Introduction to logical query processing in mysql (code example)

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Query processing can be divided into logical query processing and physical query processing.
Logical query processing indicates what results should be produced by executing the query.
Physical query processing indicates how the MySQL database obtains the result.
The two query methods may be completely different, but the results must be the same.

1. Logical query processing

In sql language, the first clause to be processed is always the FROM clause.

//示例
//前面的序号表示执行的顺序
(8) SELECT (9) DISTINCT<select_list>
(1) FROM<left_table>
(3) <join_type> JOIN <right_table>
(2)				ON<join_condition>
(4) WHERE <where_condition>
(5) GROUP BY<group_by_list>
(6) WITH {CUBE|ROLLUP}
(7) HAVING <having_condition>
(10) ORDER BY<order_by_list>
(11) LIMIT<limit_number>

You can see that FROM is executed first and LIMIT is executed last. Each operation produces a virtual table that serves as input to a process. These virtual tables are transparent to users, and only the virtual tables generated in the last step will be returned to users.

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