SQL Query Flow: Understanding Query Execution Order

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Understanding the order of execution of SQL queries is crucial for writing efficient and accurate queries. The logical processing order of a SELECT statement in SQL can be broken down into several phases. Here is the general sequence of execution:

1. FROM Clause:

• The first operation in a SELECT statement is to retrieve data from one or more tables or views specified in the FROM clause.
• Tables are joined, and the Cartesian product is created if there are multiple tables in the FROM clause.

2. ON Clause (if applicable):

• If a JOIN operation is used, the ON clause specifies the conditions for joining tables.

3. JOIN Operations:

• After the ON clause, JOIN operations are performed. The type of JOIN (INNER, LEFT, RIGHT, FULL) affects the result set.

4. WHERE Clause:

• The WHERE clause is used to filter rows based on specified conditions.
• Rows that do not meet the conditions are eliminated from further processing.

5. GROUP BY Clause:

• If a GROUP BY clause is present, rows are grouped based on the specified columns.