As an expert in the field of SQL and database management, I can provide a comprehensive explanation of the differences between the WHERE and HAVING clauses in SQL. Understanding these clauses is crucial for writing effective queries that can extract and analyze data as intended.
Step 1: English Explanation
In SQL, both the WHERE and HAVING clauses are used to filter data, but they serve different purposes and are used in different contexts within a query.

WHERE Clause:

1. Purpose: The primary purpose of the WHERE clause is to filter the rows of a table based on specified conditions before any grouping or aggregation takes place. It is used to narrow down the dataset to only those rows that meet the criteria set by the conditions in the WHERE clause.


2. Usage: It is typically used in the context of individual rows and can include a wide range of conditions such as equality (`=`), inequality (`<>`, `!=`, `>`, `<`), pattern matching (`LIKE`), and logical operators (`AND`, `OR`, `NOT`).


3. Syntax Level: The WHERE clause is positioned before the GROUP BY clause if one is present in the query. It cannot be used to filter grouped data because the grouping has not yet occurred when the WHERE clause is processed.


4. Example: Suppose we have a table of sales data with columns for `product_id`, `quantity_sold`, and `sale_date`. A WHERE clause might be used to filter out all sales that did not occur in the year 2023:
```sql
SELECT product_id, quantity_sold
FROM sales
WHERE sale_date >= '2023-01-01' AND sale_date < '2024-01-01';
```

HAVING Clause:

1. Purpose: The HAVING clause is used to filter groups of rows after the grouping has been performed by the GROUP BY clause. It is used to include or exclude groups based on the result of an aggregate function, such as `SUM()`, `COUNT()`, `MAX()`, `MIN()`, or `AVG()`.


2. Usage: It is applied to the result set after the data has been grouped and aggregated. The conditions in the HAVING clause cannot reference individual row columns but must reference aggregated columns.


3. Syntax Level: The HAVING clause comes after the GROUP BY clause. It is an extension of the WHERE clause functionality to the grouped data context.


4. Example: Continuing with the sales data example, if we wanted to find groups of products that had a total quantity sold greater than 100 units in the year 2023, we would use a HAVING clause:
```sql
SELECT product_id, SUM(quantity_sold) AS total_quantity
FROM sales
WHERE sale_date >= '2023-01-01' AND sale_date < '2024-01-01'
GROUP BY product_id
HAVING SUM(quantity_sold) > 100;
```

Key Differences:
- The WHERE clause filters individual rows before grouping, while the HAVING clause filters groups after grouping.
- The WHERE clause cannot use aggregate functions, whereas the HAVING clause is designed to work with them.
- The WHERE clause is used before the GROUP BY clause, and the HAVING clause is used after it.

Step 2: Separator
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2) WHERE clause is used for filtering rows and it applies on each and every row, while HAVING clause is used to filter groups in SQL. 3) One syntax level difference between WHERE and HAVING clause is that, former is used before GROUP BY clause, while later is used after GROUP BY clause.Aug 1, 2017read more >>