Summary  
This chapter explains the correct sequence of clauses in a SELECT SQL query—SELECT, FROM, WHERE, GROUP BY, ORDER BY, (LIMIT)—and emphasizes placing GROUP BY after filtering and before sorting to aggregate and order results properly.

General domain of usage  
Relational database querying

In our statistical analysis, we need to **count the number of stations** on each metro line and **sort them in ascending order** based on the number of stations.

This means we should first **determine the number of stations** for each metro line and then arrange them from the line with the fewest stations to the one with the most.

This information will help the construction company decide **which metro lines should be prioritized** for adding more stations.


For this, it's crucial to understand the **sequence of SQL clauses**, especially where to place the `GROUP BY` clause.

So, the order looks like this:

1. `SELECT` statement;
2. `FROM table`;
3. `WHERE` clause;
4. `GROUP BY` clause;
5. `ORDER BY` clause;
6. `LIMIT` clause.


Let's consider **an example** of such statement order using our `employee` table. Suppose we need to retrieve the **number of employees** in each `department` whose `salary` is **above 70000** and **sort them** from smallest to largest:

SELECT department, COUNT(employee_id) AS number_of_employees
FROM employees
WHERE salary > 70000
GROUP BY department
ORDER BY number_of_employees

This course is perfect for those who already have a basic understanding of SQL and want to delve into more advanced concepts to craft more powerful queries. Throughout the course, you will become familiar with data grouping and filtering grouped data. You will also learn how to work with multiple tables simultaneously, including how to combine them. Additionally, you will explore different types of table joins and how to apply them in practice.

Order of Statements

Brief Instructions

解答