditions to groups or aggregated values. Step 3: Identify the common purpose: both clauses are used to limit which data appears in the query result. Step 4: Recognize that WHERE generally cannot contain aggregate functions, while HAVING often does. Step 5: Choose the option that states that both clauses filter data but operate at different stages relative to GROUP BY.

Verification / Alternative check:
Think about an example query: SELECT deptno, SUM(sal) FROM emp WHERE sal > 1000 GROUP BY deptno HAVING SUM(sal) > 5000;. Here, WHERE sal > 1000 removes rows with salary less than or equal to 1000 before grouping. Then GROUP BY forms groups by deptno, and HAVING SUM(sal) > 5000 keeps only those departments whose total salary exceeds 5000. This demonstrates that both WHERE and HAVING are filters, but WHERE operates on individual rows before aggregation while HAVING operates on groups after aggregation. That matches the description in option b.

Why Other Options Are Wrong:
Option a incorrectly claims that both filters operate after grouping and reverses the relationship of aggregate usage. Option c states that WHERE filters columns and HAVING filters rows, which is incorrect; both filter rows or groups, not column definitions. Option d suggests a subquery versus outer query distinction, which is not the core conceptual difference between WHERE and HAVING. Option e claims HAVING is used in INSERT statements, which is wrong; HAVING is used only in SELECT statements with grouping.

Common Pitfalls:
A frequent mistake is trying to place aggregate conditions in the WHERE clause, which causes syntax errors or incorrect logic. Developers also sometimes overuse HAVING for conditions that could be applied in WHERE, causing unnecessary work by grouping more rows than required. Another pitfall is forgetting that HAVING is only meaningful when GROUP BY is present or when you rely on implicit grouping. Understanding the logical order of query processing helps avoid these issues and leads to more efficient and readable SQL.

Final Answer:
The correct choice is Both WHERE and HAVING are used to filter data, but WHERE filters individual rows before grouping, whereas HAVING filters groups after GROUP BY., because it clearly states the shared purpose of filtering and the crucial difference in when and how each clause is applied.