SQL WHERE Clause
While performing a SQL operation the WHERE clause is the most used in all of the operations where the relevant feature is available as it essentially allows for one to set a condition in any case and database. First and foremost, it deals with granularity as it aims to filter the employees in such a way that allows one to specify the row level whilst applying several criteria.For example, you can check a database of employees of a specific age. Knowing that you need only the ones that are older than thirty would mean that the employee registry carries irrelevant information. When a WHERE clause is utilized the task is completed with simple filters as it is added at the start of the task directly disagreeing the employees that are below thirty.
The WHERE clause has several functions as it can be used to check conditions ranging from simple less than and greater than conditions to complex conditions such as checking for NULL values or pattern matching.
The HAVING Clause
The HAVING clause, in contrast, is used subsequent to grouping and aggregation. The use of the WHERE clause is at the row level while the HAVING clause is at the group level that results from the GROUP BY statement. If your figures include gross profits, averages or counts, etc., you are using HAVING, which does not operate on raw data. It helps you refine the result of an aggregate operation.To comprehend the logic behind HAVING, let’s consider a situation where you have grouped your data on some columns like department or region. After you grouped the data, you might want to place a condition on the output of the aggregation. For instance, you may want to locate departments whose total salaries are above a specified limit. In this instance, the HAVING clause will definitely be required as it allows specification of conditions on the result of the GROUP BY command.
Key Differences Between WHERE and HAVING
As much as WHERE and HAVING appear to work in the same capacity of filtering data, there exist significant differences which are worth noting:1. WHEN They Are Used:
This type of clause is placed within the context of grouping level meaning that it works in a left to right manner with the data coming to it being grouped already. The clause WHERE is provided before several rows are combined and acts to confirm or filter rows that pass within that region of grouping. Unlike the HAVING clause, which appears at the end of the statement.2. What They Filter:
To this end, WHERE only filters non-aggregated columns in a table that are structured according to the selection criteria. Like the sum of values in one or more columns. There are sums however that constitute aggregates that do escape the clause and that’s WHERE clause but it still is limited. This clause is meant to act on aggregates. It uses functions such as SUM, AVG, COUNT, MIN, or MAX.3. Compatibility:
In an aggregate free query you can incorporate WHERE, but aggregation is not a requirement. For the use of HAVING, it is requisite to use WHERE because of the level of the data that it acts on.So let's say you want to use the specific data of customers that placed orders over an amount, in that case you will use the WHERE clause. On the other hand, if you want to partition your clients according to the total amount of the orders that they have done as well as filter only those who surpass a certain amount, then the clause required is the HAVING clause.
Common Use Cases
- Filtering Individual Records (WHERE Clause):
If you wish to filter individuals' records in need of an order by all means, this is when the “WHERE” clause will come in handy for various sorts of conditions. For example in cases where you want to retrieve all employees working within a specific department or all orders within a determined timeline.- Filtering Aggregated Data (HAVING Clause):
This can be applied where you have aggregated data, for example when needing to calculate the average salary of employees across different departments that have a specific average salary threshold the HAVING clause comes in handy.The Benefits of Using WHERE in Combination with HAVING
In practice, the use of the two clauses WHERE and HAVING is frequent. The sequence of filters is that WHERE clauses pre-aggregates rows into groups as per the corresponding fields of the grouping clause while the filtered superset is defined by HAVING clauses in the post aggregation stage. Therefore, it is possible to filter the data on both sides of grouping.For example, if you want to get data of employees in a company, first, you may remove employees that have spent less than five years in the company. For example, if you have a group of employees organized by department and the average salary of each department is calculated, then you would like to filter out those departments with average salaries below a certain value. This time, WHERE and HAVING both would be needed.
