How Does an SQL Query Execute: A Step-by-Step Breakdown?

Execution Order of SQL Query

The execution order of an SQL statement is crucial for optimizing its performance. Execution proceeds in specific stages, as defined by the SQL standard:

Stage 1: FROM Clause

• Identifies data sources (tables or views) involved in the query.
• This step establishes the context for subsequent clauses.

Stage 2: WHERE Clause

• Filters the data from the specified data sources based on specific conditions.
• Rows that satisfy the WHERE criteria are included in the result set.

Stage 3: ORDER BY Clause

• Sorts the result set in ascending or descending order based on specified columns (e.g., C.CustomerSalary DESC).
• This operation typically occurs after the WHERE clause to ensure that only qualifying rows are sorted.

Additional Clarifications:

• The SELECT clause is executed before the ORDER BY clause, but after the WHERE clause.
• The LIMIT or TOP clauses are typically executed after the ORDER BY clause to limit the number of rows returned.
• The query optimizer may reorder operations for efficiency, but the logical execution order remains the same.
• The physical execution order may differ due to factors such as indexing and optimization techniques employed by the database engine.

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