Introduction

SQL is a widely used programming language for managing and manipulating relational databases. SQL statements are executed in a specific order to produce the desired results. Understanding the order of SQL execution is crucial for developing efficient and effective SQL queries. In this article, we will discuss the order of SQL execution with clear examples.

SQL execution order can be divided into three major parts.

1. FROM and JOIN
2. WHERE, GROUP BY, and HAVING
3. SELECT and ORDER BY

Let’s explore each of these parts in more detail.

FROM and JOIN in SQL

The first step in SQL execution is to retrieve data from one or more tables using the FROM and JOIN clauses. The FROM clause specifies the tables from which data is retrieved, while the JOIN clause combines data from multiple tables.

For example, consider the following SQL query,

SELECT *
FROM orders
JOIN customers ON orders.customer_id = customers.customer_id

In this query, the FROM clause retrieves data from the “orders” table, and the JOIN clause combines data from the “customers” table based on the “customer_id” column.

WHERE, GROUP BY, and HAVING in SQL

After retrieving data from one or more tables, the next step is to filter the data based on certain criteria using the WHERE clause. The WHERE clause is used to specify conditions that must be met by the data being retrieved.

For example, consider the following SQL query,

SELECT *
FROM orders
JOIN customers ON orders.customer_id = customers.customer_id
WHERE customers.country = 'USA'

In this query, the WHERE clause filters the data only to retrieve orders made by customers from the United States.

GROUP BY in SQL

This clause is used to group the retrieved data based on one or more columns. The HAVING clause filters the grouped data based on certain criteria.

For example, consider the following SQL query,

SELECT COUNT(*) AS order_count, customers.country
FROM orders
JOIN customers ON orders.customer_id = customers.customer_id
GROUP BY customers.country
HAVING COUNT(*) > 100

SELECT and ORDER BY in SQL

The final step in SQL execution is to select the columns to be displayed using the SELECT clause. The SELECT clause specifies the columns to be displayed and can also include aggregate functions to perform calculations on the retrieved data.

For example, consider the following SQL query,

SELECT customers.customer_id, customers.name, COUNT(*) AS order_count
FROM orders
JOIN customers ON orders.customer_id = customers.customer_id
GROUP BY customers.customer_id, customers.name
ORDER BY order_count DESC

In this query, the SELECT clause specifies the customer ID, name, and the number of orders they have made. The ORDER BY clause orders the results by the number of orders in descending order.

Summary

Understanding the order of SQL execution is crucial for developing efficient and effective SQL queries. The FROM and JOIN clauses retrieve data from one or more tables, the WHERE, GROUP BY, and HAVING clauses filter and group the retrieved data, and the SELECT and ORDER BY clauses select and order the columns to be displayed. By following this order of execution, you can write complex SQL queries with ease and efficiency.

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Reference:

Reddy, J. (2023). Order of SQL Execution. Available at: https://www.c-sharpcorner.com/article/order-of-sql-execution/ [Accessed: 21st August 2023].