> ## Documentation Index
> Fetch the complete documentation index at: https://rajanand.org/llms.txt
> Use this file to discover all available pages before exploring further.

# SQL query order of execution

> Understand how SQL queries run internally

SQL statements are executed in a specific **lexical execution order**, which determines how SQL queries are processed internally. Although SQL is written in a declarative way (i.e., not in the execution order), the SQL engine follows a distinct sequence. Here's the **lexical execution order** for a typical SQL `SELECT` statement:

#### 1. **FROM**

* The source tables or joins between tables are identified.
* Any subqueries in the `FROM` clause are executed first.
* Temporary tables may be created to handle derived tables or subqueries.

#### 2. **WHERE**

* Filters rows from the `FROM` clause based on specified conditions.
* This step excludes rows that do not meet the condition.

#### 3. **GROUP BY**

* Groups rows into subsets based on column(s) specified.
* Aggregate functions (e.g., `SUM`, `MAX`, `COUNT`) operate on these groups.

#### 4. **HAVING**

* Filters groups formed in the `GROUP BY` clause based on specified conditions.
* Similar to `WHERE`, but operates on grouped data.

#### 5. **SELECT**

* Projects columns or expressions to return the desired data.
* Includes computed columns and uses aggregate functions.

#### 6. **DISTINCT**

* Eliminates duplicate rows in the result set, if specified.

#### 7. **ORDER BY**

* Sorts the result set based on specified column(s) or expressions.
* Can be ascending (`ASC`) or descending (`DESC`).

#### 8. **LIMIT / FETCH / TOP**

* Restricts the number of rows returned to the client.
* Syntax depends on the database system (e.g., `LIMIT` in MySQL, `TOP` in SQL Server).

***

#### Example Query Execution Breakdown

<CodeGroup>
  ```sql T-SQL theme={"system"}
  SELECT TOP 5 department, COUNT(*) AS employee_count
  FROM employees
  WHERE salary > 10000
  GROUP BY department
  HAVING COUNT(*) > 10
  ORDER BY employee_count DESC;
  ```

  ```sql MySQL theme={"system"}
  SELECT department, COUNT(*) AS employee_count
  FROM employees
  WHERE salary > 10000
  GROUP BY department
  HAVING COUNT(*) > 10
  ORDER BY employee_count DESC
  LIMIT 5;
  ```
</CodeGroup>

1. **FROM**: Reads the `employees` table.
2. **WHERE**: Filters rows with `salary > 10000`.
3. **GROUP BY**: Groups rows by the `department` column.
4. **HAVING**: Retains groups where `COUNT(*) > 10`.
5. **SELECT**: Selects `department` and the computed `employee_count`.
6. **ORDER BY**: Sorts the result by `employee_count` in descending order.
7. **LIMIT**: Returns the top 5 rows.

Understanding this order helps in optimizing queries and ensuring proper execution.