𝗦𝗤𝗟 𝗤𝘂𝗲𝗿𝘆 𝗘𝘅𝗲𝗰𝘂𝘁𝗶𝗼𝗻 𝗢𝗿𝗱𝗲𝗿!

When you write an SQL query, the database engine executes the clauses in a specific order, which is different from the sequence in which you write them.

Understanding this order is crucial for optimizing queries and debugging issues efficiently. Here’s a concise guide to the SQL query execution order:

1. 𝗙𝗥𝗢𝗠 𝗮𝗻𝗱 𝗝𝗢𝗜𝗡
Purpose: Identify and join tables.
FROM: Specifies the tables to be queried.
JOIN: Combines records from related tables.

Example:
“FROM Employees
JOIN Departments ON Employees.DeptID = Departments.DeptID
“

2. 𝗪𝗛𝗘𝗥𝗘
Purpose: Filter rows based on conditions.
Filters individual rows that meet the specified criteria.

Example:
“WHERE Employees.Salary > 50000
“

3. 𝗚𝗥𝗢𝗨𝗣 𝗕𝗬
Purpose: Group rows based on column values.
Creates summary rows using aggregate functions.

Example:
“GROUP BY Departments.DeptName
“

4. 𝗛𝗔𝗩𝗜𝗡𝗚
Purpose: Filter groups based on conditions.
Filters groups created by GROUP BY.

Example:
“HAVING COUNT(Employees.EmployeeID) > 5
“

5. 𝗦𝗘𝗟𝗘𝗖𝗧
Purpose: Select columns for the final result.
Specifies which columns to display.

Example:
“SELECT Departments.DeptName, COUNT(Employees.EmployeeID) AS NumEmployees
“

6. 𝗗𝗜𝗦𝗧𝗜𝗡𝗖𝗧
Purpose: Remove duplicate rows.
Ensures uniqueness in the result set.

Example:
“SELECT DISTINCT Employees.JobTitle
“

7. 𝗢𝗥𝗗𝗘𝗥 𝗕𝗬
Purpose: Sort the result set.
Orders the rows based on specified columns.

Example:
“ORDER BY NumEmployees DESC
“

8. 𝗟𝗜𝗠𝗜𝗧 / 𝗢𝗙𝗙𝗦𝗘𝗧
Purpose: Limit the number of rows returned.
Constrains the result set.

Example:
“LIMIT 10 OFFSET 20
“

Complete Example Query

Here’s how it looks when put together:

“SELECT Departments.DeptName, COUNT(Employees.EmployeeID) AS NumEmployees
FROM Employees
JOIN Departments ON Employees.DeptID = Departments.DeptID
WHERE Employees.Salary > 50000
GROUP BY Departments.DeptName
HAVING COUNT(Employees.EmployeeID) > 5
ORDER BY NumEmployees DESC
LIMIT 10 OFFSET 20
“