When you submit a SQL query, it travels through a series of well-defined stages before returning results. Understanding these stages helps you write more efficient queries and diagnose performance issues. The typical lifecycle of a SQL query includes:

• Parsing: checks the query's syntax and transforms it into a structured format;
• Planning: devises possible strategies for executing the query;
• Optimization: selects the most efficient plan based on available strategies and database statistics;
• Execution: carries out the chosen plan and retrieves or modifies data;
• Result Delivery: formats and sends the final results back to you.

Each stage plays a unique role in ensuring your query is valid, efficient, and produces the correct output.

When you write a simple query like SELECT * FROM employees;, the SQL engine follows a clear sequence of steps:

1. The engine receives your query as plain text;
2. Parsing analyzes the syntax, ensuring the statement follows SQL rules and converting it into an internal structure called a parse tree;
3. Planning considers possible ways to access the employees table and retrieve data;
4. Optimization evaluates different plans, often choosing the fastest or least resource-intensive option;
5. Execution runs the selected plan, reading all rows from the employees table;
6. Result delivery packages the rows and returns them to you.

This process is largely invisible, but it happens every time you submit a query, no matter how simple.

When you add a WHERE clause, as in SELECT name, salary FROM employees WHERE department = 'Engineering';, the lifecycle adapts by including additional checks and planning steps. The parser must validate the filter condition, and the planner and optimizer must consider how best to locate only those rows matching the department criteria. This can affect which indexes are used or how data is scanned, but the overall lifecycle—parsing, planning, optimization, execution, and result delivery—remains the same.