SQL queries are written in a specific syntax, but the database engine doesn't execute them in the same order they're written. Understanding the actual execution order is fundamental for writing efficient queries and avoiding logical errors. Let's explore how SQL statements are really processed by the database engine.
Here's the actual logical execution order that database engines follow. First, the FROM clause identifies the data sources. Then WHERE filters individual rows. GROUP BY groups the filtered rows. HAVING filters the groups. SELECT chooses which columns to return. ORDER BY sorts the results. Finally, LIMIT restricts the number of returned rows. This order is crucial for understanding query behavior.
Let's see how FROM and WHERE work together. The FROM clause first identifies the employee table as our data source, creating the initial dataset with all rows. Then the WHERE clause filters this dataset, keeping only rows where salary is greater than 5000. Notice how WHERE operates on individual rows, not groups. This filtering happens before any grouping or aggregation.
After WHERE filtering, GROUP BY organizes rows into groups based on department. Each group contains rows with the same department value. We can then calculate aggregate functions like COUNT and AVG for each group. HAVING filters these groups based on aggregate conditions - in this case, keeping only departments with more than one employee. Notice that HAVING works on groups, while WHERE works on individual rows.