Do you want to know about the SQL order of operations? You may know BODMAS in mathematics or Operators precedence order of execution in programming languages. Well, the same thing happens when we execute an SQL query.
It is important to understand the order of execution of the SQL operations or clauses as they significantly impact the performance and efficiency of the queries you write. In this blog post, we are going to learn about these operations and their execution order with the help of examples.
So, waiting further, let’s jump into it!
Summary Of The Article:
SQL or Structured Query Language uses various operations or clauses to perform a database-related task.
These clauses help in getting the data, arranging them in a specific order, filtering data, and performing other such operations to extract information as per our requirements.
Clauses in SQL act as conditional statements and extract the result set as per the stated conditions.
Having an understanding of how these operations work in an SQL query is vital for producing efficient results.
Does SQL Follow Order Of Operations?
To answer this question many people wonder in a very simple way…Yes! SQL does follow an order of operations. This helps in defining the sequence of how each part of the query will be processed and executed.
This order of execution is essential to obtain accurate and consistent results. Thus, it helps us in defining queries that work effectively. The general SQL order of operations includes the following:
FROM clause
WHERE clause
GROUP BY clause
HAVING clause
SELECT clause
DISTINCT clause
ORDER BY clause
LIMIT clause
We will learn about each of these in detail with the help of an example so that you can understand all these clauses and their order of execution in a better way. So, let us dive in!
Consider the following tables ‘Employees’ and ‘Orders’ for our example problem. The Employees table contains information about a company’s employees, whereas, the Orders table contains information about the order they are supposed to deliver.
With the help of the above tables, we will understand the SQL order of execution of various clauses. Along our journey, we will also understand more complex queries as well. Let us continue!
SQL FROM Clause
The FROM clause in SQL is used to specify which table/tables we need to retrieve the data from. It is also paired with the JOIN clause to derive the data from multiple tables. When we execute an SQL query, this clause is executed first.
Below is an example query that outputs the whole table ‘Employee’.
SELECT* FROM 'Employees';
In this statement, the FROM clause gets executed first, followed by SELECT. If we pair this using a JOIN, it will return the cartesian product of the two tables. The query for the same is given below.
SELECT* FROM 'Employees'
JOIN Orders ON Employees.Employee_id = Orders.Employee_id
SQL WHERE Clause
After the FROM clause is executed in a query, the WHERE clause is executed. It is used to filter the rows based on the specific conditions provided to it. For example, if we want to get the first_name and last_name of only those employees who are from the UK, we will use this clause.
SELECT first_name, last_name FROM 'Employees' WHERE country = 'UK';
While getting to know SQL order of operation, JOIN vs WHERE is a point of confusion for many people. The WHERE clause is a filtering operation that filters the row result based on specific conditions. JOIN clauses, however, are used for joining two or more tables based on related columns.
SQL GROUP BY Clause
Next in the order of execution of SQL queries, is the GROUP BY operation. This clause is mainly used for grouping individual rows that have the same values into summary rows. For instance, we want to group the items in the orders table based on their amount. The following query will be executed in this case.
SELECT item, amount FROM 'Orders' GROUP BY amount;
SQL HAVING Clause
Now that we have filtered rows and grouped them, it is time to filter the groups using the HAVING clause. It filters the grouped rows based on some conditions. Have a look at the query below.
SELECT item, amount FROM 'Orders' GROUP BY amount HAVING amount >300;
Here, only those items will be retrieved that have an amount of more than 300. These are given in the output table below.
In case, you’re facing difficulties in understanding the output tables, then you ask for SQL homework help from experts online.
SQL SELECT Clause
The SELECT clause in SQL is used to specify which columns do we want in our result. We can get one or more columns as our result as well. You can use (*) to indicate all the data present in any table or mention specific column aliases as per your requirement. Basically, it acts like a print statement.
You can also mention other clauses or aggregate functions along with this clause. We will see some of these in the later sections. Let us first see how we can select the first_name, last_name, and item, from the above tables using the SELECT clause in the following query.
SELECT first_name, last_name, item FROM 'Employees' INNER JOIN 'Orders' ON Employees.Employee_id = Orders.Employee_id;
Here is the output table we got using the select clause.
You can see that the data is listed based on the employee IDs in the result.
Some Extra Order of Execution Queries
By now, I hope you have understood what is the query order of execution in Structured Query Language. As a bonus, let me tell you about the SQL Order of Operations CASE/WHEN statement as well.
A CASE statement also returns a value when a specific condition is met. You can use it with a SELECT statement along with a GROUP BY clause, ORDER BY, or WHERE. Filter a row or group rows using the WHEN or CASE statement.
Select order_id,
CASE
WHEN order_id = 3 AND amount >=500 THEN 'Monitor'
Else 'Other'
END AS item
from Orders;
You can see the result set below when only the item with order_id is listed as its item name, the rest are named as ‘Other’ in the item column.
I hope that this article has cleared your doubts regarding the order of execution process. Let us have a quick recap.
Conclusion:
Like any other programming language, Structured Query Language also follows an order for operations. Having a knowledge of the query order is helpful for creating accurate and effective queries and extracting meaningful data from the database system.
SQL’s order of query processing is discussed above, if you are just getting started with DBMS and learning how to implement queries, you will definitely find grasping this concept helpful. We understand that DBMS can be a difficult concept as it involves coding. Then you can ask for DBMS help online from CodingZap experts.
Key Takeaways:
Clauses and Statements in Structured Query Language are used to quickly manipulate data present in the database to retrieve the information required by us.
By understanding the query order process, we can write more accurate queries that result in efficient performance and consistent data retrieval.
Aggregate function like can also be used with these clauses for efficient processing of the final result.
You can filter the same value and determine the order in which you want to get your result set as well.
FAQs (Frequently Asked Question by Students)
Any conflict between these clauses is resolved by SQL based on the precedence of these clauses and the logical requirement of the query.
Window functions are some special kinds of functions that perform certain operations on set of rows defined by a window frame.
These are executed after the GROUP BY clause and the HAVING Clause, but before the ORDER BY and LIMIT clauses.
